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Soil composition
- Mineral part of soil
Absorption capacity of soils (Русский Español)
Soil acidity
Base saturation and soil buffering
Effect of fertilizers on soil properties
Agrochemical characteristics of Russian soils

Mechanical absorption capacity

The mechanical absorption capacity of the soil is the ability to mechanically retain particles in the soil pores, stirred up in the water passing through the soil profile. It retains suspensions of aluminosilicate and organic particles, colloidal dispersed substances, which allows to retain colloidal fractions, poorly soluble fertilizers and ameliorants.

The intensity of mechanical absorption depends on soil porosity, pore size, dispersion of substance. Therefore, clayey and loamy soils are characterized by more mechanical absorption capacity than sandy and sandy loam soils.

This type of absorption capacity also plays a role in the distribution of microorganisms along the soil profile.

Physical absorption capacity

Physical absorption capacity of soils is the ability to retain mineral and organic substances on the surface of the solid phase due to the action of adsorption forces. Physical absorption is caused by the presence of highly dispersed soil phase “soil + soil solution”, which has free surface energy, due to which soil water and substances contained in it are attracted and accumulate on the surface of soil particles.

In large-grained soils, such as sandy soils, the adsorption forces are relatively small, so their physical absorption capacity is low. With increasing soil dispersion, the amount of silt and colloidal fraction, the adsorption forces increase.

An increase in the concentration of the dissolved substance in the layer of dispersion medium adjacent to the particles of the solid phase is called positive adsorption. Substances that increase the surface tension of the dispersion medium cause negative adsorption. In the case of positive adsorption, the dispersed phase attracts dissolved substances from the dispersion medium to its surface, while negative adsorption repels them. Hydrates of metal oxides and salts formed by strong bases and weak acids are positively adsorbed. Anions are adsorbed negatively.

At this type of absorption substances contained in soil solution are not changed.

Due to a large number of factors (soil composition, weather conditions) that can affect physical absorption, this type of absorption is dynamic.

Positive adsorption is important in soil processes and plant nutrition: dissolved substances are kept from leaching into deeper soil layers and different concentrations of nutrients are created, allowing plants to choose the solution with the best concentration for them.

Physical absorption in the soil can also result from coagulation (sticking) of colloidal particles. This process stops the leaching of silt and nutrients from the root zone. Physical absorption is difficult to determine because it is interrelated with chemical absorption and exchange adsorption.

Physical uptake is important for the rational use of fertilizers that include, for example, soluble nitrates and chlorides. For example, chloride ions in large quantities are toxic to many crops, so chloride-containing fertilizers are applied in autumn so that during autumn and spring precipitation it can be washed out of the arable layer. On the contrary, for nitrate fertilizers this leaching is undesirable, so they are applied in spring before sowing or in top dressing.

The physical absorption capacity of soils is of environmental importance: adsorption of pesticides reduces their penetration into the adjacent environment, including plants.

Biological absorption

Biological absorption is the absorption of nutrients, fertilizers and air by plants and soil microflora. In the process of vital activity of plants and soil microorganisms organic matter including ash elements and nitrogen is accumulated.

The peculiarity of biological absorption is selectivity: plant roots and microorganisms absorb from soil those elements which they need.

Biological uptake includes fixation of free nitrogen by nitrogen-fixing microorganisms. Nitrogen fixers convert molecular atmospheric nitrogen into forms available to plants. The nitrogen fixation process can be regulated by fertilizers, ameliorants, legume crops, and tillage practices. Nitrification bacteria oxidize ammonia nitrogen into nitrate nitrogen, which, if not assimilated by plants and microorganisms, is leached from the soil and/or subjected to denitrification. Biological uptake is the only way nitrates and chlorides are retained in the soil.

Some of the nutrients applied to the soil with fertilizer are consumed by microorganisms. If the process is severe, it can negatively affect plant nutrition. In cultivated sod-podzolic soils, microbial cell plasma contains about 125 kg of nitrogen, about 40 kg of P₂O₅ and about 25 kg of K₂O per 1 ha.

According to studies conducted by radioisotope method using 15N isotope, 10-20% of nitrogen in the form of nitrates and 20-40% in the ammonium form of nitrogen fertilizers are fixed in organic form in the soil.

The intensity of biological absorption is influenced by:

air regime of soils;
content of soil organic matter, plant residues and organic fertilizers;
thermal regime of soils;
environmental reactions.

An easily accessible source of energy material for microorganisms is plant root excretions. Therefore, microbiological processes are particularly intense in the rhizosphere zone.

Fungi and their mycorrhiza developing in the rhizosphere zone play an important role in plant nutrition. Fungi are supplied with oxygen through the root system of plants; they have the ability to decompose soil organic matter and provide the host plant with minerals and water.

Microbiological processes in the soil can be regulated through the use of organic and mineral fertilizers, liming, bacterial fertilization, crop rotation, timing and methods of soil treatment and other agricultural practices.

Chemical absorption capacity

Chemical absorption capacity of soils is the ability of the soil to retain ions through the formation of insoluble compounds as a result of chemical reactions, or, the ability of the soil to convert anions and cations of the soil solution into insoluble substances.

Nitrates and chlorides (NO₃^– and Cl^–) do not participate in chemical absorption because they do not form insoluble substances with any soil cation. Carbonates and sulphates (СO₃^2-, SO₄^2-) are chemically absorbed in soils with high content of calcium Ca²⁺ and magnesium Mg²⁺. Phosphates are fairly easily formed insoluble compounds with all the two- and trivalent cations (Ca²⁺, Mg²⁺, Fe³⁺, Al³⁺).

In neutral soils or soils with a slightly alkaline reaction, containing exchange-absorbed calcium, calcium or magnesium hydrocarbonates in the soil solution, there is a chemical absorption of phosphoric acid as a result of the formation of poorly soluble calcium and magnesium phosphates. Thus, when water-soluble phosphate fertilizers (superphosphate) are applied to soils high in calcium, reactions occur:

Са(Н₂РO₄)₂ + Са(НСO₃)₂ → 2СаНРO₄ + 2Н⁺ + СО₃^2-,

Са(Н₂РO₄)₂ + 2Са(НСO₃)₂ → Са₃(РO₄)₂ + 8Н⁺ + 4СО₃^2-.

With exchange-absorbed calcium in the soil:

(soil)=Ca + Ca(H₂PO₄)₂ → (soil)-H₂ + CaHPO₄ ↓,

(soil)=Ca + Ca(H₂PO₄)₂ → (soil)-H₄ + CaHPO₄ ↓.

The low-soluble salts formed as a result of similar chemical reactions precipitate and pass from the soil solution into the soil solid phase.

Along with the chemical absorption (retrogradation) of phosphate in soils, the reverse process – mobilization of phosphorus: its transfer from inaccessible forms into available ones – is possible. Mobilization occurs when soils are acidified, which, in particular, is possible with an increase in the concentration of carbonic, nitric and organic acids formed in the process of activity and decomposition of plants and biota. Thus, nitric acid formed in the process of nitrification converts the tri-substituted calcium phosphate into a single-substituted one:

4HNO₃ + Ca₃(PO₄)₂ = 2Ca(NO₃)₂ + Ca(H₂PO₄)₂.

In soils with hydrolytic acidity above 2.5 mg-eq/100 g plants are able to assimilate trisubstituted phosphates. Moreover, the more acidic the soil, the more intense this process is, not inferior to the absorption of monosubstituted phosphates.

In an acidic environment, poorly soluble iron and aluminum phosphates can be formed:

Fe(ОН)з + H₃PO₄ → FePO₄ + 3H₂O or

Al(ОН)з + H₃PO₄ → AIPO₄ + 3H₂O.

Unlike calcium phosphates, aluminum and iron phosphates are poorly soluble in soil solution with an acidic reaction. Therefore, the chemical uptake of phosphate occurs differently in different soils. In soils with an acidic reaction low-soluble phosphates of oxides of the type R₂O₃ are formed, and with a neutral and alkaline reaction low-soluble calcium phosphates are formed.

Freshly precipitated three-substituted iron, aluminum and calcium phosphates can be absorbed by plants due to root excretions, but their availability to plants decreases with sediment aging. Two-substituted calcium phosphate (CaHPO₄) is soluble in weak acids and therefore is absorbed by plants due to acidic root excretions. During the period from seed germination to the emergence of developed roots, plants consume only water-soluble monosubstituted phosphates. However, they are quickly chemically bound in all types of soils, and not only with soluble, but also with ions of calcium, magnesium, iron and aluminum in the absorbed state.

In order of increasing ability to chemical absorption of phosphates the soils are arranged in the sequence: black earths < gray earths < sod-podzolic soils < red earths. Intensive absorption of phosphate makes it necessary to apply and embed phosphate fertilizers to a certain depth near the root sites, because they cannot migrate. In order to reduce the chemical absorption of available forms of phosphorus fertilizers reduce the total surface of their contact with the soil by granulating the powdered forms and localization of powdered and granular forms at pre-sowing application. Thanks to this method granular superphosphate at any method of application and on all soils provides a larger increase in crop yields than powdered. Phosphorus from organic fertilizers (manure and compost) is absorbed by plants better than from mineral fertilizers.

Physico-chemical, or exchangeable, absorption capacity

Physical-chemical, or exchangeable, absorption capacity is the ability of soil to retain ions on the surface of particles, or the ability of fine colloids of soil particles with a negative charge to absorb cations from solution. Exchangeable absorption consists in the equilibrium exchange of cations between the solution and the solid phase.

K.K. Giedroytz formulated the basic law of exchange adsorption of soils: all soils have the ability to exchange cations contained in their absorbing complex for electrolyte cations, and the amount of cations absorbed by the soil is equivalent to the amount of cations displaced into the solution.

Surface energy matters in the exchange absorption of cations. In exchange absorption, the electrolytes in the soil solution react chemically with the molecules on the surface of the soil particles. This chemical reaction is made possible by surface energy on the one hand, and negatively charged soil colloidal particles that attract soil solution cations to their surface, where these cations enter into an exchange reaction with the cations on the particle surface. These exchange reactions are physical and chemical in nature.

In contrast to chemical absorption, exchange absorption does not change the concentration of the solution, but changes the composition of the cations in the solution; the concentration of the anions is almost unchanged. When a cation is absorbed from a solution, the soil gives an equivalent amount of another cation that was previously in the solid phase to the solution:

(soil)=Ca + 2NH₄NO₃ → (soil)=(NH₄)₂ + Ca(NO)₃,

(soil)=Ca + 2NaCl → (soil)=Na₂ + CaCl₂.

Depending on solution concentration, nature of cations and properties of adsorbing soil particles, dynamic equilibrium is established between solution cations and solid phase. Thus, changes in soil solution composition during fertilizer application, formation of various soluble compounds in the process of microorganisms life activity, release of carbon dioxide and other substances by plant roots are factors of equilibrium shift.

The exchange absorption in soils was first studied by K.K. Giedroytz. He called the whole totality of finely dispersed mineral and organic soil particles, which are the carrier of exchangeable soil absorption capacity, the soil absorption complex (SAC). It includes soil colloids, i.e. particles from 0.25 to 1 micron in size. With increasing size the exchange absorption capacity decreases. Soil colloids are subdivided into:

organic,
mineral,
organomineral.

Fertilizer exchange reaction with soil SAC can be represented by the following equation:

[SAC](Ca, Mg, H) + 5NH₄NO₃ ⇔ [SAC](NH₄)₅ + Ca(NO₃)₂ + Mg(NO₃)₂ + HNO₃.

Organic colloids are mainly humus substances. Mineral colloids include crystalline clay minerals of kaolinite and montmorillonite groups, hydromica and amorphous compounds (oxide hydrates of the R2O3 type, silicic acid, etc.). Particles of organic and mineral colloids are negatively charged, except for colloids of aluminum and iron hydroxides. This explains the pronounced ability of most soils to absorb cations rather than anions. Clay minerals of kaolinite group can acquire positive charges under acid reaction conditions.

Hydroxyl groups bound to aluminum in the octahedral layer located on the outer surface of microcrystalline kaolinite particles can be detached under acid reaction, forming positive charges on some parts of the particles. In this case they can absorb anions from the soil solution, exchanging them for OH-. For this reason, the more kaolinite group minerals and aluminum and iron hydroxides in the absorbing complex, the lower is the cationic exchange capacity and the higher is the exchange capacity for anion absorption. The main properties of such colloids are manifested in strongly acidic sod-podzolic soils and red soils.

In the SAC of highly acidic soils, such as bogs, podzols, red earths, yellow earths enriched with iron and aluminum hydroxides, along with acidoid colloids there are positively charged (bazoids), containing as counterions anions, displaced by other anions of the soil solution:

[Al(OH)₂]HPO₄ + (NH₄)₂SO₄ ⇔ [Al(OH)₂]SO₄ + (NH₄)₂HPO₄.

Laws of exchange absorption

The exchange reaction proceeds in equivalent ratios and is reversible. This establishes a dynamic equilibrium between the solid phase of the soil and the solution. The equilibrium depends on composition and concentration of solution, nature of anions and cations, soil properties. Fertilizers, ameliorants, degree and intensity of mineralization of organic matter, moisture, consumption of ions by plants shift equilibrium.
At constant concentration of solution amount of cations displaced from solid phase to solution increases with growth of solution volume; at constant volume amount of cations displaced from soil to solution increases with growth of solution concentration.
The rate of exchange reactions is high: for 3-5 minutes from the moment of introducing water-soluble fertilizers with SAC up to 85% of SAC cations are exchanged for an equivalent amount of ions.
Different cations are characterized by different absorption and retention energies in the absorbed state.

The higher the relative atomic mass and the greater the charge of the cation, the stronger its absorption and more difficult to displace from the absorbed state. In the series of monovalent cations, absorption increases in the order:

Li⁺ < Na⁺ < NH₄⁺ < K⁺ < Rb⁺

The relative atomic mass of these cations is 6, 9, 23, 18, 39, 85, respectively. Divalent cations are in the series: Mg2+ < Ca2+ < Co2+ (relative atomic weight 24, 40, 59, respectively), trivalent: Al3+ < Fe3+ (relative atomic weight 27 and 56).

Double-charged magnesium cations are absorbed better than single-charged sodium cations, calcium is absorbed better than potassium. To evaluate the sorbability of cations, K.K. Giedroytz introduced the concept of “cation absorption energy”.

The cations are arranged in increasing absorptivity in the series:

⁷Li⁺ < ²⁵Na⁺ < ¹⁸NH₄⁺ < ³⁹K⁺ < ⁸⁹Rb⁺ << ²⁷Mg²⁺ < ⁴⁰Ca²⁺ < ⁵⁹Co²⁺ << ²⁷Al³⁺ < ⁵⁶ Fe³⁺.

Among the monovalent cations, due to the smaller size of the ions in the hydrated state, the exceptions are ammonium and hydrogen H⁺ or H₃O⁺ (hydroxonium), which is 4 times the absorption energy of calcium and 17 times that of sodium.

The increase in the absorption energy of cations with the increase of their atomic weight is explained by a decrease in their degree of hydration. Weakly hydrated cations are more strongly attracted by the colloid surface. Minerals of kaolinite group absorb cations by the outer surface of the crystal, because they have a small free space between lattice packages (2.8 A). The free space between the packages of the montmorillonite group minerals is greater and amounts to 9.4 A in the dry state and up to 21 A when swelling.

The cation exchange can occur on the outer (extramycellular) and inner surface of colloids (intramycellular). Clay minerals of montmorillonite group with three-layer crystal lattice, expanding with moisture, can absorb cations extramicellularly and intramicellularly. When the soil dries, the interstitial space shrinks and closes the absorbed cations in hexagons. When the soil moistens, the cations become available to the plants again.

Absorption of potassium and ammonium cations increases with increasing content of montmorillonite group minerals. Application of ammonia and potassium fertilizers in layers of soil with a stable moisture content, allows to reduce non-exchangeable absorption (fixation) of cations and keep them available for plants.

Cation exchange uptake determines the reaction, buffering, structural condition and other properties of the soil. In neutral soils, the interaction occurs mainly between water-soluble forms, in acidic and alkaline soils – due to the exchange of ions between SAC and fertilizers, SAC and ameliorants.

Non-exchangeable absorption of cations

Non-exchangeable absorption of cations also occurs in the soil, as a result of which cations are fixed by clay minerals with a three-layer crystal lattice.

Non-exchange absorption of ammonium and potassium varies over a wide range, depending on the genetic properties of soils, their granulometric and mineralogical composition. It is stronger on chernozem soils than on sod-podzolic soils. It also increases under periodic wetting and drying. Therefore, shallow embedding of ammonia and potassium fertilizers in the soil layers subject to moisture and drying increases non-exchangeable absorption of ammonium and potassium. Non-exchange-absorbed ammonium and potassium are more slowly released into available forms than exchange-absorbed forms.

Capacity of cation absorption by soil

Capacity of cations absorption by soil, or absorption capacity, or cation exchange capacity (CEC), is the maximum amount of cations that can be metabolically absorbed by soil. Absorption capacity is expressed in millimoles per 100 g of soil. For example, if 100 g of soil contains 500 mg of absorbed calcium in the absence of other cations, the absorption capacity will be 500 : 20 = 25 mmol equivalent per 100 g of soil, where 20 is the equivalent mass of calcium (40 : 2 = 20), and 1 mmol equivalent of calcium is 20 mg. If all the calcium (500 mg in the example) is displaced by NH4+, 450 mg of NH4+ will be absorbed instead of calcium, since the equivalent mass of the NH4+ ion is 18. Therefore, when Ca2+ is exchanged for NH4+, 18 mg of ammonium is absorbed in place of 20 mg of Ca. The absorption capacity remains the same: in millimoles-equivalents it will also be equal to 25 mmol/100 g of soil, since 450 : 18 = 25. Thus, absorption capacity for a particular soil is a constant value.

Absorption capacity can be expressed in milligram equivalents per 100 g of soil. For example, if 100 g of soil contains 160 mg of calcium, 24 mg of magnesium, and 3 mg of hydrogen in the exchange-absorbed state, then the CEC (T) will be equal:

T = 160/20 + 24/12 + 3/1 = 13.0 mg·eq/100 g,

where 20 is the equivalent mass of Ca, 12 of Mg, and 1 of H.

The value of the absorption capacity depends on:

content of highly dispersed particles in the soil;
chemical and mineralogical composition of soil colloids;
soils reaction (pH).

Soils with high content of highly dispersed particles have high absorption capacity; clayey soils have higher absorption capacity than sandy soils.

Different groups of soil colloids differ in absorption capacity. From clay minerals, montmorillonite minerals characterized by high dispersity have the greatest capacity of cationic absorption, their absorption capacity reaches 60-150 mmol per 100 g of mineral. Kaolinite is characterized by low dispersibility and an active surface, the capacity of absorption of cations is 3-15 mmol/100 g of a mineral.

Organic soil colloids have greater absorption capacity as compared with mineral colloids. The cation exchange capacity of humic acids of different origin is determined by the content of functional groups: humic acids of podzolic soils have a capacity of about 350 mmol/100 g of substance at pH = 7.0, and humic acids of chernozems and chestnut soils – 400-500 mmol/100 g under the same conditions. Therefore, chernozems are distinguished by a higher value of absorption capacity, 40-60 mmol/100 g, as compared to sod-podzolic soils, whose absorption capacity is 10-15 mmol/100 g. Absorption capacity of soil organic matter of solid phase is 10-30 times more than mineral part, and with humus content of 5-6% its share is more than 50% of total capacity.

Absorption capacity of amorphous mineral colloids depends on the ratio of SiO2 : R2O3 in their composition. The greater this ratio and the greater the acidoid part, the higher the value of the absorption capacity. An increase in pH leads to an increase in the absorption capacity due to an increase in the negative charge of the colloids and, consequently, the absorptivity.

The exchange capacity of absorption depends on the composition of soil colloids. The higher is the share of fine fraction of organic matter and montmorillonite minerals, the higher is the absorption capacity. The ability of soil organic matter to adsorb cations is determined by the acid (acidoid) nature of humus, due to which organic colloids have a negative charge from carboxyl groups:

RCOOH → RCOO^– + H⁺.

The cation exchange reaction of organic soil colloids, can be represented as follows:

R=(COO)₂=Ca + 2KCl ⇔ R=(COOK)₂ + CaCl₂.

Ability to exchange cations at mineral particles of fine-dispersed fraction also is connected with a negative charge.

Occurrence of a negative charge at soil minerals is caused with presence of isomorphic replacements in silicate and aluminosilicate structures. The compound composition (SiO₂)_n is neutral, but if in the silica-oxygen structure part of the silicon atoms in tetrahedral coordination will be replaced by aluminum atoms, it leads to the formation of a negative charge:

[(SiO₂)_n]

neutral structure

[(SiO₂)_n-1AlO₂]^–

structure with a negative electric charge

Negative charge in aluminosilicates is balanced by corresponding amount of cations.

In clay minerals of fine-dispersed soil fraction, these cations are capable of dissociation and exchange for other cations:

[(SiO₂)_n-1AlO₂]K → [(SiO₂)_n-1AlO₂]^– + K⁺

The high absorption capacity of minerals of the montmorillonite group is explained by the structure of the crystal lattice, which consists of flat packages, the composition and structure of which has the form:

…O₃Si₂O₂OHAl₂OHO₂Si₂O₃…

This structure is neutral, the aluminum located in the central part of the package forms a layer with octahedral coordination, and in this position the negative charge is not communicated to the aluminosilicate structure. However, a charge appears if aluminum is partially replaced by a silicon atom in the silica-oxygen layer:

[…O₃Si₂O₂OHAl₂OHSiAlO₂…]^–.

Thus, the effect of aluminum on the charge of the aluminosilicate structure is determined by its position. Aluminum forming an independent alumino-oxygen or alumohydroxyl layer with octahedral coordination does not have acidoid properties. Montmorillonite can obtain a negative charge by isomorphic substitution, such as partial replacement of aluminum in the octahedral coordination with magnesium:

[…O₃Si₂O₂OHAlMgOHO₂Si₂O₂…]^–.

The high exchange capacity of minerals of this type is also due to the exchange capacity of the inner part of the microcrystalline structure, due to the fact that during swelling the solution penetrates into the interpackaging spaces of the crystal lattice.

Other minerals of the fine-dispersed fraction have a denser packing, due to which the exchange of cations occurs only on the outer surface of the microcrystals. Kaolinite belongs to such minerals, which composition and structure of packages can be represented as follows:

…O₃Si₂O₂OHAI₂(OH)₃…

Compared to the minerals of the montmorillonite group, kaolinite contains more aluminum and less silicic acid. Aluminum in not only does not report acidoid properties, but can exhibit basic properties due to the bound hydroxyl ions that exit to the outer surface of microcrystalline particles:

[…O₃Si₂O₂OHAI₂(OH)₂…]⁺ + OH^–.

To an even greater extent, the basic (bazoidal) properties of free, non-silica-bound aluminum and iron oxides, which exhibit basic properties in an acidic environment, are manifested:

[Fe(OH)₃]_n → [Fe_n(OH)_3n-1]⁺ + OH^–,

[Al(OH)₃]_n → [Al_n(OH)_3n-1]⁺ + OH^–.

With a general increase in the fine-dispersed aluminum and iron fraction and a decrease in the amount of silicic acid there is a decrease in the cation exchange capacity and a decrease in the absorption capacity.

The low absorption capacity of light sandy and organic-poor soils is explained by their low content of fine colloidal fraction. The low absorption capacity of sod-podzolic soils may be caused by the high content of free semi-hydrous oxides in the fine-dispersed fraction, and among the aluminosilicate minerals – by clays with the prevalence of kaolinite minerals.

The high absorption capacity of chernozems is due to the increased content of fine-dispersed fraction with high content of organic matter and predominance of clay montmorillonite minerals with high SiO₂ : (Al₂O₃ + Fe₂O₃) ratio.

Soil colloids exhibit acidoid properties to a greater extent in a neutral or slightly alkaline environment. Charge of soil colloids decreases under acid reaction reducing absorption capacity.

The absorption capacity of different types of soils varies from 5-10 mg-eq/100 g for light sod-podzolic soils to 20-70 mg-eq/100 g for chernozem soils. With acidification and increase of amphoteric colloids (ampholithoids) content the absorption capacity decreases.

Composition of absorbed cations

In the exchange-absorbed state all soils contain calcium and magnesium, in leached, ordinary and thick chernozems the share of calcium (30-40 mmol) and magnesium (5-10 mmol) reaches 80-90% with a small amount of hydrogen and aluminum cations.

In southern black and chestnut soils and gray earth soils calcium and magnesium dominate in cation exchange capacity, with a small share of sodium, hydrogen is absent. In saline soils and solonchaks in the exchange-absorbed state contains calcium, magnesium and sodium. Absorbed cations in red soils, yellow soils, podzolic and sod-podzolic soils include calcium, magnesium, aluminum, hydrogen and iron. Potassium and ammonium cations in exchange-absorbed state are present in small amounts in all soils.

Composition of absorbed cations influences properties of soil:

Due to the reversibility of the exchange reaction, the soil is able to regulate the composition of the soil solution, for example, KCl on black soil displaces Ca²⁺, on acidic soils it displaces Al³⁺ and H⁺. The composition of absorbed cations affects the effect of fertilizers.
Composition of absorbed cations affects state of absorbing complex, so, calcium and magnesium increase absorption capacity improving physical properties of soil, sodium worsens.

Hydrogen ions determine acidity of soil, causing gradual destruction of minerals composing absorbing complex. Therefore, displacement of absorbed calcium by hydrogen ions leads to a decrease in absorption capacity, worsening soil structure. Fertilization allows to regulate the ratio of absorbed cations.Liming of sod-podzolic soils and application of gypsum to saline soils increases calcium content in absorbing complex.

Increased cation exchange capacity

Cation exchange capacity and composition of absorbed cations play a huge role in plant nutrition and conversion of fertilizers, determine the reaction and buffer properties of solid and liquid phases, as well as the cation-anion composition and concentration of the soil solution. The higher the soil absorption capacity, the more economically advantageous and environmentally friendly one-time (reserve, periodic) application of high doses of fertilizers and ameliorants. On the contrary, the lower is absorption capacity, the more effective is fractional application of small doses of fertilizers and ameliorants.

The composition and amount of absorbed cations affect the dispersion of SAC and soil properties, plant nutrition, transformation of fertilizers and ameliorants. Cations precipitate negatively charged soil colloids in the following sequence:

Li⁺ < NH₄⁺ < Na⁺ < K⁺ << Mg²⁺ < Ca²⁺ < H⁺ << Al³⁺ < Fe³⁺.

When acidified, the precipitating effect of cations increases, when alkalized, it weakens. Cations with the same charge in an alkaline environment do not cause precipitation (coagulation) of colloids. Calcium cations precipitate soil colloids even in an alkaline environment, and magnesium cations occupy an intermediate position between single charge cations and calcium.

The calcium cation prevailing in the absorbed composition when interacting with fertilizers passes into the soil solution, precipitates organic and mineral colloids contained in it, thus accumulating and preserving them in the solid phase, increasing the absorption capacity.

The content of alkaline potassium and sodium cations in the SAC more than 3-5% of CEC leads to the dispersion of colloid and pre-colloidal fractions, dramatically worsening the properties of alkaline soils. When interacting with fertilizers, ameliorants and mineral salts of soil solution the absorbed sodium passes into solution and forms hydrolytically alkaline salts, negatively affecting the growth and development of plants:

[SAC]Na₃ + Ca(HCO₃)₂ ⇔ [SAC](Ca, Na) + 2NaHCO₃;

NaHCO₃ + H₂O = NaOH + H₂O + CO₂.

The high content of hydrogen, aluminum, iron and manganese cations in SAC, which is typical for podzolic and boggy soils, yellow soils, red earths, worsens the properties of acidic soils. Hydrogen cation gradually destroys the SAC minerals, deteriorating the soil structure, impoverishing it with colloidal fraction and reducing the CEC, which in general negatively affects the growth, development and crop yields.

Ratios and composition of exchange-absorbed cations of different soils are regulated by fertilizers and ameliorants. On acidic soils hydrogen, aluminum, iron and manganese cations are displaced from SAC when lime ameliorants are applied, on alkaline soils – sodium cations – by applying gypsum-containing ones. In both cases, cations negatively affecting crops are replaced by calcium.

The higher the cationic absorption capacity of the soil, the greater the number of different cations it can save from leaching, thereby providing the best conditions for nutrition, growth and development of plants. In addition, an increase in CEC positively affects the resistance of soils to anthropogenic influences, in particular to chemical pollution.

Sources

Agrochemistry. Textbook / V.G. Mineev, V.G. Sychev, G.P. Gamzikov et al. – M.: Publishing house of the All-Russian Scientific Research Institute named after D.N. Pryanishnikov, 2017. – 854 с.

Yagodin B.A., Zhukov Y.P., Kobzarenko V.I. Agrochemistry / Edited by B.A. Yagodin. – Moscow: Kolos, 2002. – 584 p.: ill.

Mineral part of soil

26.01.2022

The mineral part of soil is the main component of soils.

The mineral part of the soil arose during the processes of weathering of rocks and minerals of the upper layers of the lithosphere and their transformations. This is confirmed by the similar chemical composition of lithosphere and soils. Soil was formed under the combined influence of physical and chemical factors on the mineral nature, as well as living organisms, especially plants and microorganisms.

The geochemical composition of the soil is inherited from the soil-forming rocks. Thus, high content of silicon oxide determines its high content in soil. Soils enriched with alkaline earth elements are formed on carbonate rocks.

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Biological factors of soil formation

Thanks to the activity of living organisms, the carbon content of soil compared to the Earth’s crust has increased by 20 times and the nitrogen content by 10 times.

Soil formation in natural conditions proceeds very slowly. Application of fertilizers and agricultural techniques allows to accelerate this process. Thus, the application of fertilizers enhances the vital activity of plants and soil microflora, which leads to the accumulation of organic matter and biologically important elements.

Silicates and aluminosilicates

According to chemical structure minerals are divided into silicates and aluminosilicates. From silicates for all types of soils in sand and dust fractions prevails quartz – SiO₂ characterized by low absorption capacity and high water permeability. In soils, its content is usually more than 60% in sandy soils – more than 90%. Quartz is chemically inert and has high strength.

The basis of the mineral part of soils is silicon-oxygen compounds. The most common soil mineral is quartz, or silicon oxide. Aluminum and iron are predominantly composed of aluminosilicate and ferrosilicate minerals. Silicon and oxygen atoms form tightly bound SiO₄ groups that have a tetrahedral structure. Due to the tetravalence of silicon, SiO₄ groups can form various complex combinations of compounds.

In the structures of minerals of finely dispersed soil fractions, silica-oxygen tetrahedrons can be combined into layers, chains, or separate groups of SiO₄ tetrahedrons. The total oxidation degree of these groups is negative. In complex combinations of silica-oxygen tetrahedrons, some of the silicon atoms can be replaced by aluminum atoms.

In the crystal lattice of quartz, SiO₄ tetrahedrons are connected through oxygen atoms to four other SiO₄ tetrahedrons. The general formula for quartz is (SiO₂)_n. In the crystal structure of feldspars, some of the silicon atoms are replaced by aluminum. To compensate the emerging negative charge of the silica-alumina framework they include atoms of sodium, calcium and others that are embedded in the “cavities” of the lattice. Thus, albite feldspar has the formula Na[SiAlO₈].

Aluminum in tetrahedral coordination with oxygen ions or hydroxyl group OH forms octahedral groups, where the aluminum atom is surrounded by six oxygen atoms or hydroxyl groups. The formula of such a compound (layer) [Al(OH)3]-n corresponds to the mineral gibbsite (hydrargyllite).

The structure of such minerals can be represented as follows:

…[(OH)₃Al₂(OH)₃]·n…[(OH)₃Al₂(OH)₃]·n…[(OH)₃Al₂(OH)₃]·n.

The formula reflects the chemical composition of the layer (package), and the dots represent the inter-package intervals.

The mineral part of soils consists of primary and secondary minerals. In sandy and sandy loam soils mainly primary minerals prevail, loamy soils consist of primary and secondary minerals, and clayey soils mainly consist of secondary minerals with quartz admixture. Division of minerals into primary, i.e. with particle size more than 0,001 mm and secondary less than 0,001 mm is conditional, as the latter are the products of physical and chemical weathering of primary and formation of hydrates of semi-hydrated silica oxides and other compounds at that.

During weathering, hydrolysis of feldspar and mica leads to replacement of metal cations in the crystal lattices of minerals by hydrogen ions:

Physical and chemical weathering is inseparable from the biological transformation of rocks and minerals under the influence of living organisms and their products.

Primary soil minerals

Primary soil minerals are minerals that have passed from the earth’s crust into the soil without changing their structure. They include minerals of soil skeleton:

quartz and its varieties,
feldspars: orthoclases, plagioclases, mica, hornblende, augite, tourmaline, magnetite, calcite, dolomite, etc.

Primary soil minerals are part of the parent soil-forming rocks formed as a result of weathering and destruction of rocks. They are present in soils in the form of sandy particles sized 0.05 to 1.0 mm and dusty particles sized 0.001 to 0.05 mm. In small amounts are present in the form of silty particles smaller than 1 μm and colloidal particles smaller than 0.25 μm.

From primary minerals under the influence of physical and chemical processes such as hydration, hydrolysis, oxidation and life activity of soil organisms oxide hydrates such as R2O3 and silicon oxide (silica earths), mineral salts as well as secondary minerals are formed.

Destruction of feldspars and mica releases potassium, calcium, magnesium, iron and some other plant nutrients.

Secondary soil minerals

Secondary minerals, or clay minerals, are kaolinite, montmorillonite, hydromica, etc. Secondary minerals, or clay minerals, such as kaonite, montmorillonite, hydromica, etc., are mostly present as muddy and colloidal particles, less often as dusty particles.

In crystalline lattices of aluminosilicate minerals of fine-dispersed fraction of soils there are combinations of silica-oxygen tetrahedral and alumohydroxyl octahedral layers.

The crystal lattice of kaolinite is formed by packages of two layers bound together by oxygen atoms: a tetrahedral silica-oxygen layer and an octahedral alumohydroxyl layer:

…[O₃Si₂O₂(OH)Al₂(OH)₃]•n…[O₃Si₂O₂(OH)Al₂(OH)₃]•n.

The crystal lattices of montmorillonite and hydromica are formed by one alumohydroxyl layer and two silicic acid layers attached to it:

[O₃Si₂O₂(OH)Al₂OHO₂Si₂O₃]•n…[O₃Si₂O₂(OH)Al₂OHO₂Si₂O₃]•n.

The bond between the packages in kaolinite group minerals is stronger, and the inter-package spaces are small. Therefore, the interaction of microcrystalline particles with water occurs only on the outer surface.

In minerals of montmorillonite group the interpack spaces are larger, and the bond between the packages is weaker, so water molecules can penetrate into the interpack spaces. Cations located on the surface of the particles and in the interpack spaces take part in cation exchange with the soil solution of minerals of this group. This explains high exchangeable absorptivity of minerals of montmorillonite group and availability of non-exchangeable absorption of cations. This group is characterized by high dispersibility, swellability, stickiness and viscosity.

Soil clay minerals are divided into:

montmorillonite (montmorillonite – Al₂Si₄O₁₀(OH)₂·nН₂O, beidelite – Al₃Si₃O₉(OH)₃·nH₂O, nontronite, saponite, sonite, etc.).
kaolinite (kaolinite – Al₂Si₂O₅(OH)₄ and halloysite Al₂Si₂O₅(OH)₄·2Н₂O),
hydromica (hydromuscovite (illite) (К,Н₃O)Аl₂(OН)₂[Аl,Si]₄·nН₂O, hydrobiotite, vermiculite),
minerals of halved oxides (hematite, bemitite, hydrargyllite, goethite, etc.).

The greatest absorption capacity possesses montmorillonite minerals, the lowest – kaolinite. Thus, the absorption capacity of kaolinite is 8-15 times less than that of montmorillonite. This peculiarity has significance in absorption of fertilizers.

The montmorillonite Мg₃(OН)₄[Si₄O₈(OН)₂]·Н₂O is characterized by high dispersion: 40-50 % of colloid (size less than 0,0001 mm) and 60-80 % of silty (size less than 0,001 mm) particles. It prevails in chernozems. Because of high dispersibility its absorption capacity reaches 120 mg-eq/100 g, it swells under humidifying. Cations (K⁺, NH₄⁺, Na⁺, Ca²⁺, etc.) can penetrate into interplanar space of crystal structure, which when soil dehydration (drying) are fixed and become unavailable for plants until the next saturation with moisture.

Secondary aluminosilicate minerals are found in the soil in the form of fine crystals and are characterized by high absorption capacity.

The kaolinite group is less dispersed, has small swelling and stickiness, absorption capacity is no more than 25 mg-eq/100 g of soil, particle size is less than 0.001 mm, water permeability is good.

In sod-podzolic and chernozem soils formed on overlying loams, montmorillonite and hydromica prevail in the composition of highly dispersed fractions. In red soils, yellow soils and sod-podzol soils formed on the products of ancient humid weathering of granite, the content of kaolinite group minerals is much higher.

Hydromica is formed from mica, has unstable chemical composition, and by its physical properties occupies an intermediate position between montmorillonite and kaolinite. Hydromica is present in all soils in muddy and colloidal fractions. Because of high dispersibility they have big surface and absorbing ability.

Mica determine agrochemical and agrophysical features of soil. They are the source of potassium nutrition of plants, their composition includes up to 5-7% of potassium. Energy of colloidal absorption of potassium is high and as a result in absorbing complex it contains 0.510 mmol/100 g of soil. Red soils and laterites due to small content of mica and hydromica and an excess of kaolinite minerals of the low-potassium group are characterized by potassium deficiency.

Weakly crystallized minerals having significant influence on absorbing ability of the soil are allophane, free silicic acid, different acids and their salts. The mineral part of soil includes amorphous substances: hydrates of oxides of aluminum Al₂O₃ · nH₂O, iron Fe₂O₃ · nH2O and silicon SiO₂ · nH₂O. Their greatest content is noted in the red earth soils and yellow earth soils. At isoelectric points, these substances form amorphous precipitates, which form new minerals as they age:

Mineral salts

Soil contains mineral salts: carbonates, sulfates, nitrates, chlorides, phosphates of calcium, magnesium, potassium, sodium, iron, aluminum and manganese. All nitrates and chlorides, as well as potassium and sodium salts are well soluble in water, but their content in soils (except for saline soils) is relatively small. Low-soluble salts (calcium and magnesium carbonates and calcium sulfate) are found in solid phase in some soils in significant quantities, while insoluble calcium, magnesium, iron and aluminum phosphates are found in all soils.

Table. Content of trace elements in soil and lithosphere, mass %

Element	Contents		Element	Contents
	in the soil	in the lithosphere		in the soil	in the lithosphere
Manganese	0,085	0,09	Copper	0,002	0,01
Fluorine	0,02	0,027	Zinc	0,005	0,005
Wolfram	0,01	0,015	Cobalt	8·10^-4	0,003
Boron	0,001	3·10^-4	Molybdenum	3·10^-4	3·10^-4
Nickel	0,004	0,008	Iodine	5·10^-4	3·10^-5

In addition to macronutrients, there are trace elements in the soil. Soil-forming rocks are the main source of them in the soil. Thus, soils formed on the weathering products of acid rocks, i.e. granites, liparites, granite-porphyries, are poor in nickel, cobalt, copper. Soils formed on the weathering products of basic rocks (basalts, gabbro), on the contrary, are enriched with these elements.

Some trace elements, such as iodine, boron, fluorine, selenium, arsenic can enter the soil from the atmospheric air, volcanic eruptions and precipitation. For iodine and fluorine, these are the main sources.

Sources

Agrochemistry. Textbook / V.G. Mineev, V.G. Sychev, G.P. Gamzikov et al. – M.: Publishing house of the All-Russian Scientific Research Institute named after D.N. Pryanishnikov, 2017. – 854 с.

Yagodin B.A., Zhukov Y.P., Kobzarenko V.I. Agrochemistry / Edited by B.A. Yagodin. – Moscow: Kolos, 2002. – 584 p.: ill.

Soil composition

26.01.2022

Soil composition includes:

solid phase;
liquid phase, or soil solution;
gas (gaseous) phase, or soil air.

Soil is an independent natural-historical organomineral natural body, appeared on the surface of the Earth as a result of long-term influence of biotic, abiotic and anthropogenic factors, consisting of solid mineral and organic particles, water and air and having specific genetic and morphological features, properties, which create appropriate conditions for growth and development of plants. Soil is a complex self-regulating multicomponent biocomponent unified system.

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Gas phase

The gas phase is the result of interaction between atmospheric air and gases formed in the soil. In its composition there is a higher content of carbon dioxide compared to atmospheric air – 0.3-1%, sometimes up to 2-3% or more and a lower content of oxygen. Gas phase is characterized by high mobility, which depends on many conditions: the content of organic matter, weather conditions, the nature of vegetation, etc.

Sufficient oxygen content in the soil creates favorable conditions for the activity of aerobic microorganisms. On the contrary, with its deficiency there are conditions for the development of anaerobic bacteria, which are often pathogenic to plants.

The volume of soil air is in dynamic equilibrium with the liquid phase: the more water, the less air. Gas exchange processes in soil occur constantly as a result of decomposition of organic matter, respiration of plant roots and soil organisms, as well as some chemical reactions. As a result of gas exchange, the above-ground air is enriched with carbon dioxide, improving conditions for photosynthesis. When carbon dioxide interacts with water in the liquid phase, the soil solution is slightly acidified by the reaction:

CO₂ + H₂O ⇔ H⁺ + HCO₃^–.

Acidification promotes transition of some minerals of a solid phase, for example, phosphates and calcium sulfate, in the form accessible to plants. At the same time, an excess of carbon dioxide leads to a lack of oxygen and the creation of anaerobic conditions, which is observed when soils are over-watered and over-consolidated. Lack of oxygen in gas phase inhibits growth and development of microorganisms and plants, prevents assimilation of nutrients, increases reduction processes in liquid and solid phases.

Soil air concentrates in non-capillary pores, i.e. in large spaces of soil. If all pores are filled with water, the soil air is displaced, on the contrary, if the soil is dry, the air fills all pores – capillary and non-capillary.

The most optimal ratio of water and air is formed on loose structural cultivated and cultivated soils. Regulation of water and air regimes of soils by appropriate treatments in combination with application of fertilizers and ameliorants improves root and air nutrition of plants, thereby increasing quantity and quality of production, contributes to development of soil biota.

Liquid phase

The liquid phase, or soil solution, is a solution of mineral and gaseous substances soluble in water. It is the most active and dynamic phase of soil, from which plants assimilate nutrients and simultaneously the interaction of plants with fertilizers, ameliorants, solid and gaseous phases takes place.

Soil solution includes cations (Ca²⁺, Mg²⁺, H⁺, Na⁺, K⁺, NH₄⁺, etc.), anions (HCO₃^–, OH^–, Cl^–, NO₃^–, SO₄^–, H₂PO₄^–, etc.), water soluble organic compounds and soluble gases CO₂, O₂, NH₃ etc. Input of ions into the soil solution comes from solid and gaseous phases, fertilizers and ameliorants, excreta of soil biota, atmospheric precipitation and groundwater. Thus, composition and concentration, acidity, buffer and osmotic pressure of soil solution are dynamic and are determined by soil-climatic conditions and anthropogenic impact.

The concentration of salts in the soil solution depends on the properties, mineral composition, soil type, natural conditions, degree of salinity and migration of salts along the soil profile, anthropogenic impact, etc. The concentration of various salts can vary from thousandths to hundredths of a percent (10-200 mg/l) in low fertile soils to 1 and more percent (> 10,000 mg/l) in highly saline (solonchaks), in medium fertile soils – about 500 mg/l. Excess salts over 2,000 mg/L usually adversely affect crops, especially in the first 2 to 4 weeks after seed germination. Tolerance to high concentrations increases with age.

The properties of the liquid phase are generally determined by the water regime of the soil.

Solid phase

The solid phase of soil consists of:

mineral part, the share of which ranges from 90% to 99.5%;
organic, or organic matter of the soil, which accounts for 0.5% to 10%.

The mineral part is the debris and particles of primary rocks and minerals, secondary, i.e. newly formed minerals, oxides, salts and other compounds formed in the process of weathering and soil formation. The mineral part includes all ash substances, 1-3% of nitrogen from the total amount.

Oxygen, silicon, aluminum, and iron account for nearly 93% of the solid phase, carbon, potassium, and calcium for 4.6%, and 2.5% for all the remaining elements. Carbon, oxygen, hydrogen, phosphorus, and sulfur are in the mineral and organic parts, while nitrogen is almost entirely in the organic part.

The organic part, or organic matter of the soil, is the remains of plant and animal organisms and products of their decomposition and neosynthesis, among which humus prevails.

Table. Average chemical composition of the solid phase of soil (% of mass) by A.P. Vinogradov

Element	Contents	Element	Contents	Element	Contents
Oxygen	49,0	Barium	0,05	Gallium	0,001
Silicon	33,0	Strontium	0,03	Tin	0,001
Aluminum	7,1	Zirconium	0,03	Cobalt	8·10^-4
Iron	3,7	Fluorine	0,02	Thorium	6·10^-4
Carbon	2,0	Chrome	0,02	Arsenic	5·10^-4
Calcium	1,3	Chlorine	0,01	Iodine	5·10^-4
Potassium	1,3	Vanadium	0,01	Cesium	5·10^-4
Sodium	0,6	Rubidium	0,006	Molybdenum	3·10^-4
Magnesium	0,6	Zinc	0,005	Uranium	1·10^-4
Hydrogen	0,5	Cerium	0,005	Beryllium	1·10^-4
Titan	0,46	Nickel	0,004	Germanium	1·10^-4
Nitrogen	0,10	Lithium	0,003	Cadmium	5·10^-5
Phosphorus	0,08	Copper	0,002	Selenium	1·10^-6
Sulfur	0,08	Bor	0,001	Mercury	1·10^-6
Manganese	0,08	Lead	0,001	Radium	8·10^-11

Sources

Agrochemistry. Textbook / V.G. Mineev, V.G. Sychev, G.P. Gamzikov et al. – M.: Publishing house of the All-Russian Scientific Research Institute named after D.N. Pryanishnikov, 2017. – 854 с.

Yagodin B.A., Zhukov Y.P., Kobzarenko V.I. Agrochemistry / Edited by B.A. Yagodin. – Moscow: Kolos, 2002. – 584 p.: ill.

Agrochemical indicators of soil fertility

25.01.2022

Agrochemical indicators of soil fertility – a set of properties that characterize the ability of the soil to provide plants with elements of nutrition and the optimal nutrient regime.

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Agrochemical indicators of soil fertility (Русский Español)

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Agrochemical indicators of soil fertility (Русский Español)

Nutrient regime of soils

Main article: Plant life factors: Nutrient regime of soils

Nutrients come from the soil solution, which is in constant equilibrium with the soil solid phase. The rate of this process is very high and depends on the concentration of substances. As a consequence the composition of soil solution is highly dynamic.

Content of available forms of nutrients is influenced by their gross stock in soil. Soil microflora, especially living in the root zone (rhizosphere) has a significant impact on the translation of gross stocks into available forms.

Soil composition

Main article: Soil composition

Soil composition largely determines the agrochemical indicators of soil fertility. The composition is usually divided into three phases:

gas, or gaseous, phase;
liquid phase, or soil solution;
solid phase, which is divided into mineral part and organic part (soil organic matter).

Nitrogen content in soil and its availability

Sources of nitrogen intake and its transformation in the soil

Natural sources of nitrogen intake are: activity of nitrogen-fixing free-living and nodule bacteria and intake with atmospheric precipitation.

The process of nitrogen fixation is carried out by free-living anaerobic bacteria Clostridium pasterianum, aerobic Azotobacter croococcum and Rhizobium nodule bacteria living in symbiosis on the root system of legumes. Their activity and efficiency of nitrogen fixation are influenced by the provision of carbohydrates, phosphorus, calcium and other elements, the reaction of the soil environment, temperature, moisture. They accumulate 5-15 kg of nitrogen per 1 hectare during the year. Some algae and fungi in symbiosis with plants also have nitrogen fixation ability.

Bacteria of Azotobacter group thrive in aerated, cultured, well-warmed, neutral soils with phosphorus and calcium. Under favorable conditions, they accumulate up to 30 kg of nitrogen per hectare.

Bacterial strains and races of Rhizobium group are specific for each legume species. Nitrogen fixation efficiency depends on plant species, agrotechnics, soil and a number of other conditions. Under optimal conditions these bacteria can accumulate in symbiosis with: alfalfa – 250-300 (to 500) kg of nitrogen per ha, lupine – 160-170 (to 400), clover – 150-160 (to 250), soybeans – 100, vetch, peas, beans – 70-80 kg of nitrogen per 1 ha. Their activity is positively affected by the introduction of organic and phosphorus fertilizers and liming of the soil.

Introduction of leguminous crops into crop rotation contributes to the increase of nitrogen reserves in the soil.

With atmospheric precipitation annually in the form of ammonia and nitrates formed under the influence of lightning discharges, 2-11 kg of nitrogen per 1 ha is received.

Natural sources of nitrogen are of practical interest, but their quantity is much less than the amount of nitrogen taken out with a crop. Therefore, the reproduction of soil nitrogen reserves requires the application of organic and mineral fertilizers.

Humus reserves, which contain about 5% of nitrogen, play an important role in providing plants with nitrogen. Mineral forms of nitrogen account for about 1-3%. According to the data of I.V. Tyurin, the reserves of humus in a meter layer of soil per 1 ha are: gray soils – 50 tons, light chestnut – 100, dark chestnut and southern chernozems – 200-250, ordinary chernozems – 400-500, heavy chernozems – 800, leached chernozems – 500-600, gray forest-steppe – 150-300, sod-podzolic – 80-120 tons. Arable layer has the largest share of humus, which is enriched with microflora and from which the main part of mineralized nitrogen for plant nutrition comes.

Ammonification is a microbiological process of transforming organic matter nitrogen into ammonium compounds. Ammonium salts are oxidized by nitrifying bacteria (Nitrosomonas and Nitrobacter) into nitrates and nitrites. Normal life activity of these groups of bacteria requires optimal conditions: temperature 25-32 ° C, sufficient oxygen and water, soil acidity close to neutral. This is achieved by loosening the soil, applying organic fertilizers and liming acidic soils. Carrying out these methods allows to activate the processes of nitrogen transformation from organic matter and reduce its losses. Violation of these requirements leads to the opposite effect – the transition of nitrogen compounds in gaseous ammonia and nitrogen, that is, activates the process of denitrification.

Another method of regulating the balance of nitrogen in the soil is the use of bacterial preparations (rhizotorfin).

Nitrogen losses

Nitrogen content in mineral form is very dynamic and depends on the activity of soil microflora, moisture, and the phase of plant development.

Nitrogen losses consist of:

immobilization, which is the absorption of nitrogen by the soil microflora;
leaching, i.e. leaching of nitrogen, mainly in nitrate forms into the ground water;
volatilization in the form of ammonia into the atmosphere;
fixation of ammonium forms by soil or non-exchange absorption.

The process of immobilization proceeds especially intensively with the introduction of organic fertilizers with a wide ratio of carbon to nitrogen – 20-25:1. Microbial plasma contains much more nitrogen (10:1), due to which the consumption of nitrogen by microflora occurs at the expense of organic matter and mineral reserves of the soil. Which worsens the nitrogen nutrition of cultivated plants.

In order to compensate the effect of nitrogen immobilization by microorganisms, when plowing straw or other plant residues rich in cellulose before sowing subsequent crops additional about 1% of mineral nitrogen is added.

Nitrogen immobilization can have a positive effect on light soils with sufficient moisture by fixing the mobile forms of nitrogen in the conditions of their strong leachability. Further, during decomposition of microorganisms’ residues, a part of fixed nitrogen is bound by humus compounds, and another part goes to mineral forms.

Leaching of mobile forms of nitrogen, mainly nitrates, is especially relevant on light soils of granulometric composition with low levels of organic matter under conditions of sufficient, excessive moisture and irrigation. Continuous crops reduce this effect due to intensive nitrogen uptake, while in fallow fields the leaching effect is intensified.

Nitrogen loss in the form of gaseous matter occurs due to denitrification, i.e. the reduction of nitrate nitrogen to ammonia and gaseous nitrogen due to the activity of denitrifying microorganisms. Denitrifier activity is activated by anaerobic conditions, when microbes are forced to use oxygen in the nitrate form for respiration, reducing nitrogen to the free form. The denitrification process is stimulated by the creation of anaerobic conditions, alkaline reaction of the medium, excessive organic matter with high glucose and fiber content, and high soil moisture.

Another way of nitrogen loss in the form of gaseous forms (nitrogen dioxide and nitrogen monoxide) is the decomposition of nitrous acid at soil acidity 6 and below.

The total loss of nitrogen can reach 50%. The decomposition of 1 ton of humus produces 50 kg/ha of nitrogen, but part of it is lost to the atmosphere in the form of ammonia gas escaping into the atmosphere. It is especially important if the storage and application technologies of manure, slurry and other organic fertilizers are not complied with, with losses reaching 30-40%.

A significant portion of nitrogen is consumed by weed plants, and this amount can exceed the consumption of cultivated plants.

Nitrogen fixation by soil

Some nitrogen can be absorbed by some minerals of the hydromica group. When moistened, the crystalline lattice of these minerals exchangeably absorbs ammonium nitrogen, but when it dries, it binds it, making it inaccessible for plants and microflora.

According to A.V. Peterburgsky and V.N. Kudeyarov, the arable layer contains from 130 to 350 kg/ha of fixed nitrogen depending on the type and variety of soil. The top layer contains 2-7% of fixed ammonium of the total amount, in the subsoil its share increases to 30-35%. This is explained by the reduction of humus content in the deep layers and, consequently, nitrogen in the organic matter.

The ability of soils to bind ammonium without necessarily affecting the type of clay minerals, ambient temperatures, humus content, the reaction of the soil solution, microbiological activity, and moisture. Ammonium fixation increases with increasing temperature, pH (maximum on salts), humus content (chemical binding). Nitrogen fixation is influenced by the content of clay minerals with three-layer crystal lattice, primarily vermiculite.

Fixed ammonium can be displaced back into the soil under certain conditions, such as the introduction of calcium, magnesium, sodium cations into the crystal lattice, becoming available to plants.

Phosphorus content in soil and its availability

The content of phosphorus (P₂O₅) in soils ranges from 0.01% for poor sandy soils to 0.20% for thick highly humus soils. In the upper layers of the soil more P₂O₅ is concentrated, which is associated with its accumulation in the zone of the dying away of the main mass of the roots. The amount of P₂O₅ decreases with soil depth. Phosphorus is present in organic and mineral forms.

Organic phosphates are part of the humus, in the decomposition of which it becomes available to plants.

Some plants assimilate simple organophosphorus compounds, due to their decomposition by the enzyme phosphatase secreted by the root system. Such plants include peas, beans, corn and other crops.

Mineral forms are represented by calcium salts, predominant in neutral and alkaline soils, iron and aluminum oxide phosphates in acidic soils. Calcium phosphates are more soluble, and therefore more accessible to plants than aluminum and iron salts.

The main source of phosphorus for plant nutrition is the salts of orthophosphoric (H₃PO₄) and metaphosphoric (HPO₃) acids. Phosphates of monovalent metals, due to their greatest solubility, are the most accessible. Single-substituted (dihydroorthophosphates) of calcium and magnesium are less soluble, but also well available for absorption. Metaphosphates are poorly soluble in water.

Two-substituted calcium and magnesium salts (hydroorthophosphates) are insoluble in water, but well soluble in solutions of weak acids, which makes them also accessible to plants, due to the creation of a weakly acidic reaction by the root system in the rhizosphere.

Orthophosphates of divalent and trivalent metals are insoluble in water, so they are inaccessible to most plants. Lupine, buckwheat, mustard, alfalfa and clover are the most adapted to the assimilation of hard-to-reach forms of phosphorus. Peas, melilot, sainfoin, hemp, rye, and corn show this property to a lesser extent (E. Rübenzam and K. Rauer, 1960).

In contrast to nitrogen, due to weak mobility, there are no natural ways of phosphorus loss as well as natural sources of replenishment.

The optimum level of phosphorus supply for most crops in mobile forms is considered to be 150-250 mg/kg of soil for gray forest and sod-podzolic soils (according to Kirsanov) and 45-60 mg/ha for chernozems (according to Machigin).

The regulation of phosphorus content in the soil is carried out mainly by applying organic and phosphorus fertilizers. To increase the content of phosphorus in the soil by 1 mg it is required depending on the granulometric composition and type of soil from 40 to 120 kg P₂O₅/ha.

Potassium content in soil and its availability

The gross potassium content often exceeds that of nitrogen and phosphorus, and is determined by the granulometric composition. Especially rich in potassium are clay and loamy soils, where the content reaches 2-3%. Sandy, sandy loam and peaty soils are poor in potassium, up to 0.1%.

However, gross potassium content in view of the peculiarities of exchange reactions, does not mean sufficient supply to plants, as only about 1% of its gross content is available to plants. Therefore, the characteristic feature of potassium availability is its amount in mobile forms.

In terms of availability to plants, all potassium compounds in soil are divided into five groups:

Potassium, which is part of the soil minerals of aluminosilicates. Hardly available form of potassium. However, some minerals (muscovite, biotite and nepheline) can transform into accessible form some part of potassium under the influence of carbon dioxide and some organic acids produced by plant roots. The rate of transition of potassium from non-exchangeable to exchangeable forms depends on soil type. For sod-podzolic soils it is 15-30 kg/ha per year, for leached chernozems – about 60 kg/ha.
Absorbed, or adsorption-bound by soil colloids, potassium is the main source of plant nutrition. The content in the soil can be from 50 to 300 mg per 1 kg of soil. During vegetation, plants use only part of the exchangeable potassium, which is determined by soil properties, biological features of plants, weather conditions, etc.
Water-soluble forms of potassium are the most available form. They make up 10-20% (about 1% according to E. Rübenzam and K. Rauer) of exchangeable potassium. According to the Moscow Agricultural Academy in unfertilized sod-podzolic soil during the spring-summer period the content of water-soluble forms of potassium was 1.5-5 mg/kg of soil. It is formed as a result of chemical and biological effects on minerals. It partially passes into the water-soluble form from the exchange state as a result of displacement from the soil absorbing complex as well as from fertilizers.
Biogenically bound potassium, that is included in the biomass of soil bacteria, plant residues and biota. Its proportion may reach, for example, in sod-podzolic soils 40 kg K₂O per 1 ha. It passes into available form only after dying off and mineralization of residues.
Potassium fixed by soil. Potassium can be fixed in the mineral part of the soil in a non-exchangeable state. The process proceeds most actively in conditions of alternating soil wetting and drying and prevails in soils of heavy granulometric composition containing clay minerals montmorillonite and hydromica, which are characterized by intracrystalline adsorption of cations, in contrast to kaolinite.

Potassium fixation into non-exchangeable form intensifies in alkaline environment and prevails in solonets. Black soils fix potassium to a greater extent than sod-podzolic soils.

Soils have a certain limit of potassium fixation from fertilizers: for sod-podzolic soils it rarely exceeds 200 kg/ha, for black earth can reach 300-700 kg K₂O per 1 ha. The use of potassium fertilizers allows to achieve complete saturation of potassium fixation capacity.

The optimum content of exchangeable potassium in the soil, which observed the maximum crop yield is for sod-podzolic and gray forest soils – 170-225 kg/ha.

In the main subtypes of black earths optimal content of mobile potassium depending on the soil, culture and method of definition is Chirikov 130 to 200 mg/kg, by Machigin – up to 400 mg / kg.

Micronutrient content in soil

The removal of micronutrients with a crop from 1 hectare of soil ranges from fractions of grams (molybdenum) to several hundred grams (manganese, zinc).

As a rule, soils have a certain reserve of trace elements, which can be replenished by organic fertilizers, microfertilizers, and admixture with mineral fertilizers, in which they are in mobile form. Mobility of trace elements in manure is inferior to mineral fertilizers and is no more than 25%.

In order to obtain stable high yields, the plants’ need for micronutrients must be satisfied in full.

Reaction of soil environment

Main article: Acidity of soils

For most cultivated crops and microorganisms living in the soil, a slightly acidic or neutral soil reaction is optimal. Certain crops may exhibit special requirements for the pH interval. For example, alfalfa, sugar beets, and cotton cannot tolerate acidic soils, while others, on the contrary, prefer a slightly acidic reaction: lupine, potatoes, buckwheat, and flax.

Table. Optimal and permissible reaction of soil solution for the main crops (according to Shishkov and Mineev, 1987)^[1]Farming. Textbook for universities / G.I. Bazdyrev, V.G. Loshakov, A.I. Puponin et al. - M.: Publishing house "Kolos", 2000. - 551 p.

Crop	Optimal pH	Permissible pH	Crop	Optimal pH	Permissible pH
Lupine	4-5	4-6	Clover	6-6,5	3-8
Potatoes	5	4-7	Peas	6-7	5-8
Oats	5-6	4-8	Corn	6-7	5-8
Rye	5-6	4-7	Wheat	6-7	5-8
Flax	5-6	5-7	Sugar beet	7	6-8
Buckwheat	5-6	5-7	Alfalfa	7-8	6-8,5

Indirectly, the reaction of the soil environment affects fertility due to the increased mobility of humus compounds and their leaching beyond the arable layer and the harmful effects of hydrogen ions H⁺ on the mineral part, which leads to the depletion of soil colloidal particles.

Soil acidification causes a lack of exchangeable calcium and magnesium, which sharply deteriorates the physical and physicochemical properties of the soil (structure, absorption capacity, buffering). Acidic environment increases the concentration of aluminum and manganese toxic for plants and reduces the mobility of some elements, such as molybdenum, creating violations in the nutrient regime of soils.

Increased acidity depresses soil biota, especially nitrifying and nitrogen-fixing bacteria, earthworms, mites, and cattails. In general, the biological activity of acidic soils is significantly lower than that of neutral soils.

The main factors of soil acidification are removal with the crop and washout of calcium and magnesium. Leaching of calcium and magnesium can reach tens to hundreds of kilograms per 1 ha. Cabbage, alfalfa, and clover, which are highly sensitive to soil acidity, have the highest removal rates.

Loss of calcium and magnesium from leaching is influenced by leaching water regime of soil depending on abundance of precipitation. For example, on acidic soils leaching can be 200-300 kg/ha, on carbonate soils – significantly more. Soils with light granulometric composition lose much more calcium and magnesium under washing water regime.

The main agronomic method of regulation of soil acidity is liming – the introduction of calcium carbonate CaCO₃. In addition to normalizing the acidity, lime displaces hydrogen ions from the soil absorbing complex, neutralizes the organic acids. That leads to the elimination of exchange acidity and significantly reduces the hydrolytic acidity, increases the degree of saturation of the bases and increases the absorption capacity of the arable layer.

For alkaline saline soils the application of calcium sulfate CaSO₄ (gypsum) is used. Introduction of gypsum eliminates the alkaline reaction, improves physical, chemical and biological properties of soil, aeration, facilitates tillage, increases microbiological activity and, in general, increases fertility. The effectiveness of gypsum depends on the depth of plowing, snow retention, fertilizer application, irrigation. The effect of gypsum persists for 8-10 years.

Degree of base saturation and soil buffering

Main article: Degree of base saturation and soil buffering

The degree of base saturation of soil is the sum of absorbed bases, expressed as a percentage of the cation exchange capacity (T). It is an indicator of the need for liming of soils. The lower it is, the higher the need for lime application.

Buffer capacity of the soil (soil buffering) is the ability to withstand changes in the reaction of the environment. Buffering capacity is characterized by the value of cationic absorption capacity (T), the composition of absorbed cations and the cationic-anionic composition of the soil solution. The indicator is used to calculate the optimal doses, forms, timing and methods of applying fertilizers and ameliorants to crops. The higher is the value of cationic absorption capacity, the higher is the soil buffering.

Absorption capacity of soil

Main article: Absorption capacity of soil

Absorption capacity is the ability of soil to absorb and retain solid, liquid and gaseous substances.

Absorption capacity of the soil keeps nutrients from leaching in forms that are accessible to plants. Soils with high absorption capacity accumulate moisture in the necessary amounts and keep it from seeping deep into the soil profile, being washed away on the surface and evaporating into the atmosphere, providing favorable water and air regimes. The high content of humus and texture of the soil provides a high moisture capacity.

Effect of fertilizers on soil properties

Main article: Effect of fertilizers on soil properties

Agrochemical methods and means have a complex effect on soil fertility and properties:

acidify or alkalize the soil solution;
change agrochemical properties;
influence on biological and enzymatic activity of soil;
strengthen or weaken physicochemical and chemical absorption;
affect mobilization or immobilization of toxic elements and radionuclides;
increase mineralization or synthesis of humus;
influence the intensity of nitrogen fixation from the atmosphere;
strengthen or weaken the effect of other soil nutrients and fertilizers;
influence the mobility of biogenic macro- and microelements in the soil;
cause antagonism or synergism of ions when absorbed by plants.

Agrochemical characteristics of soils in Russia

Main article: Agrochemical characteristics of soils in Russia

Reproduction of agrochemical indicators of soil fertility

The main method of reproduction of agrochemical indicators of soil fertility is the introduction of mineral and organic fertilizers.

On chemically poor soils extended fertility reproduction is necessary, i.e. introduction of nitrogen, phosphorus, potassium with crops should exceed their removal with crops. When optimal levels are reached, reproduction can be simple, i.e. aimed at maintaining optimal levels. Creation of deficit-free balance of nutrients is especially relevant on sod-podzolic soils characterized by low fertility potential.

In conditions of intensive farming, the use of fertilizers is indispensable. The effectiveness of their use is determined by a number of factors: doses of application, the coefficient of use by cultivated plants, agricultural techniques, forms of fertilizers, etc.

Sources

Farming. Textbook for universities / G.I. Bazdyrev, V.G. Loshakov, A.I. Puponin et al. – M.: Publishing house “Kolos”, 2000. – 551 с.

Diagnosis of plant nutrition

24.01.2022

Diagnosis of plant nutrition – a set of methods aimed at determining the availability of plant nutrients.

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Chemical composition of plants
Air nutrition of plants (photosynthesis)
Mineral (root) nutrition of plants
Nitrogen in plant life
Phosphorus in plant life
Potassium in plant life
Diagnosis of plant nutrition (Русский Español)

The purpose of plant nutrition diagnostics is to continuously monitor growing conditions and, if necessary, correct plant nutrition during vegetation.

Diagnosis of plant nutrition can be:

comprehensive, involving regular agrochemical soil analyses, including an annual one to assess nitrogen availability;
operational, which involves diagnosing nutrition during the growing season.

Methods of plant nutrition diagnostics:

soil diagnostics – determining the quantitative content of nutrients in soils.
plant diagnostics – determining the composition of chemical substances in the plant organism.

When correcting plant nutrition with the help of diagnostic methods we should take into account:

high ordering of vital processes and their localization inherent in plants;
the rate of plant growth and the onset of phases of development, which are determined by genetic factors;
growing conditions;
the impact of nutritional disturbances on the development and chemical composition of vegetative organs, which correlates with the chemical composition of reproductive organs;
nutrients are activators, inhibitors or stabilizers, due to the lack or excess of which the processes of biosynthesis of physiologically active substances and their metabolism are disturbed;
if the function of the nutrient is known, it is possible to control the reaction by dosage and ratio of nutrients;
introduction of nutritional elements leads to changes in the chemical composition of plants, and it is possible to increase the content of other elements that were not introduced;
decomposition of organic matter produces carbon dioxide, water and minerals that affect plant nutrition;
correction of nutrition and cultivation technology in the early stages of development is most effective. Sufficient and balanced supply of nutrition helps to accelerate the initial stages of growth, which leads to a prolongation of each individual leaf;
when distributing assimilates between competing same-type organs, the larger and closer to the source have an advantage.

Plant diagnostics

Plant diagnostics – determining the availability of chemical elements in plants by their chemical composition, taking into account the biological characteristics of the variety, growth rate and duration of growing season.

Diagnosis of plant nutrition is carried out taking into account the history of the field, soil and agrochemical maps, the results of experiments and zonal recommendations on the use of fertilizers for a particular crop. Diagnosis of plant nutrition is carried out taking into account the analysis of the chemical composition of leaves and roots. Normal provision of plants with nutrients is considered a state of internal saturation, accumulation in the reserve zones of the reserve of chemical elements.

For different conditions of soil and climatic zones the general optimal parameters of NPK content in grain crops using the methods of plant diagnostics have been developed.

Plant diagnostics includes:

visual,
chemical (tissue and leaf),
functional, or physiological.

Visual diagnostics

The method of visual diagnosis is based on changes in the morphological signs of plants caused by deficiency or excess of nutrients in the soil.

The accuracy of the visual method is reduced due to the fact that often a sharp deficiency or excess of elements caused in the characteristic signs is rare, while a partial deficiency or excess may not be externally manifested. In addition, similar visual signs can be caused by deviations in temperature or water regimes, pest or disease damage.

Any disturbance of plant life processes is reflected in its appearance, which can be detected in various organs. But for each disorder, there are the most characteristic indicator organs, by which diagnosis is easier to make.

Plant starvation is noted when there is a short-term shift in the optimum ratio of elements, it can occur even on a high nutrient background with a combination of unfavorable external growth factors – light exposure, humidity, temperature, aeration.

In practice, quite often an excess of some nutrients, such as ammonium nitrogen, chlorine, and manganese, is observed in the plant.

The nutrient requirements of different crops vary, so in the same field rye can give a good yield without showing signs of potassium starvation, while potatoes cannot develop normally.

Indicator-plants – plants, by the appearance of which it is easiest to determine the lack or excess of any element of mineral nutrition.

Visual diagnosis of plant nutrition is one of the simplest, requiring no equipment method that allows in a relatively short time to draw a conclusion about the violations of plant nutrition, the causes (soil and weather factors), and to make recommendations for changes in cultivation technology.

In the visual diagnosis, in addition to the general provisions take into account the following:

When assessing the plant is considered simultaneously in three temporal aspects: in the past, reflecting a set of measures for its cultivation, including environmental factors, in the present, assessing the growth rate and degree of development, and in the future, that is, making a forecast of the size and quality of the crop.
Signs of starvation or excess of nutrients are more often manifested not on the entire area, but on individual plots, which is associated with different soil fertility, terrain features, the use of fertilizers, treatments.
Examine the appearance of the plant better with the root system or its part for damage from pests, fungi, bacteria, viruses, pesticides and growth agents.
Often the signs of deficiency and excess have outward similarities. However, deficiency is characterized by clearer signs.
Plant damage caused by viruses often has similar signs to mineral nutritional deficiencies, but is characterized by a clearer boundary of the affected area.
A final conclusion about the cause of the disturbance is made after eliminating it with appropriate treatments or fertilizers. On severely damaged leaves, external signs may persist.

In case of deficiency or excess of the element, the external signs may differ depending on the species and variety. However, there are common signs.

Many nutrients such as nitrogen, phosphorus, potassium, and magnesium have the ability to be reutilized, that is, reused. Deficiencies of these elements appear primarily on the lower, older leaves. Ability to reutilize calcium, sulfur, chlorine, boron and others is weaker, so their deficiency is manifested in the points of growth and on young leaves.

For visual diagnosis they evaluate:

the general condition of the plants of the plot, field;
weight, height of plants, correspondence of development to the period (phase) of vegetation;
length of internodes (young plants have shorter internodes);
stems are complete, resilient and mature, e.g. with a balanced nutrition the stem is more complete in a circle; maturity is estimated by the color of the cut at the level of the 3rd internode from above;
leaf elasticity, coloration of leaves by tier, and nature of disturbances within the tier;
development of roots, presence of root hairs, coloring of roots.

Based on the evaluation results, conclusions are made and recommendations are developed to change the cultivation technology. It is possible to correct unbalanced nutrition only partially, since the appearance of external signs of the lack of a mineral nutrient element indicates profound changes in plant metabolism, which subsequently cannot be eliminated completely.

In order to confirm the suspected nutritional disturbance in plants, injection and spraying methods are used for visual diagnosis. By spraying a leaf or by injection, the presumed missing element is injected into the stem (leaf gland). Observations are then made for several days. If the missing element is correctly identified, the signs of deficiency disappear, but not on the leaves on which it is identified, but on the newly formed ones.

To eliminate nutritional element deficiency, 0.5% solutions of potassium and calcium salts, 0.1% solutions of urea, sodium monophosphate, magnesium sulfate, 0.02-0.1% solutions of salts of trace elements are used.

Nitrogen

Nitrogen deficiency manifests itself:

in the form of inhibition of vegetative growth and reproductive development,
severe reduction in yields,
leaves become light green, then yellow-green to yellow, with severe deficiency signs of nitrogen starvation spread to the whole plant. The aging of the lower leaves may be due to a lack of water, but at the same time there are signs of excess nitrogen.

Nitrogen starvation can easily be corrected by applying appropriate doses of fertilizer. If there is a lack of nitrogen in the soil, but with periodic fertilization, the plants are not deficient in this element.

Normal or increased nitrogen content in the soils of protected ground in spring and summer leads to increased pigmentation, can contribute to overheating of plants, closure of stomata and the termination of the entry of carbon dioxide, the increased decay of organic compounds.

Excess nitrogen is manifested by:

a longer growing season;
prolongation of the vegetative period, and its strong vegetative growth and, if there is a large excess of nitrogen, a complete halt to growth or even death of the plant;
a reduction in resistance to disease;
increased concentration of low-molecular nitrogen compounds, which worsen the fodder quality. For example, it was found that when nitrates are present in the feed more than 0.20%, milk yields decrease (Harker and Kaman, 1961), and at 0.34-0.45%, animal death is possible;
formation of broad, succulent, dark green to bluish-green leaves (if the excess nitrogen is not due to lack of water);
an increase in plant weight;
poor development of the reproductive organs;
damage to the product during storage;
a gradual weakening of carbon dioxide uptake;
decreased protein content and increased carbohydrate content;
retardation of aging processes;
increased outflow of nitrogen from old to young organs;
decomposition of plasma proteins, the content of nucleic acids remains unchanged.

Cucumber and zucchini can be used as indicators of nitrogen excess, and white and cauliflower cabbage, corn, potato, black currant, apple tree, plum can be used as indicators of nitrogen deficiency.

Phosphorus

Phosphorus deficiency manifests itself as:

decreased activity of the tricarboxylic acid cycle, protein synthesis,
Increased accumulation of non-protein nitrogen compounds,
decreased synthesis of starch and cellulose, and in severe deficiency the formation of sugars is slowed down,
increased accumulation of sugars and anthocyanin.

Phosphorus deficiency appears well on tomato, apple, gooseberry, rutabaga, turnip.

Deficiency of phosphorus leads to inhibition of cell division, limitation of plant growth. Leaves turn dark green, dirty green, then reddish to purplish. Older leaves are the first to suffer. Forming leaves are small, ugly, and flowers are small. Premature fruit drop is noted in fruit and citrus crops, and crop thinning in cereals.

In field crops, the signs of phosphorus deficiency are more difficult to determine compared to others. Phosphorus deficiency can also be observed in soils with sufficient content, but with changes in other indicators, such as pH, humus and fine fraction, aluminum, calcium, iron. Phosphorus intake decreases during droughts and low temperatures, lack of oxygen.

Against a background of high doses of nitrogen and high yields, plant demand for labile phosphorus increases, especially during the phase of maximum growth.

Excess phosphorus is often observed in indoor conditions, much less often in field crops, leads to early aging of plants, starting with yellowing and dying off of old leaves, accelerated transition to the development of reproductive organs.

Application of high doses of phosphorus creates a deficit of calcium and trace elements such as, zinc, iron, boron, copper, manganese, the intake of toxic elements – aluminum and heavy metals is reduced.

Potassium

Potassium deficiency is often evident on light, acidic or high in three-layer clay minerals soils, which lose and fix potassium during intensive use. Potassium uptake worsens with desiccation and high doses of ammonium fertilizers that block potassium in three-layer minerals like vermiculite. Potassium deficiency can be caused by antagonism with calcium and ammonium.

Potassium deficiency leads to profound disturbances in structure and metabolism due to the participation of potassium in enzymatic processes and biological colloids. Hydrolysis processes are intensified, enrichment with low-molecular compounds of carbon and nitrogen increases, cell walls become thinner, water loss increases and its intake decreases.

Potassium-loving crops such as cabbage, potatoes, gooseberries, beets, alfalfa, beans, red currants and apple trees are most sensitive to potassium deficiency.

Signs of potassium deficiency include:

stunted plant growth;
normally colored or light green leaves are firm in the morning hours, wilting as light or temperature increases;
young leaves small;
leaves of lower tiers with normal or dark green coloration, become cup-shaped, dome-shaped, more often with marginal undergrowth. With severe potassium deficiency, these signs spread to leaves of the middle and upper tiers. In some plant species, pitting necroses along leaf margins are noted, which later merge into areas of light and dark brown color.

Excess of potassium is rare. Its signs are more often accompanied by signs of chlorine excess. Excess potassium may also appear as a lack of calcium and magnesium. High potassium content leads to lower intake of boron, zinc, manganese and ammonium, and increased intake of iron.

Calcium

Calcium deficiency increases synthesis of phenolic compounds. Cell membrane permeability increases, resulting in outflow of ions from the cell and further disturbance of nucleus structure, decrease in chromosome stability. Calcium is especially important for meristem tissue and its differentiation, directed action of phytohormones.

Plant species and varieties differ in their need for calcium and in their ability to absorb it from the soil. Disturbance of calcium nutrition in plants is often the cause of non-parasitic diseases.

Calcium accumulates in vegetative organs and in limited amounts in fruits. Its content in fruits and hoarding organs decreases with decreased transpiration, but high transpiration does not guarantee sufficient supply of calcium and water to plants.

Calcium nutrition of the plant is related to boron nutrition, the signs of deficiency of which are similar.

High calcium concentrations due to antagonism reduce the supply of other cations, which is important to consider in the presence of heavy metals in the soil. Calcium has a positive effect on elevated concentrations of soil solution macronutrients and most trace elements except molybdenum.

Calcium deficiency increases nitrate accumulation in plant tissues. The apical meristem, shoot and root, flowers and fruits are the first to suffer. Old leaves turn dark green, then turn yellow and die off. Roots remain short, slough, turn brown, and die off. In the upper, young leaves, the tip turns white at first, and the edges are affected with major disturbances.

With reduced transpiration, reduced calcium inflow leads to breakage of shoots of outwardly normal and intensively developing plants.

In fruit plants, when the ratio of ammonium nitrogen to calcium is increased, flower dieback occurs; increased potassium content intensifies this process. When calcium content in leaves is less than 3.0% and in fruits less than 0.15%, wilting of flowers begins.

Sufficient calcium content in leaves and fruits is not a guarantee of optimum conditions for further plant growth. Calcium must be in the form of free ions in the soil solution. The need for it increases with increasing light exposure.

An excess of calcium is rare, as a result of nutrient disturbance during liming. At the same time there may be a lack of potassium, boron, manganese, zinc, copper, sometimes magnesium, and an excess of chloride and sulfate. In such cases, in order to maintain the planned yields, the doses of all elements are increased and physiologically acidic fertilizers are envisaged.

Magnesium

Sufficient potassium nutrition of plants increases the content of magnesium in seeds and fruits; high doses of potassium fertilizers, on the contrary, suppress this process. A high potassium to magnesium ratio increases the manifestation of chlorosis, even if there is enough magnesium in the soil. High doses of ammonium have a similar effect.

Magnesium deficiency is observed in many soils, especially on dealluvial sandy, highly leached, acidic soils of high bogs, as well as on soils after liming in connection with the antagonism of calcium and magnesium. Lack of magnesium nutrition at optimal levels in the soil may be due to antagonism with hydrogen, potassium, ammonium, calcium, and manganese ions (Bergman, 1983).

Cabbage, potatoes, apple trees, gooseberries, blackcurrants, and grapes are susceptible to magnesium deficiency. Magnesium deficiency causes orange coloring of millet leaves, and purple-red coloring of blackcurrant and cottonseed.

Magnesium deficiency causes magnesium to drain from older leaves. Healthy plants have more magnesium concentrated in the lower leaves than in the upper leaves. Interstitial chlorosis is observed on the lower leaves, followed by brown and dark brown necroses. Magnesium deficiency reduces starch accumulation in potatoes, sugar in sugar beets, fat in oil-bearing plants, and protein.

Magnesium deficiency in vegetative organs leads to an increase in phosphorus, in seeds – to a decrease, and later in leaves. Nitrate reduction and phytohormone synthesis are slowed. In severe deficiency, carbon dioxide fixation stops, leaves become brittle, and accumulation of proteins, carbohydrates or fats in fruits decreases.

An excess of magnesium can be observed when the calcium-magnesium ratio is disturbed, especially when the root system is specifically damaged due to the lack of calcium, the yield is reduced, growth is slowed, potassium content and magnesium intake is reduced. Excess magnesium is also affected by high levels of nickel and chromium. Magnesium intake is facilitated by the nitrate ion.

Bor

Deficiency of boron leads to disturbances in the metabolism of nucleic acids, proteins and carbohydrates, respiration and photosynthesis processes; synthesis of phytohormones is reduced. Visual identification of boron deficiency due to its involvement in many metabolic processes and plant development is difficult.

Signs of boron deficiency are manifested primarily on young leaves and on the tips of growing shoots and roots. Boron content in older leaves is always higher.

Signs of boron deficiency may include:

chlorosis, yellowing and then turning brown on the tips of young leaves; in tomato, blackening of the stem growth point;
die-off of the growth cone, stunted formation of roots, flowers, seeds;
desiccation of leaves, shredding, stopping the dominant development of the central shoot and sprouting of lateral shoots and roots.

Boron deficiency is stronger on rutabaga, turnip, sugar and fodder beets, sunflower, cauliflower and fodder cabbage, legumes, fruit crops, tomato, celery, flax, and rye.

Excess of boron leads to white-white edges of leaves, later they turn brown; sometimes pinpoint chlorosis appears on old leaves first. Excess of boron can coincide with signs of potassium deficiency.

Molybdenum

Molybdenum deficiency is manifested in the form of light coloring of leaves, especially along the central vein, similar to the signs of nitrogen deficiency and nitrate nitrogen excess, i.e. darker coloring and whitening of the leaf margin. Nitrogen supply to the reproductive organs is slowed, resulting in yield loss. Excess molybdenum leads to severe growth inhibition.

The deficiency is pronounced on cauliflower, legumes and green crops, tomato, and citrus. Most crops develop yellow leaf spotting, in cucumber – chlorosis of the edge of leaf blades.

Copper

In soil, copper is accumulated in organomineral complexes and partially in the exchange-absorbed state. Availability of copper decreases when the pH rises from 5.5 to 6.0. Deficiency of copper is clearly expressed on uncultivated wastelands, on light soils and soils of high bogs, sometimes on lowlands. Lack of copper in the feed leads to animal diseases.

Copper deficiency is more pronounced on clover, meadow millet, legumes, vegetables, oats, barley, wheat, cereal grasses, hemp, flax, forage and table root crops.

Copper deficiency is manifested by white tips of leaves, which later dry up; plants throw out panicles with high hollowness with a long delay; grains are formed sparsely.

Excess copper and phosphorus leads to zinc deficiency, sometimes iron deficiency. Excess appears on young leaves.

Iron

Iron deficiency is noted in soils rich in calcium and having an alkaline reaction, can occur in acidic soils with high magnesium content. Iron is absorbed by the plant throughout the growing season because it is not recycled from old leaves.

Iron deficiency shows up on young leaves, on fear, only when very deficient. Light green coloring of young leaves appears first, followed by yellowing and whitening. The veins and adjacent tissues remain green. The appearance of chlorosis decreases from top to bottom.

Excess iron is extremely rare, with leaves becoming dark green and bluish-green in color due to limited growth of shoots, leaves and roots. Symptoms of excess often coincide with symptoms of phosphorus deficiency, especially at low pH values.

Manganese

Manganese is contained in the humus layer and the silt fraction of the soil. In acidic soils, it is present in the form of a low-mobile divalent poorly accessible to plants. Mobility of manganese increases with the application of ammonia fertilizers.

Manganese deficiency manifests itself as point chlorosis, turning into necrosis on young leaves, and in excess – on old leaves.

Manganese deficiency is often observed in oats, wheat, potatoes, corn, table and forage root crops, cabbage, legumes, sunflower, fruit, citrus and vegetable crops. For example, oats have gray leaf spotting and sugar beets have spotting jaundice.

Excessive manganese content is eliminated by liming or high doses of iron.

Zinc

Zinc deficiency is manifested as reduction of growth, asymmetry of leaves, corrugation of leaf blade, inter-vein chlorosis.

Fruit crops, citrus, corn, soybeans, beans, buckwheat, beets, potatoes, meadow clover, and hops are susceptible to zinc deficiency.

Zinc deficiency is especially common in neutral and slightly alkaline soils. Systematic application of manure greatly reduces the risk of zinc deficiency. One way to prevent zinc deficiency is to plow weeds under corn.

An excess of zinc is extremely rare. Appears as chlorosis associated with iron deficiency, coloration of leaf veins is the same as in phosphorus deficiency; individual chlorosis along veins is located closer to leaf margins; leaf marginal chlorosis.

Chemical diagnostics

The method of leaf or tissue diagnosis is based on the relationship between changes in the nutritional regime and the chemical composition of leaves or tissues, the most responsive organs. Optimal concentrations of nutrients have been determined for different plants, at which crops show maximum productivity.

The accuracy of these methods for predicting fertilizer requirements is higher than soil analyses, because when determining the amount of nutrients in the soil, it is difficult to predict how much of them will reach the plants under changing plant life factors.

The disadvantage of chemical diagnostics, as well as of visual diagnostics, is the delay in obtaining information. Chemical diagnosis of plant nutrition under adverse meteorological conditions also gives distorted data on plant nutrition due to.

Tissue diagnosis

Tissue diagnosis of plant nutrition involves determining the content of nitrates, phosphates, sulfates, potassium, magnesium and other nutrients in tissues or extracts from plants. Tissue diagnosis can be carried out in the field with portable devices – portable laboratories, and in the laboratory. It is used for express analysis of nitrate, phosphate and potassium content in raw plant samples by the method of V.V. Zerling and to determine the ripeness of grain. For the same purpose you can use a portable express laboratory – a field bag of K.P. Magnitsky.

Determination of nitrate nitrogen in plant tissues under field conditions can be done by reaction with diphenylamine. This method is used to assess the need for nitrogen dressing.

Table. Determination of nitrate nitrogen

Average field score	Proportion of nitrogen, kg/ha a.d.m.
1,0-1,8	60
1,9-2,5	30
2,6-3,0	Feeding is inexpedient

Indicator paper “Indam” can be used to diagnose nitrogen nutrition of winter cereals. Diagnostics is performed in tillering, heading, earing, and flowering phases. The following nodes are analyzed: in tillering phase – the tiller node, the second stem node – the second stem node, earing and flowering – the last stem node before the ear.

Table. Estimation scale of nitrogen availability of winter cereal crops

Indicator colour	Score	Nitrogen availability	Average score	Nitrogen rate, kg/ha a.d.m.
				tillering-tubing	earing-flowering
White, pink-white	1	Low	Up to 1,8	60-80	Ineffective
Pink	2	Medium	Up to 1,9-2,5	30-40	40-50
Pink intense, crimson	3	High	2,6	-	0-30

The method of determination in tissue slices is less accurate than in extracts.

Observations made under field conditions by B.A. Yagodin in 1993 of nitrate nitrogen availability in winter wheat in phases from the beginning of emergence to the beginning of grain formation using the method of tissue diagnosis allowed to establish the periods of highest demand for nitrogen fertilizers. It was found that the place of nitrate localization in the stem within the same phase of development is not constant, so it is necessary to pre-determine the stem node over which to make a cut for tissue diagnosis.

In this work, tissue diagnosis was performed according to the method of Wolring, Wehrmann (1981, 1983) using 0.5 g diphenylamine in 100 ml of concentrated H₂SO₄, which is similar to the method of Zerling (1978). A four-point scale was used to assess assurance:

0 – no staining,
1 – blue,
2 – light blue,
3 – dark blue.

The necessity of the second foliar nitrogen top dressing in the phase of emergence of the tube was justified by a decrease in the nitrogen content below 2 points. The dose was determined according to recommendations (Vielemeyer und a. 1985; Jakob und a., 1986): with an average score at the stage of 0 to 1.4, the dose is 40-50 kg N/ha, with 1.5-2.4 points – 30-40 kg N/ha. The third feeding with nitrogen fertilizers was carried out at the stage of milky ripeness in a dose of 30 kg/ha.

Leaf diagnostics

Leaf diagnosis of plant nutrition consists of gross analysis of the chemical composition of leaves of the whole plant or individual organs and comparison of the data obtained with reference values, the results of which make a conclusion about the availability of mineral nutrition, taking into account the state, growth and development of plants in a particular phase.

Plant samples are selected from areas typical for the given field, i.e. with a characteristic soil cover and the state of plants in certain phenophases.

Early diagnostic control taking into account the specific needs of crops by vegetation periods is the most informative.

When analyzing seedlings, sprouts or young plants, the whole above-ground part is determined, in adult plants, the lower part of the stem or petioles of the lower leaves are used to determine nitrates. To determine the total nutrient export all plant organs are analyzed. For leaf diagnostics, the indicator organs subject to the greatest changes in chemical composition depending on nutritional conditions can be analyzed. For example, in field tests with grain crops, a mixed sample is made up of 50-70 indicator leaves.

Table. Optimal gross content of nitrogen, phosphorus and potassium in plants, % on absolutely dry mass

Crop	Development phase	Part of the plant	N	P₂O₅	K₂O
Winter wheat	Tillering	Above-ground part	4,0-5,9	0,44-0,65	3,3-4,2
	Tillering	Leaves	4,0-5,9	0,44-0,65	3,3-4,2
	Tubing	Above-ground part	3,8-5,0	0,52	2,5-3,3
Barley	Tillering	Above-ground part	4,7-5,0	0,52-0,78	4,2
	Tillering	Leaves	4,7-5,0	0,52-0,78	4,2
	Tubing	Leaves	4,7	0,52	4,0
Clover	Budding	Above-ground part	3,5-4,0	0,26-0,39	2,9
	Blooming	Above-ground part	2,5-3,5	0,17-0,26	2,2
	Blooming	Leaves	3,8	0,22	2,9
Corn	Sprouting	Above-ground part	4,3	0,52	5,2
	Phase 3-5 leaves	Above-ground part	3,0-3,6	0,30-0,65	2,8-3,3
	Phase 3-5 leaves	Leaves	3,8-4,0	0,35-0,57	3,2-4,2
	Phase of 6-10 leaves	Leaves	3,5-4,0	0,30-0,52	3,5-4,2
Sugar beet	Phase 4-6 leaves	Leaves	5,2-5,5	0,44-0,52	4,1-6,0
	Phase 10-18 leaves	Leaves	3,7	0,35	-
	Row closing	Middle leaves	3,6-4,0	0,33-0,40	4,0
Potatoes	Before budding	Above-ground part	5,2-6,0	0,39-0,61	4,2
Potatoes	Before budding	Leaves	4,5-5,0	0,26-0,57	4,2

To determine the insufficiency of elements capable of reutilization, the upper, fully formed leaf is used, for the elements with low ability to reutilization the lower leaves are analyzed. Roots are analyzed in parallel and the ratio of mineral nutrients in the leaves and roots is determined.

For express analysis by the method of Zerling, a mixed sample is prepared from 10-20 whole plants in the tillering and trumpeting phase and from 20 plants in the earing and flowering phases. For biometric control of plant growth and development, 20 plants, including roots, are taken from each experimental plot, and 70-100 plants from each plot are taken for gross analysis in production crops; for biometric control, 25-30 plants are taken. Sampling is carried out in the morning hours, walking along the plot diagonals, no rainfall or irrigation for 2-3 days before sampling.

Storage, transportation, sample preparation and chemical analyses of plants are carried out in accordance with established methods.

The allowable content of nitrates in crop products is established by the state sanitary rules. For example, for potatoes, the allowable content of nitrates is 80 mg/kg raw weight or less, for cabbage no more than 300 mg/kg, tomatoes no more than 60 mg/kg, cucumbers no more than 150 mg/kg, carrots no more than 300 mg/kg, melons no more than 45 mg/kg, water melon no more than 45 mg/kg, table beets no more than 1400 mg/kg, onions no more than 60 mg/kg, chives no more than 400 mg/kg raw weight.

Particularly careful control of nitrate content should be carried out in the early phases of plant development and in leaf and green crops. By the end of the growing season, the nitrate content decreases. Their concentration is higher in the petioles and central veins of leaves which are analyzed. In reproductive organs and meristematic tissues, nitrate content is minimal.

Conclusions about the availability of plant nutrients are made on the basis of the relative content of nutritional elements, as well as the total accumulation by leaves or the whole plant by comparing with reference data.

To determine the removal of elements in plants, the content of nutrients is multiplied by the dry weight of the crop per 1 hectare. Also determine the ratio between the elements to establish the degree of balance of nutrition and compare them with the reference data.

Doses accepted in the system of fertilizers for the planned yield are specified by the results of plant diagnostics:

where D – adjusted dose of fertilizer, kg a.s./ha; A – average set dose, kg/ha; C_opt – optimum content of nutrient in plants, % of dry matter; C_fact – actual content of nutrient in plants, % of dry matter. The C_opt/C_fact ratio reflects the degree of plants’ need for a nutrient.

If the ratio of nutrients is unbalanced, the dose of one nutrient can be refined relative to the other.

For example, with a lack of nitrogen and an excess of phosphorus, the specified dose of nitrogen (D_N) can be calculated by the formula:

Similarly, the dose of phosphorus (D_P) in relation to potassium can be calculated by the formula:

Integrated diagnosis and recommendation system (DRIS)

The integrated diagnosis and recommendation system (DRIS) was developed in the United States. It is based on a probabilistic approach, based on the fact that the balance of nutrients in the tissues and organs of plants is subject to characteristic patterns. At the same time, it is assumed that the ratio of elements has diagnostic informativeness and better reflects the provision of plants.

In our country the first experiments with the use of this system, called the integrated system of operational diagnostics (ISOD), were carried out in the Soil Institute of them. V.V. Dokuchaev.

The integrated system of operational diagnostics is a set of methods used to diagnose fertilizer requirements, predict crop productivity and develop models of highly fertile soils. Influences of each factor on productivity indicators are expressed in indices. The basic basis for calculating the index is the optimal level of the factor under study.

The methodology of the system is reduced to determining the actual ratio of the amounts of nutrition elements (N : P, N : K, K : P, N : Ca, N : Mg, etc.) in the leaves and comparing the data obtained with the standards, which are constant for different types of soils. The index shows the degree of deviation of the factor under study from the optimum. The value and sign of the indices show the level of deficiency of an element of a nutrient.

Example. Norms of N:P, N:K, K:P ratios were obtained for the Moscow region by Elnikov et al., 1986. Corn was grown on leached chernozem in different weather conditions in different years. All leaves of corn in the flowering phase were taken for analysis. According to the method in 1963 with a favorable moisture regime in the variant without fertilizers, the soil was insufficiently provided with nitrogen, with an index of – 13.6, excessively provided with potassium (index +11.2) and close to optimal – with phosphorus (index +2.4). Thus, there was a strong imbalance of nutrients – the sum of the indices without taking into account the sign (13.6 + 2.4 + 11.2) = 27.2. Introduction of N60 reduced nitrogen deficiency, but at the same time increased the phosphorus deficiency (index – 5,8), and the total imbalance remained high (sum of indices 18,6). The addition of P₆₀ increased the phosphorus supply and increased the nitrogen deficiency. Potassium application balanced nitrogen and phosphorus deficiency, but the overall imbalance remained.

Table. Fertilizer doses and N, P, K deficiency in corn leaves (by DRIS indexes)

Option of experience	Content in leaves			DRIS Indexes			Grain yield, t/ha
	N	P	K	N	P	K
Experience 1963.
Control (without fertilizer)	2,53	0,29	2,50	-13,6	+2,4	+11,2	3,40
N₆₀P₀K₀	2,78	0,28	2,49	-3,5	-5,8	+9,3	3,41
N₀P₆₀K₀	2,68	0,33	2,50	-16,0	+11,3	+4,7	3,88
N₆₀P₆₀K₀	2,80	0,31	2,45	-7,9	+3,6	+4,3	4,53
N₀P₀K₆₀	2,71	0,28	2,66	-8,0	-17,2	+15,2	3,68
N₆₀P₆₀K₃₀	2,82	0,31	2,51	-8,1	+2,4	+5,7	4,56
N₉₀P₆₀K₃₀	3,00	0,30	2,40	0	-2,2	+2,2	4,77
N₁₂₀P₆₀K₃₀	3,22	0,29	2,32	+8,3	-7,7	-0,6	5,00
N₆₀P₉₀K₃₀	2,85	0,31	2,40	-5,7	+3,4	+2,3	4,64
N₆₀P₁₂₀K₃₀	2,80	0,31	2,41	-7,4	+4,2	+3,2	4,94
N₁₂₀P₁₂₀K₁₂₀	3,05	0,33	2,50	-4,8	+3,8	+1,0	5,42
Experience 1964.
Control (without fertilizer)	2,60	0,30	2,74	-16,0	+0,5	+15,5	2,71
N₆₀P₀K₀	2,70	0,28	2,74	-9,1	-8,3	+17,4	2,72
N₀P₆₀K₀	2,78	0,38	2,82	-25,7	+18,8	+6,9	2,98
N₆₀P₆₀K₀	2,87	0,29	3,15	-10,4	-14,4	+24,8	2,93
N₀P₀K₆₀	2,91	0,28	3,07	-6,4	-17,3	+23,7	3,39
N₆₀P₆₀K₃₀	2,90	0,36	3,15	-21,5	-6,5	+15,0	3,48
N₉₀P₆₀K₃₀	3,12	0,36	2,82	-11,0	+6,2	+4,8	3,67
N₁₂₀P₆₀K₃₀	3,48	0,33	2,57	+4,8	-4,0	-0,8	3,79
N₆₀P₉₀K₃₀	2,86	0,39	2,90	-25,5	+18,5	+7,8	3,83
N₆₀P₁₂₀K₃₀	2,89	0,40	2,99	-27,2	+19,4	+7,8	3,99
N₁₂₀P₁₂₀K₁₂₀	3,11	0,37	2,83	-7,7	+5,3	+2,4	4,17

Note. The indices are calculated according to the norms: N/P = 10.0, N/K = 1.26, K/P = 7.50 with coefficients of variation 10, 19, 15.

In a dry year, the elimination of excess potassium or reducing its content to a minimum in the absence of nitrogen deficiency (options N₁₂₀P₆₀K₃₀ and N₁₂₀P₁₂₀K₁₂₀) led to the highest corn yield, which fully corresponds to the characteristics of soil fertility in the option without fertilization. In the other variants a higher deficit of nitrogen was noted, which agrees with the position of the low efficiency of nitrogen fertilizers in years unfavorable in terms of moisture. In the experiments the corn yield was determined by the balance of nutrition elements, which was manifested in the conditions of relatively favorable moisture.

Functional diagnosis of plant nutrition

The uptake of nutrients is not always a consequence of their need for the plant. This is the main drawback of chemical diagnostic methods. In addition, a deficiency or excess of elements can lead to an impairment of plant uptake of other nutrients. For example, phosphorus deficiency leads to an accumulation of nitrates, and boron deficiency leads to a deficiency. However, this has nothing to do with nitrogen nutrition.

Functional diagnostic methods allow you to assess the need for plant nutrients by determining the intensity of physiological and biochemical processes. Thus, the level of supply and need for nitrogen is determined by the ability of tissues to reduce nitrates into nitrites, that is, by the activity of the enzyme nitrate reductase (Muravin, Slipchik, Pleshkov, 1978).

A.S. Pleshkov and B.A. Yagodin (1982) developed a diagnostic method for determining photochemical activity of chloroplasts, which was based on measuring photochemical activity of chloroplast suspension of an average leaf sample of diagnosed plants, followed by analysis with addition of nutrients. When the photochemical activity of the suspension increases with the addition of a nutrient compared to the control, the conclusion is made about the lack of this element, when it decreases – about the excess, and when the activity is the same – about the optimum content.

The proposed method allows for 40-50 minutes to establish the need of plants in 12-15 macro- and microelements and to make recommendations for root and foliar feeding. The method was introduced in 80 greenhouse farms, including Moscow (farm “Belaya Dacha”) and Ivanovo regions (farm “Teplichny”).

Soil diagnosis

Granulometric composition, organic matter content in soil, gross nutrient content, absorption capacity (AC) change slowly and serve as a characteristic of a particular soil difference.

The content of mobile forms of nutrients, soil reaction, composition of absorbed cations, the degree of saturation with bases change faster, especially under the influence of ameliorants and fertilizers. Therefore, agrochemical surveys of soils by these indicators are carried out at certain periods, as a rule, one in two years, or more frequently, depending on the amount of fertilizers and ameliorants applied. The results of such studies are presented in the form of agrochemical maps, field passports, or cartograms, and are used to determine the optimal norms, forms, timing, and methods of fertilizer application, as well as the need and doses of ameliorants.

Soil diagnosis of plant nutrition – periodic determination of relatively rapidly changing agrochemical indicators of soil. Soil diagnosis allows the most rational use of fertilizers and ameliorants, maximize their agrotechnical, economic efficiency and environmental safety.

The most dynamic indicator determined by soil diagnosis, which can change over several days, is the content of mineral forms of nitrogen. For this reason, this indicator is not used for agrochemical maps, cartograms and field passports. However, for economical and environmentally safe use of nitrogen fertilizers annual information about the reserves of mineral forms of nitrogen is necessary.

For regions of sufficient and excessive moisture soil diagnosis of nitrogen forms is carried out before the application of nitrogen fertilizers, in zones with insufficient moisture and arid climate – before the fall application. Depending on the depth of penetration of root systems of crops, water and air regimes of soils during the growing season, the content of mineral forms of nitrogen is determined in the soil layers to 100, 150, 180 cm. For the majority of agricultural regions of the country it is experimentally established that 60-80% of mineral nitrogen of 0-180 cm soil layer is concentrated in 0-60 cm layer. Conversion factors for mineral nitrogen reserves for 0-40, 0-60 cm layers in 0-100, 0-150 cm layers, etc. have been established for some regions.

There are several variants of correction or calculation of doses of nitrogen fertilizers based on the results of mineral nitrogen nutrition diagnostics. In all variants ammonia and nitrate nitrogen stocks in a definite layer are recalculated in kg/ha and taking into account possible nitrogen use coefficients by definite crops the received value is deducted from the value of crops general demand.

A simplified modification of nitrogen fertilizer doses correction was proposed by Y.P. Zhukov from the Agrochemistry Department of the Moscow Agricultural Academy. Mineral nitrogen is determined before the application of nitrogen fertilizers in the arable soil layer, i.e. 0-20 or 0-30 cm, the received result is recalculated in kg/ha and deducted from the set dose or the total crop requirement in nitrogen. This approach makes it possible to reduce the time for deep soil studies, moreover, the uncertainty of weather conditions does not allow us to make an accurate prediction regarding the stability of nitrate and ammonia forms of deep layers, especially in the first month after sowing the roots do not have time to penetrate to a sufficiently large depth.

Sources

Yagodin B.A., Zhukov Y.P., Kobzarenko V.I. Agrochemistry / Edited by B.A. Yagodin. – Moscow: Kolos, 2002. – 584 p.: ill.

Potassium in plant life

Potassium, a chemical element, along with nitrogen and phosphorus, is the most important element of plant nutrition. Attempts to replace it with closely related elements (sodium, lithium, rubidium) proved unsuccessful.

The need for potassium for plants was first suggested by Sosur in 1804 on the basis of chemical analysis of plant ash, in which potassium was always present. Later, Liebich concluded that potassium fertilizer was necessary. The first experimental confirmation of the necessity of potassium for plants was obtained by Salm-Gorstmar in 1846.

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Potassium content in the plant organism

Potassium in plants is mainly concentrated in the cytoplasm and cell vacuoles in ionic form. It is not part of organic compounds, but is involved in photosynthesis. About 80% of potassium is in plant cell sap and can be easily washed out by water, such as rain and especially from old leaves, while the remaining 20% is retained in the exchange-absorbed state by colloids of cytoplasm. It enhances the hydration of cytoplasmic colloids, increasing the water-holding capacity and drought tolerance of plants. About 1% is absorbed by mitochondria.

During the daytime, potassium, while remaining mobile, is retained in plant cells. At night, when photosynthesis stops, some potassium may be excreted through the root system, but when the first rays of the sun appear, it is absorbed by the plant again.

Young plant organs contain 3-5 times more potassium than old ones: its content is higher in organs and tissues, where metabolic processes and cell division are intense. Therefore, potassium is also called the element of youth. Potassium is contained in large amounts in pollen. For example, pollen ash from corn contains up to 35.5% of potassium, while calcium, magnesium, sulfur and phosphorus account for 24.7% in total. The mobility of potassium determines its reutilization by moving it from old leaves to young ones. Therefore, its distribution in plants is characterized by a basipeptal concentration gradient, that is, its content in leaves and stem per unit of dry matter increases from bottom to top.

In the cell sap, the content of potassium is much higher than other cations and exceeds the concentration in the soil solution by 100-1000 times.

Unlike nitrogen and phosphorus, potassium is concentrated in vegetative organs and not in reproductive ones. For example, in cereal straw potassium is 2 times more, and in corn stalks – 5 times more than in grain. Therefore, the removal of potassium with the nonreproductive part of the crop is usually greater than with the marketable part, with the exception of leguminous plants.

Potassium content in plants may vary depending on climatic conditions, the applied agricultural techniques, and soil fertility.

Grain crops contain 15% of the total amount of potassium in the crop, and straw contains 85%. Potato tubers contain up to 95% and haulm up to 5% of the total potassium content.

Таблица. Average content K₂O in the yield of some crops, % on absolutely dry matter (by Peterburgsky)

Crop	Products	K₂O	Crop	Products	K₂O
Winter cereals	Grain	0,65	Potatoes	Tubers	2,40
Winter cereals	Straw	1,10	Potatoes	Above-ground part	3,70
Spring cereals	Grain	0,67	White cabbage	Heads	4,60
Spring cereals	Straw	1,30	Carrots	Roots	3,20
Corn	Grain	0,43	Cucumber	Fruits	5,65
Corn	Stems	1,93	Tomato	Fruits	5,60
Peas	Grain	1,46	Flax	Straw	1,10
Peas	Straw	0,60	Cottonwood	Fiber	1,00
Sugar beet	Roots	1,00	Meadow clover	Hay	1,80
Sugar beet	Above-ground part	3,00	Lucerne	Hay	1,80
Fodder beet	Roots	3,50	Vicia	Hay	1,20
Fodder beet	Above-ground part	2,63	Timothy	Hay	2,42

Importance of potassium

Potassium regulates photosynthesis, increases outflow of carbohydrates from leaf lamina to other organs, participates in synthesis of sugars and high-molecular weight carbohydrates – starch, cellulose, pectin substances, xylans.

Potassium promotes the accumulation of monosaccharides in fruit and vegetable crops, sucrose in root crops, starch in potatoes, thickens cell walls of cereal straws, increases resistance to lodging, in flax and hemp it improves fiber quality.

Table. Effect of potassium on the content of reducing sugars, sucrose and starch in tomato leaves and petioles, % (by Bagaev)

Indicator	Leaves		Petioles
	with potassium	without potassium	with potassium	without potassium
Reducing sugars	2,34	2,01	1,56	1,00
Sucrose	1,20	0,35	0,00	0,00
Starch and dextrins	2,48	1,00	4,22	0,96

By accumulating carbohydrates in plant cells, potassium increases the osmotic pressure of the cell sap, thereby increasing the cold tolerance and frost tolerance of plants.

Potassium accumulation in chloroplasts and mitochondria helps to stabilize their structure and formation of ATP. It increases the hydrophilicity of protoplasm colloids and decreases transpiration, which helps plants to better tolerate short-term droughts.

Potassium is involved in protein synthesis and metabolism. If it is lacking, synthesis is reduced with the simultaneous breakdown of old protein molecules. Amino acids accumulate in plants. Optimized potassium nutrition leads to an increase in the proportion of protein in wheat plants. Asparagine and glutamine synthesis is enhanced. The positive effect of potassium on protein synthesis is associated with its effect on the accumulation and transformation of carbohydrates (carbohydrates during respiration form keto acids, from which amino acids are synthesized), as well as with an increase in the enzymatic activity of protein synthesis.

Potassium catalyzes the synthesis of vitamins thiamin and riboflavin, regulates the functioning of the stomata closing cells of leaves.

Potassium is absorbed by plants as a cation and in this form remains in cells and is the main counterion of negatively charged cell anions. Potassium creates an electrical potential difference between the cell and the environment.

By participating in the most important biochemical processes, potassium increases resistance to various diseases during the growing season and in the post-harvest period, improves the storability of fruits and vegetables.

The critical period of potassium consumption by plants occurs in the first 15 days after sprouting. The period of maximum consumption often coincides with the period of intensive growth of biological mass. In some crops, such as flax, for example, the intake of potassium stops at the phase of full flowering or at flowering – the beginning of ripening, as in cereals and legumes. In other crops, the intake is more protracted and occurs throughout the growing season, as in potatoes, sugar beets, and cabbage.

In areas where the effect of potassium fertilizers is the most effective, their use provides an increase in yield for each kilogram of potassium fertilizer: 2-3 kg of grain, 20-33 kg of potatoes, 35-40 kg of sugar beets, 1-1.5 kg of flax fiber, 20-33 kg of hay of sown grass and 8-18 kg of hay of meadow grass.

Potassium deficiency

Potassium deficiency leads to decreased enzymatic activity, impaired carbohydrate and protein metabolism, and increased consumption of carbohydrates for respiration.

As a result, plant productivity and product quality decrease. Grain crops form puny grains, reduced germination and seed viability. Bread straws often lodge due to reduced strength. The starch content of potato tubers, sucrose content of sugar beet roots, pectin content of fruits and berries, and vitamins in products are reduced. The incidence of diseases increases. Fewer storability during storage.

Outwardly, potassium starvation is manifested on leaves of the lower layer: they turn yellow prematurely, starting from the edges; subsequently, the edges turn brown, then die off and are destroyed, so they look like burnt. This phenomenon is called “marginal burn”. Potassium deficiency leads to decreased turgor, and leaves wilt and droop. Most often, the lack of potassium occurs during intensive growth (in the middle of the growing season), when its content in the cells decreases by 3-5 times the norm.

Potassium deficiency is most strongly reacted by potassium-loving crops.

An excess of potassium also negatively affects growth and development. It manifests itself in the appearance of pale mosaic spots between leaf veins, with time they turn brown, and leaves fall off.

Potassium cycling and balance in agriculture

Potassium cycling in biocenoses is intense. The content of potassium in the biomass of biocenoses varies from 20 ka/ha for deserts to 2000 kg/ha for oak forests.

The closed cycle of the nutrient cycle in natural biocenoses due to the accumulating activity of plants leads to the accumulation of potassium within the root layer and the gradual enrichment of the upper horizons with this element.

In agrocenoses, the circulation and balance of potassium is mainly influenced by economic activities: the provision of fertilizers, the specialization of farms.

Gross reserves of potassium in soils are 5-50 times higher than the reserves of nitrogen and phosphorus. D.N. Pryanishnikov when estimating the balance of potassium for the country as a whole allowed its deficit of 20-22 kg/ha per year.

The main expense item of the balance of potassium in agriculture is the economic removal with the products.

With a crop of plants can be taken out annually from 40 to 310 kg/ha of potassium. These figures are calculated, in particular for cereals, for average yields, with increasing productivity they will naturally increase.

Table. The content of potassium in the yield of the most important crops^[1] Yagodin B.A., Zhukov Yu.P., Kobzarenko V.I. Agrochemistry / Edited by B.A. Yagodin. - Moscow: Kolos, 2002. - 584 p.: ill.

Crop	Product yield, t/ha		Total removal K₂O, kg/ha
	commercial	by-products
Cereals	2,0-2,5	4,0-6,0	45-77
Buckwheat	2,0	6,0	150
Flax and hemp	1,0	4,5-6,5	50
Sunflower	1,8	7,5	360
Potatoes	20,0	12,0	200
Sugar beet	30,0	20,0	175
Cabbage	70,0	40,0	310
Leguminous	2,0	3,0	40
Clover (hay)	6,0	-	90
Alfalfa (hay)	10,0	-	up to 150
Meadow grasses (hay)	6,0	-	up to 120
Timothy (hay)	6,3	-	up to 86

For an objective assessment of the balance of potassium it is necessary to take into account the distribution of removal between marketable and non-marketable parts of the product. For example, the grain of wheat contains 15% of the total economic removal of potassium, and straw contains 85% of potassium. The less potassium is contained in the marketable, alienated from the farm part of the crop and more in the non-marketable, remaining in the field or farm, including feed, the less potassium is alienated from the on-farm cycle. Thus, farm specialization determines the on-farm potassium balance.

Part of potassium can be lost from the root layer of soil as a result of infiltration: on light soils about 5%, on heavy soils – 2% of the amount applied with fertilizers. The intensity of leaching is influenced by the granulometric composition of the soil, water regime, fertilizer doses, biological features of crops.

Part of potassium can be lost through water and wind erosion. According to average data, this value is 4-8 kg/ha. It is considered that the expenditure items of losses from erosion are compensated by the receipt with seeds (about 2 kg/ha) and precipitation (2-6 kg/ha).

Some part of exchangeable potassium can pass in the soil in the fixed (non-exchangeable-absorbed) state, becoming inaccessible to plants. Subsoil layers are also involved in supplying potassium to plants, thus reducing the consumption of potassium in the arable layer. Thus, in experiments on sod-podzolic soils, sunflower and lupine on average about 32% of the total potassium removal was consumed from the subsoil horizons.

Sources

Yagodin B.A., Zhukov Y.P., Kobzarenko V.I. Agrochemistry / Edited by B.A. Yagodin. – Moscow: Kolos, 2002. – 584 p.: ill.

Evtefeev Y.V., Kazantsev G.M., Bases of agronomy: textbook. – M.: FORUM, 2013. – 368 p.: ill.

Agrochemistry. Textbook / V.G. Mineev, V.G. Sychev, G.P. Gamzikov et al. – Moscow: Publishing house of the All-Russian Scientific Research Institute named after D.N. Pryanishnikov, 2017. – 854 с.

Phosphorus in plant life

Phosphorus is a chemical element, known in several modifications: white, red, black and metallic, which are solid substances of corresponding color. It was first isolated by the Hamburg pharmacist Henning Brandt in 1669 from. Its role in plant life was first mentioned by Dendonald in 1795. The Swiss naturalist Sossure found calcium phosphate in the ash of all the plants he analyzed a little later.

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Phosphorus content in plant organisms

Phosphorus consumption by plants is less than nitrogen; it accounts for 0.2-1.0% of dry matter mass. The distribution of phosphorus in plants is the same as that of nitrogen: most of it is accumulated in reproductive organs and organs, where the processes of synthesis of organic matter occur intensively. Nitrogen and phosphorus in plant organisms are characterized by a fairly stable ratio in the yield.

The ratio of nitrogen and phosphorus for grain, roots, tubers, and hay is approximately 1:0.3, whereas between nitrogen and potassium it can vary from 1:0.6 to 1:1.4. In vegetation experiments, changing the ratio of nitrogen and phosphorus in nutrient media can achieve different ratios of these elements in plants, but under field conditions this ratio is stable due to the property of soil to regulate plant nutrition.

Table. Average ratio of the main nutrients in the yield of plants, %^[1]Yagodin B.A., Zhukov Yu.P., Kobzarenko V.I. Agrochemistry / Edited by B.A. Yagodin. - Moscow: Kolos, 2002. - 584 p.: ill.

Crop	N	P₂O₅	K₂O
Winter wheat, grain	100	32	60
Sugar beet, roots	100	29	106
Potatoes, tubers	100	30	140
Meadow clover, hay	100	31	901

Phosphorus in plants is represented in the mineral (5-15%) and organic (85-95%) forms. Mineral phosphorus compounds are phosphates of potassium, calcium, magnesium and ammonium. Organic compounds: nucleic acids, nucleoproteins and phosphatoproteins, adenosine phosphates, sucrose phosphates, phosphatides, phytins.

Nucleic acids, ribonucleic acid (RNA) and deoxyribonucleic acid (DNA), are high-molecular-weight compounds shaped as helical strands (25 A in diameter) and composed of combinations of nucleotides. Nucleotides include nitrogenous bases, sugars, and phosphoric acid. The carbohydrate component of RNA is ribose; in DNA it is deoxyribose.

Connecting together in various combinations, nucleotides form nucleic acids. A single nucleic acid molecule can have thousands of combinations of nucleotides joined together by acidic phosphoric acid residues. The combinations of nucleotides in nucleic acids form a kind of code that records the hereditary properties of an organism. Thanks to an almost infinite number of combinations of nucleotides, a huge variety of species of all living things is created.

DNA is the molecule that stores all the information about the genetic properties of the organism, while RNA is directly involved in the synthesis of proteins. Phosphorus in nucleic acids accounts for about 20%. Nucleic acid molecules are present in all plant tissues and organs, in every plant cell. In plant leaves and stems nucleic acids account for 0.1-1.0% of dry weight, more in young leaves and shoot growth points, less in old leaves and stems. The highest content of nucleic acids is in pollen, seed germ, and root tips.

Nucleic acids can form complexes with proteins – nucleinoproteins that are part of cell nuclei.

Phosphorus is involved in energy metabolism of plant cells due to adenosine phosphates that can release energy during hydrolysis. Adenosine monophosphate (AMP), adenosine diphosphate (ADP) and adenosine triphosphate (ATP) are distinguished by the number of phosphoric acid residues. An ATP molecule consists of a purine base (adenine), a sugar (ribose), and three orthophosphoric acid residues:

Energy-intensive phosphate macroergic bonds (wavy line) contain 50280 J of energy, and 31,425 J are released when they are broken. One acidic residue of phosphoric acid is lost, and ATP is converted to ADP. ADP can also participate in this circuit with the formation of AMP.

Adenosine phosphate compounds in the plant cell are energy accumulators that are used in many vital processes of the cell, such as biosynthesis of proteins, fats, carbohydrates, amino acids and other compounds. The formation of ATP in plants occurs through respiration processes. In addition to adenosine phosphate compounds, other macroergic compounds that include phosphorus are known.

Phosphatides, or phospholipids, are also found in any plant cell. They are esters of glycerol, high molecular weight fatty acids, and phosphoric acid. They are part of phospholipid membranes, regulate the permeability of cell organelles and plasmalemma. For example, the cytoplasm of plant cells contains lecithin – phosphatide, a fat-like substance derived from diglyceride-phosphoric acid.

Sucrose phosphates, or phosphorous esters of sugars, are present in plant tissues. More than ten such compounds are known. They are involved in plant respiration, conversion of simple carbohydrates into complex carbohydrates during photosynthesis, and mutual transformations. Phosphorylation is a reaction for the formation of saccharophosphates. The content of saccharophosphates in plants ranges from 0.1 to 1.0% of dry weight, depending on the age and nutritional conditions.

Phytin is calcium-magnesium salt of inositphosphoric acid. Phytin ranks first among other organophosphorus compounds by its content in plants.

Table. Forms of phosphorous compounds in plants, % P₂O₅ to dry matter^[2]Yagodin B.A., Zhukov Yu.P., Kobzarenko V.I. Agrochemistry / Edited by B.A. Yagodin. — М.: Колос, 2002. — 584 с.: ил.

Crop	Total phosphorus content	Including organic phosphorus					Mineral phosphorus	In % of total phosphorus
		lecithin	phytin	nucleoproteins	others	total		organic	mineral
Wheat, grain	0,860	0,032	0,609	0,130	-	0,771	0,089	89,6	10,4
Clover, hay	0,554	0,050	0,300	0,050	0,084	0,484	0,070	87,0	13,0

Phytin is contained in young organs and tissues of plants, most of all in seeds. For example, in the seeds of legumes and oilseeds it accounts for 1-2% of dry weight, in the seeds of cereals – 0.5-1.0%. In seeds, phytin is a reserve of phosphorus for germination and emergence of young seedlings.

Most of it in plants is concentrated in reproductive organs and young growing parts. Phosphorus accelerates the formation of the root system. Maximum consumption of phosphorus falls on the first phases of growth and development. Later it is easily reutilized, i.e. it moves from old tissues to young tissues and is reused.

Importance of phosphorus

Phosphorus contributes to:

economical consumption of moisture by plants;
increased drought tolerance;
improvement of carbohydrate metabolism, which contributes to sugar content of beets and starchiness of potatoes);
increasing the content of sugars in the bush nodes of winter crops and tissues of perennial grasses, which increases frost-resistance and winter-hardiness;
resistance to lodging of grain cereals;
resistance to diseases;
processes of flower fertilization, ovary formation, fruit formation and ripening.

Spinning crops produce a long thin and strong fiber.

Excess phosphorus leads to premature development and early fruiting, thereby reducing yields.

Lack of phosphorus causes slower growth and development of plants, reduced synthesis of protein and sugars, leaves form small and narrow, delayed flowering and fruit ripening. The lower leaves become dark green in color with a red-purple, purple, bluish or bronze tinge, the edges are curved upward.

There is a relationship between nitrogen and phosphorus nutrition of plants: lack of phosphorus slows down protein synthesis in tissues, while increasing nitrate content. This occurs most often in unbalanced plant nutrition, i.e. excessive doses of nitrogen.

Plants are most sensitive to phosphorus deficiency at a young age, when underdeveloped root system does not have sufficient absorption capacity. The deficit during this period can not be compensated later, even with optimal phosphorus nutrition.

The maximum absorption of phosphorus occurs during the period of intensive growth of vegetative mass.

Sources of phosphorus nutrition for plants

In natural conditions, the source of phosphorus nutrition of plants are salts of orthophosphoric acid – phosphates, as well as pyro-, poly- and metaphosphates after hydrolysis. The latter are not present in the soil, but can be a part of complex fertilizers.

Orthophosphoric acid during hydrolysis dissociates into anions H₂PO₄^–, HPO₄^2- and PO₄^3-. According to calculations by B.P. Nikolsky, under conditions of weakly acidic soil reaction, the most common and available is H₂PO₄^–, to a lesser extent – HPO₄^2-, PO₄^3- is almost not involved in the nutrition of most plants, except for lupin and buckwheat, to a lesser extent mustard, pea, melilot, sainfoin and hemp.

All salts of orthophosphoric acid and monovalent cations (NH₄⁺, Na⁺, K⁺) found in the soil are well soluble in water. Also soluble are one-substituted salts of divalent calcium cations Ca(H₂PO₄)₂ and magnesium Mg(H₂PO₄)₂. Two-substituted salts of calcium CaНРO₄ and magnesium MgНРO₄ are poorly soluble in water, but soluble in weak acids, including acidic root excretions and organic acids formed during the activity of microorganisms. Therefore, dihydroorthophosphates (one-substituted) and hydroorthophosphates (two-substituted) are a source of phosphorus for plants.

Table. Forms of phosphorous compounds in plants, % P₂O₅ to dry matter^[3]Yagodin B.A., Zhukov Yu.P., Kobzarenko V.I. Agrochemistry / Edited by B.A. Yagodin. — М.: Колос, 2002. — 584 с.: ил.

Acid, anion	рН
	5	6	7	8
H₃PO₄	0,10	0,01	-	-
H₂PO₄^-	97,99	83,68	33,90	4,88
HPO₄^2-	1,91	16,32	66,10	95,12
PO₄^3-	-	-	-	0,01

Three-substituted phosphates (orthophosphates) of divalent cations are insoluble in water and inaccessible to most. However, freshly deposited tri-substituted calcium phosphate, formed from mono- and divalent phosphates during chemical absorption by the soil, is slightly better absorbed by plants in its amorphous state. As they age, these amorphous triphosphates change to crystalline forms and lose their availability to plants.

Trivalent cations of orthophosphoric acid [AlPO₄, Al(OH)₃PO₄, FePO₄, Fe₂(OH)₃PO₄, etc.] are not available to plants, account for most of the mineral phosphates of acidic soils.

As a source of phosphorus nutrition of plants is phosphate in the exchange-absorbed (adsorbed) soil colloids state. These anions are displaced by anions of mineral and organic acids (citric, malic, oxalic acids). The soil in the solid phase-solution system contains anions in sufficient quantities. In the process of breathing, roots emit carbon dioxide, which acidifies the reaction and forms hydrogen carbonate ions when dissolved. The latter displace the adsorbed phosphorus in solution from the PPC.

It has been experimentally confirmed that exchange-absorbed phosphoric acid anions are close to water-soluble phosphates in terms of availability to plants. However, the amount of the latter in the soil is small, so adsorbed phosphates are of great importance in the balance of phosphorus nutrition of plants.

Some plants have the ability to assimilate phosphate-ion organic compounds, such as phytin and glycerophosphates, due to root excretions containing the enzyme phosphatase. Under the action of phosphatase, the phosphoric acid anion is detached from the organic compounds and absorbed by the plant. Such plants include peas, corn, and beans. Phosphatase activity increases under conditions of phosphorus deficiency.

During phylogenesis, plants have adapted to nutrition from solutions with very low concentrations. In the studies of M.K. Domontovich, all experimental plants (oats, corn, wheat, peas, mustard and buckwheat) could absorb phosphorus from solutions with concentrations from 0.01 to 0.03 mg P₂O₅ per 1 L. It is generally accepted that the optimal concentration of phosphorus for plant nutrition is 1 mg/L.

Phosphorus absorbed by the roots is quickly included in the synthesis of complex organic compounds directly in the roots. In experiments with pumpkin, 30% of labeled phosphorus ³²P was found in the composition of organic compounds after 30 minutes of absorption, and after 3-5 minutes – 70% of absorbed phosphorus. Phosphorus is primarily consumed for nucleotide synthesis. To transport phosphorus to other parts of the plant, phosphorus is again transformed into mineral compounds.

Phosphorus cycle and balance in agriculture

In natural biocenoses, phosphorus has no sources of recharge in the soil, while at the same time, its natural reserves in soils are significant. According to A.V. Sokolov, the one-meter layer of soil contains from 10 to 35 t/ha of various phosphorus compounds. Due to the fact that the roots of many field crops penetrate to a depth of 0.9 to 2.8 m, and perennial grasses – to 3-5 m, the mobile forms can be used by plants. The consumption of P₂O₅ by plants in subsoil horizons is experimentally confirmed, which can account for up to 30% of the total removal with the crop.

Phosphorus removal with agricultural products averages 25-40 kg/ha per year. Thus, the natural reserves in the soil significantly exceed the removal.

In natural biocenoses with their characteristic closed cycle of nutrients, phosphorus slowly accumulates in the upper layers of the soil due to its redistribution from the activities of plants.

Table. Content of gross phosphorus and organic phosphate in various soils, mg/100 g (according to generalized data of Ginzburg)

Soddy-medium-podzolic loamy soils			Gray forest loamy soils
Horizon	Gross phosphorus	Phosphorus organic	Horizon	Gross phosphorus	Phosphorus organic
A₁	159,7	70,6	A_пах	156,3	59,8
A₂	83,7	26,8	A₂	125,5	29,2
A₂B	78,6	23,3	A₂B	104,1	27,7
B	107,5	13,4	B	108,6	16,5
C	100,9	8,6	C	110,5	5,7

A feature of the phosphorus cycle in agrocenoses is that most of it is concentrated in the harvest, for example, up to 2/3 of all phosphorus absorbed by plants is concentrated in grain, the remaining 1/3 in the non-commodity part – straw. Given that only a small part of the grain remains in the farm, the alienation of phosphorus from farms is significant. In addition, phosphorus is also contained in livestock products, which should also be taken into account in the external balance.

Table. The content of phosphorus in the harvest^[4]Fundamentals of agronomy: textbook / Yu.V. Evtefeev, G.M. Kazantsev. - MOSCOW: FORUM, 2013. - 368 p.: ill..

Crop	Type of commercial products	Removal P₂O₅ per 100 kg of marketable crop with the corresponding amount of non-marketable part, kg
Winter rye, oats, barley	Grain	1,0
Spring wheat	Grain	1,0 - 1,2
Corn	Grain	0,7 - 0,9
Peas	Grain	1,5
Sunflower	Seeds	2,6
Flax fiber	Fiber	До 2,6
Hemp	Fiber	До 6,2
Tomatoes	Fruits	До 0,11
Sugar beet	Roots	До 0,18
Potatoes	Tubers	До 0,15
Meadow clover	Hay	До 0,55

In agrocenoses, the phosphorus cycle is relatively easier than the nitrogen cycle.

Phosphorus losses can be associated with soil erosion in the form of losses of the solid part with wind erosion and runoff with water erosion. On average, losses can be up to 11 kg/ha per year. On soils of medium and heavy granulometric composition infiltration as a rule does not exceed 1 kg/ha per year, on light and peaty soils – up to 3-5 kg/ha.

An insignificant amount of phosphorus enters the soil with seeds of plants, atmospheric precipitation and dust.

For these reasons, compensation of expenditure items of the balance of phosphorus in agriculture is possible through the use of organic and mineral fertilizers.

In the 70-80s, a positive balance of phosphorus was formed in the USSR: in many regions there was an increase in its content in the soil. Thus, in the Central region of the Nonchernozem zone the amount of mobile phosphate in the soil increased from 5.3 to 12.5 mg/100 g, in the Moscow region – from 6.4 to 20.6 mg/100 g.

Sources

Yagodin B.A., Zhukov Y.P., Kobzarenko V.I. Agrochemistry / Edited by B.A. Yagodin. – Moscow: Kolos, 2002. – 584 p.: ill.

Evtefeev Y.V., Kazantsev G.M., Bases of agronomy: textbook. – M.: FORUM, 2013. – 368 p.: ill.

Nitrogen in plant life

Nitrogen is a chemical element, an inert gas without color or odor, discovered by French chemist Lavoisier in the second half of the 18th century, and is the main component of atmospheric air (78.08%). The name means “non-living,” as it does not support combustion or respiration. However, further research has shown the enormous role of nitrogen in plant life and the entire organic world.

Nitrogen is part of:

proteins, peptides, and amino acids, which are an integral part of the protoplasm and nucleus of plant cells;
nucleic acids (DNA and RNA), carriers of hereditary properties of living organisms and involved in metabolism;
chlorophyll molecules;
enzymes;
phosphatides;
hormones;
most vitamins.

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Nitrogen nutrition of plants

All enzymes are protein substances, so when plants are insufficiently supplied with nitrogen, enzyme synthesis slows down, which leads to disturbances in the processes of biosynthesis, metabolism, eventually reducing yields.

Regulating nitrogen nutrition of plants, you can influence the yield of crops, taking into account other factors of life. Maximum yield is achieved when plants are sufficiently provided with all the conditions of their growth. Academician D.N. Pryanishnikov wrote that the whole history of farming in Western Europe shows that the main condition determining the average yield height at different epochs was the degree of provision of agricultural plants with nitrogen.

Optimal nitrogen nutrition promotes the synthesis of protein substances, plants form powerful stems and leaves with intense green coloration. Powerful assimilation apparatus allows plants to accumulate more photosynthesis products, increasing yield and, as a rule, its quality.

One-sided excessive nitrogen feeding, especially in the second half of the growing season, leads to delayed maturation of plants; a large vegetative mass is formed, but the yield of reproductive organs does not have time to form.

Lack of nitrogen leads to a strong slowdown in plant growth. First of all, it affects the development of vegetative mass: leaves become small, light green, turn yellow earlier, stems are thin, weakly branching. The formation of reproductive organs decreases, and the yield decreases sharply. Nitrogen starvation in cereal crops leads to weakening of tillering, the number of grains in an ear decreases, and grain protein content decreases.

Nitrogen content in plants

In terms of chemical composition, nitrogen in plants accounts for 0.5-5.0% of the air-dry weight, the main amount is in the seeds. Protein content clearly correlates with the amount of nitrogen in plants. Nitrogen content is lower in vegetative organs: in legume straw 1.0-1.4%, in cereal straw 0.45-0.65%. Even less nitrogen is accumulated in root and tuber crops and vegetable crops: potatoes (tubers) 0.32%, sugar beets (roots) 0.24%, cabbage 0.33% raw matter.

Nitrogen content in plants depends on age, soil and climatic conditions, nutritional regime, in particular the provision with nutrients.

Table. Content of protein and nitrogen in the seeds of various crops, %^[1] Yagodin B.A., Zhukov Y.P., Kobzarenko V.I. Agrochemistry / Edited by B.A. Yagodin. - Moscow: Kolos, 2002. - 584 p.: ill.

Crop	Protein	Nitrogen
Soybean	29	5,8
Peas	20	4,5
Wheat	14	2,5
Rice	7	1,2

The nitrogen content in young vegetative organs is higher. As they age, the nitrogenous substances migrate to the emerging leaves and shoots.

Table. Nitrogen content in the vegetative mass of cereal crops by phases of development, % on air-dry substance^[2]Yagodin B.A., Zhukov Yu.P., Kobzarenko V.I. Agrochemistry/ Edited by B.A. Yagodin. - Moscow: Kolos, 2002. - 584 p.: ill.

Crop	Plant development phase
	tillering	booting	earing	blossom
Winter wheat	5,0-5,4	3,0-4,5	2,1-2,5	2,0-2,4
Spring wheat	4,5-5,5	3,0-4,4	2,5-3,0	1,8-2,5
Oats	5,5-5,9	2,9-3,9	2,2	1,3-1,7

Input and transformation of nitrogen into protein substances

The rate of accumulation of organic matter by plants outstrips the intake of nitrogen and other nutrients. There is a “growth dilution” of the content of nutrients. At ripening, there is a pronounced movement of nitrogen to reproductive organs, where it is accumulated in the form of spare proteins.

Nitrogen enters plants mainly in nitrate and ammonium forms, but some soluble organic compounds, such as urea, amino acids, and asparagine, are also able to assimilate.

Of the nitrogen compounds from the soil to plants, only ammonia is directly used for amino acid synthesis. Nitrates and nitrites are included in amino acid synthesis only after reduction in plant tissues.

The reduction of nitrates to ammonia begins already in the roots with the help of flavin metalloenzymes:

In excess, some of the nitrates enter the leaves unchanged, where they are reduced according to the same scheme.

The formation of amino acids (amination) occurs as a result of the interaction of ammonia with keto acids: pyruvic acid, oxalic acid, ketoglutaric acid, etc., formed in the process of carbohydrate oxidation. Amination is regulated by enzymes. Thus, when pyruvic acid interacts with ammonia, alanine is formed:

CH₃—CO—COOH + NH₃ + 2H → CH₃—CHNH₂—COOH + H₂O

Similarly, the interaction of ammonia with oxalic acetic acid leads to the formation of asparagic acid (СООН-СН₂-СНNН₂-СООH), with ketoglutaric acid – glutamic acid (СООН-СН₂-СН₂-СНNН₂-СООH).

Amino acids contain nitrogen in the form of an amino group (-NH₂). The processes of amino acid formation occur in the roots and in the above-ground parts of plants.

Experiments using labeled atoms show that just a few minutes after feeding plants with ammonia fertilizers, in tissues amino acids synthesized from the ammonia added to the dressing can be detected. The first amino acid formed is alanine, followed by asparagine and glutamic acids.

Nitrate nitrogen can accumulate in plants in large quantities without harming them. Ammonia in free form in tissues is contained in insignificant amounts. Its accumulation, especially with a lack of carbohydrates, leads to ammonia poisoning of plants.

However, plants have the ability to bind excess free ammonia: its part interacts with synthesized asparagine and glutamine amino acids, forming the corresponding amides – asparagine and glutamine:

СООН-СН₂-СНNН₂-СООH + NH₃ → CONH₂-CH₂-CHNH₂-COOH + H₂O

СООН-СН₂-СН₂-СНNН₂-СООH + NH₃ → CONH₂-CH₂-CH₂-CHNH₂-COOH + H₂O

The formation of asparagine and glutamine allows plants to protect themselves from ammonia poisoning and create a reserve of ammonia; in addition, amides participate in protein synthesis.

In 1937, biochemists A.E. Braunstein and M.G. Kritzman discovered the reamination reaction, which consists in the transfer of an amino group from an amino acid to a keto acid to form other amino and keto acids. The reaction is catalyzed by the enzymes transaminases or aminoferases.

For example, the addition of an amine group from glutamic acid to pyruvic acid (СН₃-СO-СООH) leads to the formation of alanine (CH₃-CHNH₂-COOH) and ketoglutaric acid (COOH-CO-CH₂-CH₂-COOH):

СН₃-СO-СООH + COOH-CH₂-CH₂-CHNH₂-COOH → COOH-CO-CH₂-CH₂-COOH + CH₃-CHNH₂-COOH

A significant number of amino acids are synthesized through overamination. In plants, glutamic and asparagic acids are the most easily overamined.

Amino acids are the components of polypeptides and proteins. 20 amino acids, asparagine and glutamine participate in the construction of protein molecules in different ratios and spatial orientations, which leads to a huge variety of proteins. More than 90 amino acids are currently known, about 70 of them are present in plants in a free form and are not included in proteins.

Plants synthesize amino acids that cannot be formed in humans and higher animals, but are essential for their lives. These include lysine, histidine, phenylalanine, tryptophan, valine, leucine, isoleucine, threonine, and methionine.

Non-protein organic nitrogen in plants accounts for 20-26% of the total amount. Under unfavorable conditions, such as potassium deficiency or insufficient light, the amount of non-protein nitrogenous compounds increases.

In plant tissues, proteins are in dynamic equilibrium with non-protein nitrogenous compounds. Simultaneously with the synthesis of proteins and amino acids, there is a process of their decomposition: the detachment of the amino group from the amino acid to form keto acids and ammonia. This process is called deamination. The keto acids released are used by plants to synthesize carbohydrates, fats, and other substances; ammonia reenters the amination reaction of other keto acids to form new amino acids, with an excess of asparagine and glutamine.

Thus, the entire cycle of nitrogenous compound transformations in plants begins (amination) and ends (deamination) with ammonia.

“Ammonia is the alpha and omega in the metabolism of nitrogenous substances in plants.”
D.N. Pryanishnikov.

A large number of protein compounds are synthesized during the entire growing season of a plant, and the exchange of nitrogenous substances occurs differently during different periods of growth.

During germination of seeds, tubers, and bulbs, there is a breakdown of stored proteins. Decomposition products are used to synthesize amino acids, amides, and proteins in the tissues of seedlings before they reach the soil surface. Then, as the root system and leaf apparatus form, protein synthesis proceeds at the expense of mineral nitrogen absorbed from the soil.

In young plants, protein synthesis predominates. As plants age, protein breakdown begins to predominate. Decomposition products from aging organs migrate to young, intensively growing organs, where they are used to synthesize new proteins at growth points. As plants mature and reproductive organs form, proteins decompose in vegetative parts, and the decomposition products migrate to reproductive organs, where they are used to form spare proteins. By this point, the nitrogen supply to plants from the soil slows down significantly or stops completely.

Peculiarities of ammonium and nitrate nutrition of plants

At the end of the 19th century, the leading role in agronomic science was played by the theory of nitrate nutrition of plants, and the role of ammonia as a source of mineral nutrition was denied.

The role of ammonia as a source of mineral nutrition was denied:

experiments in water crops: good plant development was observed against the background of nitrate salts, against the background of ammonium salts the development was poor;
discovery of nitrification in the soil; this was the basis for believing that when ammonium fertilizers are put into the soil they are converted into the nitrate form that is assimilated by plants;
application of Chilean nitrate (NaNO3) significantly increased crop yields.

However, at the end of the century, P.S. Kossovich showed in experiments with sterile crops that plants could also assimilate ammonia nitrogen without oxidation into the nitrate form. The same conclusion was reached by French researcher Mazet in 1900. After that, the conditions and peculiarities of feeding on ammonium and nitrate forms of nitrogen were studied. Fundamental research on this issue was carried out by D.N. Pryanishnikov. He showed that the efficiency of using various forms of nitrogen depends on the reaction of the environment: in neutral reaction ammonium nitrogen is better absorbed, in acid reaction – nitrate nitrogen.

During the initial phases of growth, biological features are of significant importance. When germinating seeds with little carbohydrate reserves, such as in sugar beets, and thus organic keto acids, the excessive supply of ammonium to plants has a negative effect. Ammonium nitrogen does not have time to be used for amino acid synthesis, it accumulates in plant tissues and causes plant poisoning. In this case, nitrate forms of nitrogen fertilizers are used, as they also accumulate in plant tissues, but do no harm. Seeds and crops with large carbohydrate reserves, such as potatoes, use ammonium nitrogen for amino acid synthesis without restriction. Therefore, for these crops, the ammonium and nitrate forms are of equal value in the initial stages of growth.

The absorption of nitrate and ammonium nitrogen is influenced by the supply of other nutritional elements. The increased content of potassium, calcium and magnesium in the soil promotes the absorption of ammonium. With nitrate nutrition, the provision of plants with phosphorus and molybdenum is important. Molybdenum deficiency leads to a delay in the reduction of nitrates to ammonia and contributes to the accumulation of nitrates in plant tissues.

Considering that the ammonium form of nitrogen when entering plants can be used immediately for the synthesis of amino acids, while the nitrate form is used only after reduction to ammonia, ammonium is more energy-saving form.

Nitrogen cycling and balance in agriculture

The mineral nitrogen compounds in the soil are mobile. Their stock and transformation are the result of numerous, often interdependent physical, physical-chemical and biological processes of nitrogen cycle.

In virgin soils of natural biocenoses there is a closed cycle of nitrogen and other biogenic elements cycle. It includes incoming items:

nitrogen inputs with plant fallout and root residues;
excrement and animal remains;
biological fixation of atmospheric nitrogen by microorganisms;
inputs with atmospheric precipitation (formation of NO₃^– and NH₄⁺ by lightning discharges and industrial emissions).

Consumption items:

nitrogen intake by plants;
infiltration;
denitrification;
losses from water and wind erosion;
nitrogen immobilization – conversion of mineral nitrogen into organic nitrogen (in this case nitrogen is not lost, but passes into a form inaccessible to plants).

The ratio of inputs and outputs in the nitrogen cycle constitutes the balance of this element. If consumption exceeds accumulation, it is negative; if intake exceeds expenditure, it is positive; if balance parts are equal, it is balanced, or zero.

Natural biocenoses are usually characterized by balanced balance of nitrogen and other biogenic elements. Its losses from leaching and denitrification are compensated by its inflow with precipitation and biological nitrogen fixation.

As a result of plowing soils, the nitrogen regime undergoes changes: the expenditure parts of the balance increase sharply. A large amount of nitrogen is alienated with agricultural products. Intensive tillage leads to increased mineralization of organic matter, increased losses due to infiltration, denitrification and erosion.

Under conditions of sufficient and excessive moisture, irrigation, especially on light soils, nitrate consumption increases for leaching together from the root-containing layer into drainage water. Loamy and clayey soils rich in humus retain water better, so nitrate leaching is insignificant – up to 3-5 kg/ha of nitrate nitrogen per year. On light soils of granulometric composition, especially in the fallow field, nitrate nitrogen losses from leaching reach 20-30 kg/ha per year.

Losses of gaseous forms of nitrogen from denitrification are also significant. The process of denitrification takes place under anaerobic conditions. It is logical to assume that plowing eliminates these conditions, however, loosened soil consists of separate structural aggregates (clumps) of different sizes. Anaerobic conditions are created within each soil aggregate, and the more intense the more nitrates not absorbed by plants are accumulated.

Sources

Yagodin B.A., Zhukov Y.P., Kobzarenko V.I. Agrochemistry / Edited by B.A. Yagodin. – Moscow: Kolos, 2002. – 584 p.: ill.

Mineral nutrition of plants

Mineral nutrition of plants, also plant root nutrition, is the process of absorption of water and nutrients from the soil by the root system of a plant.

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Chemical composition of plants
Air nutrition of plants (photosynthesis)
Mineral (root) nutrition of plants (Русский Español)
Nitrogen in plant life
Phosphorus in plant life
Potassium in plant life
Diagnosis of plant nutrition

History

The theory of mineral nutrition of plants was confirmed in 1858 in experiments, when under the conditions of artificial nutrient medium of water culture for the first time it was possible to bring the plant to full maturity. Later, a complete nutrient mixture was proposed for growing plants in sandy crops.

About the nature of the absorption of substances by living cells certain statements were made as early as Dutroche in 1837. He believed that the penetration of water and dissolved substances into the cell occurs by diffusion through porous cytoplasmic membranes.

Sachs used the concept of “accumulative diffusion” because he believed that chemical processes occurring in the cell upset the equilibrium of concentrations of substances inside the cells and in the surrounding solution.

Proponents of the diffusion-osmotic theory include Pfeffer, De Fries, Meyer, and other scientists. According to this theory, nutrients are sucked into plants through the root system along with water, and water constantly evaporates. Consequently, the supply of nutrients is in direct correlation with the intensity of water evaporation by plants. However, the available data on the patterns of nutrient inflow did not fit into the framework of the ideas of the diffusion-osmosis theory.

K.A. Timiryazev also noted the lack of certain dependence between water and nutrients inflow to plants. He wrote that plants do not need those enormous amounts of water that they evaporate in order to nourish themselves. This position was developed in his works by D.A. Sabinin. He showed that with small amounts of substances in the nutrient solution they are well concentrated in the plant paste.

At the end of the 19th century Overton put forward the lipoid theory, according to which the penetration of substances into the cell occurs due to their dissolution in the lipid components of cytoplasmic membranes. He noted a correlation between the rate of penetration of basic aniline dyes into plant cells and their solubility in lipids.

Traube and Ruland proposed the ultrafiltration theory according to which penetration of nutrients through the cytoplasmic membrane is determined by the size of pores and molecules. Dravet established the dependence of the penetration of acidic dyes studied on the size of their molecules. But such facts as the absorption of amino acids, phytin and other organic substances with large molecule sizes could not be explained by the proposed theory.

In the early twentieth century. Devaux established the ability of plant cells to rapidly bind cations from highly dilute solutions, which contributed to the development of the adsorption theory. He showed that bound cations are displaced back from tissues by other cations due to mutual equivalent exchange, and the intensity of the process depends on the concentration and time.

Many studies by D.A. Sabinin and other scientists trace the connection of substances absorption with activity of cells, the role of root system in the process of substances absorption is shown. Concentration of substances in passococcus depends on the level of provision of plants with nutrients, species characteristics and age of plants. Different physiological activity of cells and tissues causes differences in chemical composition and differences in electrical properties.

The rate of tissue metabolism determines the level of absorption of substances. Stuard, Lundegaard, Bjurström and other scientists established the connection of respiratory processes in tissues with the process of absorption of mineral salt ions. Hoagland and Breuer have shown that increase in absorption of substances by plant cells and tissues is observed under good aeration of nutrient solution, introduction of glucose, increase in temperature and other conditions contributing to activation of respiratory processes.

D.A. Sabinin proved the relationship between plant nutrition and the formation and development of individual organs.

The above-mentioned theories had their significance in the development of views on the process of substance entry into plants. They, in essence, simplistically reflect the different sides of the influx of elements of nutrition. The modern views on the mineral nutrient inputs include a number of basic concepts from the previously proposed theories.

Root

The root is a specialized part of the plant that performs the functions of fixation of the plant in the soil and absorption of nutrients: primary assimilation, inclusion in metabolism; distribution and transport of water and minerals. Numerous biosynthetic processes take place in the root and some special functions are performed.

The power and character of development of the root system of a plant determines the ability to assimilate nutrients, most of which are absorbed by the young, growing parts of the root.

The growth zone of the root is the division zone, or meristem. In the meristem, cells are not differentiated into tissues. The stretching zone and root hairs zone are distinguished, having developed elements of xylem and phloem and epidermis with root hairs. The most intensive uptake occurs by cells of root hairs zone.

The root system of field crops has a huge absorptive surface, which reaches its maximum, including the active, as a rule, during the flowering period.

Table. Changes in the surface area of roots of spring wheat^[1] Yagodin B.A., Zhukov Yu.P., Kobzarenko V.I. Agrochemistry / Edited by B.A. Yagodin. - Moscow: Kolos, 2002. - 584 p.: ill.

Development phase	Root surface of one plant, m²		Ratio of active surface to inactive surface
	общая	деятельная
Tillering	9,60	4,91	1,05
Booting	29,39	10,81	0,59
Beginning of blooming	36,73	17,07	0,86
End of blooming	40,09	24,76	1,30
Waxy ripeness	30,86	14,80	0,92

Root hairs greatly increase the absorptive surface of the root. It is generally accepted that the root hairs zone is the absorption zone. However, nutrient uptake can occur at a distance of 0.5 m along the root and 1 m from the root tip, that is, where root hairs are absent.

Russell and Clarkson showed that phosphate movement in barley from an area further than 40 cm from the root tip remains the same as in the root hairs area.

In Clarkson’s experiment, it was shown that root hairs have no special absorption properties. Barley grown in an aqueous culture with strong aeration of the solution does not form root hairs on the roots, but the absorption of ions is quite intense. Probably the main role of root hairs is to maximize the root surface to supply plants primarily with phosphorus.

The movement of phosphorus in the soil is very slow, but the rate at which it is absorbed by the plant is high. In a short period of time, the roots absorb the phosphorus around them and increase their surface area to absorb the next amounts of this element. Other ions are more mobile and root hairs are less important for their absorption. In the soil, due to chemotropism, the root grows and advances toward a higher concentration of nutrients.

Depending on the biological characteristics of the crop and growing conditions, the root system develops different capacities. On poor soils and in arid conditions, plants produce a relatively large mass of roots in search of food and water. Application of fertilizers reduces the ratio of the mass of the roots and the above-ground part of the plant, but increases the total mass and depth of root system spreading.

Root cell membrane

The membrane determines the ability of the root cell to selectively absorb ions. Reactions of metabolism and energy flow on its surface. The contact of a cell with the soil environment is carried out by means of cytoplasmic membrane, or plasmalemma.

The cell membrane consists of two layers of phospholipids with hydrophobic ends. Molecules of carrier proteins are embedded in certain areas of the phospholipid layer. The mosaic structure of the cytoplasmic membrane consists of positively and negatively charged sites through which cations and anions from the external environment are adsorbed (Bergelson, 1975).

Phospholipids form several types of liquid-crystal structures.

Phospholipid molecules consist of polar “heads” – hydrophilic groups, and non-polar “tails” – long hydrocarbon hydrophobic chains. Phospholipids are poorly soluble in polar solvents (water) due to non-polar “tails” and in non-polar media (oil) due to polar “heads”. The monomolecular layer at the interface creates a limitation of permeability of substances (Bergelson, 1975).

An example of a phospholipid molecule is phosphatidylcholine.

The thickness of the bilayer membrane is 10-12 nm. The presence of charged “heads” in the lipid micelle of the membrane creates a potential difference at the interface between the lipid and water, so that positive ions are attracted to the membrane surface and negative ions are repelled.

Membrane proteins are used to build pores and channel walls in the membrane. In the cytoplasmic membrane, absorbed ions are involved in metabolic reactions. Some of the membrane proteins have enzymatic activity. Membrane proteins in the form of protein globules have a hydrophobic anchor, due to which molecules are able to interact with lipids. Proteins can be bound to polar membrane lipid heads via bridges formed by divalent metal cations.

The protein globule moves in the liquid bilayer of the membrane. The molecules of the dissolved substance move continuously in the solution.

If the membrane separates two solutions with different concentrations, then, depending on its permeability, the solvent or dissolved substance will pass through by diffusion and a concentration equalization will occur. At the beginning of this process, pure matter moves across the membrane (I) and then a dynamic equilibrium is established (Clarkson, 1978).

The figure below shows how the net movement of water molecules (light circles) first occurs in chamber A, since the membrane is impermeable to the dissolved substance. Once the steady state (II) is reached, a certain difference in solution levels between chambers A and B is established. The value of the level difference is proportional to the initial concentration difference of the solute (osmotic potential), (Clarkson, 1978).

Passage of nutrients through the membranes of root cells by the mechanism of passive transport is possible in the presence of hydrophilic openings (pores) in the membranes. Non-specific passive transport of ions and molecules occurs by diffusion through the membrane pores for hydrophilic substances, whereas for neutral molecules, by dissolution of the penetrating substance in the membrane (for substances soluble in oils).

According to calculations, the pore area in the membrane is no more than 0.1% of its surface.

In the case of electrically charged particles, the process of passing through the membrane depends on the concentration difference and the electric potential.

Root nutrition of plants

At present, the composition, quantity, and forms of compounds in the form of which minerals enter the plants are well enough defined. This information has been obtained by numerous experiments in growing plants in aquatic and sandy crops and in hydroponics, where nutrient mixtures of mineral salts are used to produce record high yields.

Plants can also use organic compounds for nutrition, such as amino acids, organic acids, sugars, and sucrose phosphates. Nitrogen from amino acids in plants undergoes deamination, and the released ammonia is included in the processes of nitrogen nutrition.

Table. The main forms of consumption of nutrients by plants (Kidin, 2008)^[2]Agrochemistry. Textbook / V.G. Mineev, V.G. Sychev, G.P. Gamzikov et al; ed. by V.G. Mineev. - M.: Publishing house of the All-Russian Scientific Research Institute named after D.N. Pryanishnikov, … Continue reading

Element	Main ions	Element	Main ions
Nitrogen	NH₄⁺, NO₃^-, NO₂^-	Manganese	Mn²⁺, MnO₄^-, Mn⁴⁺
Aluminum	Al³⁺, Al(H₂O)₆³⁺	Copper	Cu²⁺, Cu⁺
Bor	H₂BO₃^-, H₄BO₇^2-	Molybdenum	MoO₄^2-
Iron	Fe²⁺, Fe³⁺, Fe(H₂O)₆³⁺	Sodium	Na⁺
Potassium	K⁺	Selenium	SeO₄^2-
Calcium	Ca²⁺	Sulfur	SO₄^2-, SO₂
Cobalt	Co²⁺, Co³⁺	Phosphorus	H₂PO₄^-, HPO₄^2-
Silicon	H₃SiO₄^-, HSiO₃^-	Zinc	Zn²⁺
Magnesium	Mg²⁺

By the end of the XX century, the study of the process of mineral nutrition of plants began to be studied not only in conditions of deficiency of one or another nutrient, but also under increased supply. This is due to the need to determine the conditions of mineral nutrition, under which the potential of plant productivity is achieved.

A number of characteristics of agronomic practices depend on the function of different parts of the root: the depth of tillage and fertilization. Thus, the root of cereal crops has three main zones:

zone of growth and extension with a length of 1.5 mm, due to the division of cells in the apical meristem the growth of the root occurs;
root hairs zone, or suction zone, which has root hairs up to 1 mm long, the length of the zone itself is 1-2 cm;
lateral root zone.

In the field, the root hairs zone, also called the absorbing zone, is of primary importance in the mineral nutrition of plants.

Using labeled carbon ¹⁴C, it was found that carbon dioxide is transferred from roots to leaves in 10-15 min. The transfer rate of photosynthetic products from leaves to the root system is 40-100 cm/h. The speed of transfer of nutrients, including those applied with fertilizer, from the root to the parts of the plant is even higher. Thus, when barley roots were immersed in a solution containing labeled phosphorus ³²P, its appearance in leaves was recorded after 5 min, and from roots of fourteen-day-old corn it reached leaves after 2 min. Similar results were observed in experiments with other crops.

The rate of nutrient uptake varies with root age. For example, as corn plants age from 20 to 80 days, the rate of absorption of nitrogen, phosphorus, potassium, calcium, and magnesium decreases tenfold.

A study of the function of germinal and knot roots made it possible to determine the important role of the tiller node in the distribution of water and minerals. The tiller node consists of loose, porous parenchyma tissue that facilitates the easy movement of nutrients. The roots of the tillering node are characterized by high absorption capacity and play a significant role in plant nutrition than the primary (germinal) roots. Their role especially increases during tillering, accompanied by increased branching of knot roots.

Depending on the nature of the energy expended, the absorption of nutrients can be active and passive.

Active absorption occurs with the expenditure of metabolic energy. Passive absorption occurs through thermal diffusion energy or solar energy.

In all movements and transformations of nutrients, the protein plasma of the cell, which has a dual nature, basoid and acidoid, plays a role. In protein plasma, acidoid and bazoid parts are arranged mosaically, so that during the movement of nutrients from cell to cell, there is a gradual exchange and interaction of cations and anions with positively charged (bazoid) and negatively charged (acidoid) parts of protein plasma molecules.

Active uptake and movement of ions occurs through a system involving protoplasts of cells connected by plasmodesms. In the case of passive absorption, ions reaching the root surface by mass overflow or diffusion get into the free space of the root and move around the plant with the transpiration current. Plant cells have loose cellulose membranes connected with each other and forming a continuous system – apoplast. Through this system, the transpiration of water by the leaves causes the movement of substances dissolved in it.

Ions in free space can also move by diffusion – from a zone with a higher concentration to a lower one. The process is relatively slow, e.g. fluorescein dye diffuses in 1 h by 5 mm, in 24 h – by 25 mm, and in a year – by 50 cm. For this reason, the importance of diffusion in the movement of soluble substances is not significant.

Ions in contact with the root are adsorbed by cell walls. The process of adsorption is an exchange process. The high intensity of exchange, the rate of ingress and movement of substances is explained by the adsorption exchange between the root system and soil colloids, as well as the soil solution.

Thanks to well-developed root hairs, which enmesh the soil and its colloidal particles in a dense network, a huge area of contact between the root system and the soil is created, which determines the efficiency of adsorption of nutrients. Thus, the number of root hairs, the lifetime of which is several days, reaches 425 per 1 mm² of root surface in the maize root, on the average for most crops – 200-500.

The exchange process of adsorption consists in the ionic exchange of nutrients in the form of ions such as K⁺, Ca²⁺, Mg²⁺, NH₄⁺, NO₃^–, H₂PO₄^–, SO₄^2-, for ions H⁺, HCO₃^–. The latter are excreted by the root surface during respiration. Root system is capable of releasing large amounts of carbon dioxide, so mustard plants in 85 days of vegetation emit 2.25 tons of CO₂ from 1 hectare.

Emerging carbon dioxide reacts with water to form carbonic acid:

CO₂ + H₂O → H⁺ + HCO₃^–.

As a result of the respiratory process, H⁺ and HCO₃^– ions appear on the surface of root hairs. The former are exchanged for soil cations K⁺, Ca²⁺, Mg²⁺, NH₄⁺, while NO₃^–, H₂PO₄^–, SO₄^2- anions are exchanged and displaced into solution by HCO₃^– anion.

On the surface of the root hair, cations and anions interact with the basoid (basic) and acidoid (acidic) parts of the cell plasma and are included in the transfer process in the plant.

The exchange absorption of nutrient ions by the root system can occur not only for H⁺ and HCO₃^– ions, but also for ions of organic and mineral compounds excreted by the roots. For example, plant roots can secrete citric acid, malic acid, oxalic acid, and other organic acids that dissociate into H+ and organic anions.

Closer contact of the root system with the soil absorbing complex contributes to the intensification of absorption processes. The best feeding conditions are created when there are enough ions in the soil solution and in the adsorption-bound state.

Adsorption absorption of nutrients is characterized by the absorption capacity of cations and anions of root hairs as well as of soil colloids. Root absorption capacity depends on plant species, feeding conditions and other factors. Thus, the capacity of cationic absorption per 100 g of root dry matter for legumes is 40-60 mmol, for potatoes and tomatoes, 35-38 mmol, and for cereals, 9-29 mmol.

Root cationic uptake capacity increases with nitrogen nutrition, which seems to be related to protein synthesis. Hydrogen ions (H⁺) account for the major part of cationic absorption capacity.

Anionic absorption is similar to cationic absorption. Hydrocarbonate-ion HCO₃^– plays the main role. In many crops, anion exchange quantitatively exceeds cation exchange, which indicates the presence of more colloids of active part of roots with positive charge.

The main barrier for ion absorption is the surface membrane, or plasmalemma. The ability of plants to absorb or exchange ions with the external environment depends on the properties of the membranes.

Attempts to influence the root membranes to regulate the supply of ions to the roots have some importance in the management of mineral nutrition of plants. Membrane-active compounds that affect ion transport in plants are of interest in this regard. These include valinomycin, gramicidin (antibiotics), 2,4-dinitrophenol, dimethyl sulfoxide. The latter is the most promising because it has a milder effect by increasing the conductivity of the sulfolipid layer of membranes.

The treatment of sugar beet plants with 2.5-5% aqueous dimethylsulfoxide solution under field conditions resulted in increased absorption and movement of nitrates and phosphates in the plants, promoted the use of soil nutrients and fertilizers, activated growth and increased the root crop by 3-6 t/ha, increased sugar yield by 0.5-1.0 t/ha.

Mechanisms of nutrient transport to plant roots

Nutrient absorption is a complex physicochemical and metabolic process involving:

diffusion,
adsorption,
metabolic transfer of substances against the electrochemical gradient.

Diffusion is important in the movement of soluble nutrients in the soil to the roots. Exchange adsorption is when nutrients enter the plant through the root system. The incoming nutrients interact with the cell protoplasm in a metabolic or non-metabolic way.

Metabolic uptake and movement of nutrients depend on aerobic respiration, temperature, and soil aeration.

Non-metabolic, or passive, absorption may be unrelated to plant activity, so it depends little on temperature and other plant conditions. An example of non-metabolic absorption is pinocytosis, i.e., the capture of a part of a nutrient solution, in which young root cells absorb ions and solution droplets.

Three mechanisms of nutrient delivery to the root surface are distinguished:

root interception,
mass flow,
diffusion.

Not only the outer walls of rhizoderm cells, but also the mucilage layer covering the root surface is important in the feeding process. Mucilage is the layer of mucus around the root hairs and young parts of the root, which participates in the process of contact exchange.

Different nutrient ions are involved in the transport mechanisms in different ways. For example, phosphorus and potassium are delivered to the roots mainly by diffusion, while calcium and magnesium are delivered by mass flow.

Mass flux becomes important at high nitrate concentrations in the soil solution (about 10 mol/L). At low nitrate concentrations, the diffusion process predominates.

Nitrate concentration in the root layer of soil is 3-4 times higher than in the soil foliar layer, which is especially noticeable at high doses of nitrogen – up to 120 kg/ha. This increase in nitrate concentration in the soil adjacent to the root can be explained by the fact that nitrates move to the root due to mass flow, but the rate of water absorption by roots is higher than the rate of nitrate uptake, which leads to the accumulation of nitrates in the root zone.

Cell membranes constitute the main part of the apoplastic compartment of tissues, through which the movement of substances occurs. Before ions are absorbed by the plasmalemma and involved in the symplast, they must pass through the free space of root cell apoplast.

Root interception

Root interception is the process of taking up nutrients by the root as it grows in the soil and comes into contact with nutrients. The share of root interception in nutrition with the volume of root system in the soil at a depth of 15 cm does not exceed 0.5-2% of the total soil volume.

The role of root interception increases when the soil contains nutrients in large quantities compared with the needs of the plant.

Mass flow

Mass flow is the process of transferring nutrients in the soil with the soil solution. As plant roots absorb water, it creates a movement of soil solution in the soil column to the roots and with it nutrients.

Depending on the type of plant and weather conditions, the intensity of mass flow can vary greatly, but as a rule it is in the range from 1 to 10 cm³ of water per 1 cm² of the root surface per second.

Diffusion

Diffusion is the absorption of a nutrient by the root, accompanied by a change in concentrations at the root surface, which creates a concentration gradient, which is the driving force of the process. The rate of diffusion through soil depends on the type of soil and the nature of the absorption of ions by the soil. For ions not adsorbed by soil, such as nitrates, the concentration gradient, that is, the ratio between ions in solution and ions adsorbed on the surface of soil particles, can be as high as 1. For strongly absorbed soil ions, such as phosphate, this ratio can be 10^-4.

For a root to be able to absorb a nitrate ion, it must be no more than 1 cm away, and for phosphate, no more than 0.1 cm. At the same time, the roots of annual plants are often placed at an average distance of 0.5 cm from each other.

Nutrient intake theory

The exchange of substances between the plant and the environment occurs through the surface cells of the root system and above-ground organs. Plant cell walls are easily permeable, since the radius of mineral ions is 0.4-0.6 nm, and the average radius of channels in the cell wall is 5-20 nm. However, if cell walls were the only barrier between the plant root and the outer nutrient solution, there would be an equalization of the concentration of substances by diffusion. But in plant organisms, nutrients are in higher concentrations than in the outer nutrient solution. The entry of individual substances and their concentration are different and do not correspond to the ratio of concentrations in the nutrient solution. This is due to the presence of a plasmalemma which prevents the loss of substances and at the same time ensures the penetration of water and nutrients.

The entry of substances against the concentration gradient is associated with energy expenditure.

There are a number of theories of the entry of substances into the cell. The transport of mineral salts into the cell is ensured by two mechanisms – the passive flow of substances along the electrochemical gradient and their active transport in the direction against the electrochemical gradient.

The electrochemical gradient is the distribution of mineral salt ions between the cell and the external environment under the action of the difference in electrical potentials and the difference in concentrations. The movement of ions along the electrochemical gradient is passive and against it is active.

The ratio of mineral salt transport mechanisms varies in plant ontogenesis and depends on a number of conditions. For example, passive transport of ions prevails at high concentrations of salts in the external environment, which is formed under conditions of salinization or local fertilization.

Free space. Ions pass through the cell membrane to the plasmalemma as a result of the diffusion process or together with the aqueous solution.

Diffusion is the movement of molecules of gases, liquids or dissolved substance in the direction of the concentration gradient and depends on the concentration difference and the area through which the substance and ions pass.

The free space is the part of the total volume of root system tissues into which ions enter and from which they are released due to diffusion. Free space accounts for about 4-6% of the total root volume. Free space is localized in the loose primary sheath of cell walls outside the protoplast outside the plasmalemma.

The free space is divided into an aqueous space, from which ions pass by diffusion into water, and a donnan space, from which ions are released into the salt solution by exchange. The root cell membrane consists of a loose surface layer with a system of pores or channels about 20 nm in diameter. In relation to the external solution, the root cells are negatively charged. The walls of free space pores or channels, carrying a negative charge, attract and hold cations and repel anions. In the central part of such a channel, the concentration of cations is equal to the concentration in the outer solution. This part refers to the aqueous space.

Exchange or donnan space is located closer to cell walls. Concentration of cations occurs here due to negative charge of walls.

The process of substances entering the free space occurs rapidly with an increase in concentration according to the saturation curve. The process is characterized by weak selectivity, its magnitude remains constant under the action of inhibitors or lower temperatures. The process is reversible.

The negative charge of the membrane is connected with negatively charged carboxyl groups of protopectin and plasmalemma embedded in cell walls. Exchange adsorption is important at this stage of absorption. For example, a root that has been incubated in a solution of calcium ions and transferred to a solution containing potassium ions loses adsorbed calcium through exchange adsorption.

The network of intercellular cells present in plant tissue provides additional transport. The loose cellulose covers of cells create a mass of micro-parts for adsorption and passive flow of substances through the tissue.

Substances entering the free space due to diffusion can undergo primary exchange adsorption. In this case they can be displaced from the free space into the outer solution or into the inner part of the cell. The process of adsorption consists in binding of mineral salt ions by cell membranes and their retention by electrostatic attraction forces.

Consequently, the free space is an area of the nutrient medium and simultaneously the absorption zone. Further, depending on the intensity and type of metabolism, selective absorption occurs. It is more correct to consider the entry of nutrients into the free space as the first preparatory stage of absorption.

Cellulose membranes of plant cells are united with each other by a system of vessels called apoplast. As a result of leaf transpiration and water flow, water and mineral salts are sucked into the intercellular spaces in the roots. To reach the xylem vessels for transport to the terrestrial organs, nutrients must travel a considerable distance in a radial direction across the root. This pathway of ions movement along the apoplast is interrupted by Caspari belts, watertight areas in which intercellular walls contain suberin, a substance that does not allow water solutions to pass through. However, some of the ions enter the conducting vessels through extracellular pathways – “passer cells”.

In the growing endoderm root tip, the Caspari bands do not have time to form, so ions can pass through the extracellular pathway. It should be noted that only a small number of ions enter the plant via the apoplastic pathway, and its role in balancing nutrients under normal life conditions is probably insignificant.

After ions cross the cell membrane (plasmalemma), their further movement occurs along a single cell system, the symplast, from cell to cell due to protoplasts linked by bridges through plasmodesmata, which ensure contact of cell plasmalemma. The mechanism of the plasmodesmata allows regulation of the rate of transport of substances through the symplast pathway, which is 2-4 cm/h.

Stages of entry:

Saturation with ions of the free space of the apoplast by exchange adsorption, diffusion, and passive adsorption.
Overcoming of membrane barrier by ions and their transfer to symplast.
Radial movement of ions in root tissues and vascular-fiber bundles.
Incorporation of ions into metabolism.
Vertical movement of ions along the above-ground parts of the plant.
Inflow into photosynthetic cells, utilization and reutilization, outflow.
Transport of assimilates and ions down phloem to roots.

Carrier theory and ion pumps

Nutrients enter the root in the form of ions with their obligatory transfer through the cell plasmalemma. This transfer can be passive, i.e. along the electrochemical gradient, and active – against the electrochemical gradient. The latter, the mechanism of active transport of ions through the phospholipid membrane, is the most important.

According to the transporter theory, the ion crosses the membrane in the form of a complex with a transporter molecule. On the inside of the membrane, the complex dissociates and releases the ion inside the cell. The transport of ions is carried out with the help of different types of transporters.

The driving force of transport involving the carrier is the chemical concentration gradient of substances in the case of passive transport and electrochemical potential. In this case the process is called facilitated diffusion. Functioning of the system with facilitated diffusion leads to equalization of gradients and establishment of equilibrium in the system.

In facilitated diffusion, substances move along the concentration gradient at a high rate.

In fungi, algae and bacteria, some antibiotics, such as valinomycin, whose molecule consists of 6 carbonyl groups, can be carriers. The complex of valinomycin with potassium is strongly hydrophobic and readily soluble in the hydrocarbon part of the membrane. Therefore, valinomycin strongly increases the permeability of membranes to potassium by forming a “potassium hole” in them.

The ionic permeability of phospholipid membranes is increased by some antibiotics produced by bacteria and fungi, such as Gramicidin and Nystatin.

Transport of substances inside root cells is stimulated by the fact that in the cytoplasm ions are involved in biosynthetic processes, as a result of which the concentration of ions inside cells decreases.

Two systems of ion transport have been established. The first system is characterized by a higher selectivity and is activated in vivo at low external ion concentrations of less than 1 mmol. Increasing the concentration of ions in the external solution leads to saturation of the first system and engages the second, less selective system.

The first system is localized in the plasmalemma, the second system is probably in the plasmalemma, but more likely in the tonoplast.

Protein globules with a diameter greater than the thickness of the cell membrane can be used as carriers. In this case, the movements of the globule around its axis ensure the transfer of ions from the outside to the inside. An example of such transport is the ion pump represented by the transport potassium-sodium ATPase. ATPases are so called because they can break down adenosine triphosphoric acid (ATP) molecules.

The energy released is used for transport and the transport ATPase is reversibly phosphorylated. The molecular weight of the transport ATPase is 200,000-700,000. Simultaneously with phosphorylation and dephosphorylation of the transport ATPase, ion binding and release and simultaneous conformational changes of the ATPase molecule occur, allowing the ion to be transported inside the cell.

The active site of the ATPase molecule consumes metabolic energy, and the M⁺ cation is attached to the exchange site. In the activated pump, the exchange site changes its orientation and faces the external environment. Under these conditions, its electric field strength changes and the C⁺ cation binds to it preferentially. Relaxation is the transition from the activated state to the non-activated state. Then the original orientation of the pump is restored: the exchange site with C⁺ cation turns to the internal environment, where M⁺ is preferentially bound to the exchange site, while C⁺ is released.

Ion pump is an active transport of ions into cells by ATPases. Its task is to maintain a constant ionic composition inside the cell.

Transport ATPases in animal cells were discovered in 1957, in plant cells – in 1964. In addition to the potassium-sodium pump, there is also a proton pump that pumps out H⁺ ions, which creates a negative charge in cells.

The transport of substances against the electrochemical gradient requires a constant supply of energy. The need for energy explains the dependence of nutrient uptake on the metabolic processes of respiration and photosynthesis, during which macroergic compounds are formed.

The rate of transport of ions due to transporters is determined by the rate of their turnover, which depends in turn on temperature, oxygen content, the presence of an inhibitor, the number of binding sites of the transporter, and the occupancy of active sites.

The theory of carriers made it possible to explain selectivity of absorption, interaction of ions and inhibition of the process. The theory suggests the presence of a control mechanism regulating the entry of substances into the cell according to the feedback principle. According to Pitman’s data, the maximum content of ions in the root cells is 80-90 mg-eq/g raw weight. For various plants this value does not depend on the concentration of ions in the solution, and is reached by absorption from 10 mM solution in 10-15 h, from 1 mM solution in 20 h, and from 0.1 mM solution in 36 h.

Some substances can be transported across the membrane by low-molecular-weight fat-soluble transporters. The carrier captures the ion, penetrates through the phospholipid membrane, and releases the ion inside the cell. The transport of substances inside the cell through membranes can occur through pores in the plasma membrane. Such pores can exist either temporarily or permanently. Their diameter is supposedly 0.5-0.8 nm. For comparison, the diameter of hydrated potassium ion is 0.34 nm.

The presence of single channels responsible for the transport of calcium ions in plant cells has been established.

The transfer of substances through hydrophilic channels or pores can proceed by channel selectivity. The relay mechanism is the passage of substances through the membrane, in which an ion or molecule is sequentially transferred from one carrier to another. In this process, the molecules of the carrier are embedded in the membrane in sequence and transfer ions or molecules on a relay. If the carrier moves with the ions being transferred, the transfer mechanism is called a shuttle mechanism.

Symporter and antiporter

Hydrogen ions are pumped out of the cell through a proton pump, which, by pumping protons out of the cell, creates a concentration and electrical gradient.

In doing so, alkalinization occurs inside the cell, as a result of which the carrier can transport back into the cell under the action of the electrochemical gradient, both protons back and other anions. This transfer mechanism is called symporter. The opposite process – pumping the H⁺ proton out of the cell and delivering it inside the cell to keep the ion with the same charge electroneutrality – is called antiporter. The terms symporter and antiporter were suggested by Mitchell.

Nitrogen entry is much easier. The NH₄⁺ cation in aqueous medium is in equilibrium NH₄⁺ ↔ NH₃ + H⁺. The NH₃ molecule enters the cell a thousand times faster than other electroneutral molecules except water. As it passes through the membrane, ammonia takes the hydrogen ion H⁺ from water, forming the ammonium ion NH₄⁺ and the hydroxide ion OH^–:

The cytoplasm is alkalized, which prevents proton pumping and proton pump operation, while at the same time alkalization promotes phosphorus influx.

NO₃^–, CN^–, I^–. anions pass through the membrane 100-1000 times harder than K⁺ and NH₄⁺. These anions in high concentrations can destroy the membrane structure and are called chaotropic.

Proof of the passage of nitrate through the membrane is the following experience. If in the medium where the liposome – a ball bounded by a two-layer artificial membrane – is placed, to put KNO₃ and valinomycin, a potassium “hole” is formed and K⁺ and NO₃^– ions enter the liposome, it quickly swells up. If KCl is used instead of KNO₃, the membrane is impermeable to KCl. A small amount of potassium will enter the liposome due to valinomycin and the process will stop due to the diffusion potential, the liposome will not swell.

Water molecules have the highest rate of passage through membranes.

Pinocytosis

Many cells can absorb solids and droplets from the environment. In the case of the absorption of solids the phenomenon is called phagocytosis, and in the case of the absorption of liquid droplets it is called pinocytosis.

By pinocytosis, substances can enter plants. At the beginning of the process, the absorbed particles are adsorbed on the cell membrane, then the membrane is drawn inside; its edges close together at the place of retraction. Thus, a pinocytic vesicle is formed, which detaches from the outer membrane and migrates inside the cell.

Two mechanisms of pinocytosis are possible.

In the case of the first mechanism (I), the membrane is retracted inside the cell and forms a narrow channel. From the end of the channel the vesicles with the captured substance are laced off.

In the case of the second mechanism (II-VII), the section of the membrane on which the micromolecules were adsorbed (III) is retracted inside (IV). At the place of retraction, the membrane edges close (V), and the resulting pinocytic vesicle is detached from the cell membrane (VI). In the thickness of the cell membrane, the vesicle is destroyed by enzymes (VII), and the trapped particles enter the cytoplasm. The process of vesicle formation and its detachment from the outer membrane is associated with energy expenditure, which is provided by ATP.

The pinocytic vesicle is destroyed by contact with the lysosome, which contains hydrolytic enzymes that break down macromolecules.

The phenomenon of pinocytosis is initiated by certain areas of the membrane with the appropriate substances. Reverse pinocytosis, i.e., the process of carrying the substance out of the cell to the outside, is possible.

Molecules or ions coming from the external solution, regardless of the method of transport through the plasma membrane, practically do not enter into reactions of exchange at the level of the plasma membrane. After entering the inner space of the cell they may:

be included in the cycle of metabolic transformations, with subsequent incorporation into the organic compounds of the structural elements of the cell;
concentrate in the vacuoles of root cells, creating a stock of ions;
to be transmitted by xylem vessels to the above-ground parts of plants;
to be excreted from the organism into the environment.

The effect of the plant root system on the soil

The roots of plants affect the soil by releasing organic and mineral substances such as sugars, organic acids, nitrogen-containing organic compounds, vitamins, enzymes, etc. into the external environment. These excreta serve as a nutrient medium for soil microorganisms. In turn, in the process of life, microorganisms contribute to the mobilization of soil nutrients, increasing their availability, provide plants with physiologically active substances (auxins, vitamins, antibiotics). This form of impact of the root system on the soil is indirect.

The direct impact lies in the ability of plants to influence the hard-to-reach forms of nutrients (phosphorus). Even D.N. Pryanishnikov proved the ability of buckwheat, lupine, mustard to use phosphorus of tri-substituted phosphates or natural phosphorites, which is related to the acidity of root secretions. Thus, the acidity of the solution surrounding the root hairs of lupine is 4-5, that of clover is 7-8.

Synthetic capacity of roots

For many years, it was believed that only in leaves the formation of complex organic compounds was possible. However, thanks to the method of labeled atoms, it was found that active synthetic processes also take place in the roots.

“We can say that in the life of the leaf the very essence of plant life is expressed, that the plant is the leaf.”
K.A. Timiryazev

The plant absorbs more of those substances which it needs more, which is explained by the physiological laws of the living organism. For example, when sodium nitrate NaNO₃ is applied to the soil, the plant absorbs the anion NO₃^– and to a lesser extent the cation Na⁺. When adding ammonium sulfate (NH₄)₂SO₄, the plant absorbs more NH₄⁺ cation and less SO₄^2- anion. Substances that are physiologically necessary for the plant are assimilated (assimilated) through the roots, while substances that are not needed remain in the mineral, easily soluble ionic form in the same form as they were before entering the plant.

According to numerous studies, the root system performs not only the functions of absorption, accumulation and transfer of nutrients, but also the function of synthesis of organic substances. In a study of plant root resin extraction (pasoka) the following were found in it: amino acids, oligopeptides, sugars and growth substances. From the lower, dying leaves, assimilants flow into the roots in the form of sucrose, which is used by the root for metabolism, growth and maintaining mature, functioning cells in a physiologically active state; for the synthesis of substances released by the root into the external environment and substances that enter the above-ground organs.

D.A. Sabinin made a great contribution to the study of the physiology of the root system and its properties. He was one of the first to put forward the assumption of synthetic ability of the root system. In 1940, he proposed the concept of transformation of substances during their passage through the root. This concept was further developed to the position of the synthetic activity of the root. The main provisions of this concept:

The root is capable of absorbing mineral substances, fully or partially processing them, and feeding them to the above-ground organs in a modified form.
The synthetic activity of the root is carried out at the expense of assimilates flowing into the roots, that is, it depends on photosynthesis.
The root influences the above-ground organs by providing water, minerals and products of specific metabolic reactions occurring in the roots – phytohormones of non-auxin nature.

One of the main growth substances found in Pasok are cytokinins, which contribute to intensive metabolism of leaves and delay their aging. Cytokinins are synthesized mainly in the root and partially in the leaves.

Other growth substances synthesized by the root are gibberellins, necessary for stem growth. Stopping the growth of above-ground organs when the roots are removed is associated not only with the deterioration of nutrient supply, but also with the cessation of cytokinin and gibberellin synthesis.

As plants age, the concentration of calcium increases in their cells, while the concentration of potassium decreases. Young and actively functioning plants are characterized by high concentrations of K⁺ ions. Treatment of plants with the growth agent kinetin leads to an outflow of Ca²⁺ ions from the cells and an increase in the concentration of K⁺ ions. Yellowed leaves become green, the destruction of subcellular structures stops and protein biosynthesis increases.

In 1965, F.V. Turchin, using a stable nitrogen isotope ¹⁵N, determined that almost all the ammonium (¹⁵NH₄⁺) and a significant portion of nitrate (¹⁵NO₃^–) nitrogen absorbed by the root system is located in the root passover as nitrogen-containing organic compounds. About 18 amino acids are synthesized in the roots of various crops.

Studies using labeled atoms of phosphorus (³²P), sulfur and other elements have shown that in plant roots the phosphoric acid anion (H₂PO₄^–) is incorporated into organic compounds through ether bonds, while the absorbed sulfate ion (SO₄^2-) is reduced and incorporated into sulfur-containing amino acids: cystine, cysteine and methionine.

The synthetic ability of the root system was proved by Academician A.A. Shmuk and his collaborators in 1941. They found that nicotine in tobacco plants is synthesized in the root system, not in the leaves. If tobacco is grafted onto tomato, there is virtually no nicotine in the tobacco leaves, and conversely, up to 3-4% nicotine accumulates in the leaves of tomato grafted onto tobacco.

Root excretions of plants use relatively small amounts of assimilates. For example, 0.5-0.7% of carbon from the roots of beans is excreted into the leaves in the form of carbon dioxide.

The synthetic activities of the leaf and root of plants are closely related.

The discovery of the synthetic ability of the root system is one of the major achievements of science in the XX century in the field of physiology of plant root nutrition.

Excretory function

Root excretions include sugars, amino acids, organic acids, and, in small amounts, vitamins, enzymes, and volatile organic substances such as ethylene. Root excretions vary in quantity and composition and are determined by the species and varieties of crops.

Root excretions are related to the ability of plants to regulate the nutrient regime of soils. For example, lupine, due to the acidifying effect of root excretions, is able to assimilate hard-soluble forms of phosphorus.

Accumulation of root excretions when growing isolated roots under sterile conditions leads to growth suppression due to accumulation of amino acids, the main component of root excretions, to toxic concentrations for plants.

Nutrient absorption selectivity

The plant absorbs more of those substances that it needs more, which is explained by the physiological laws of the living organism. Thus, when sodium nitrate NaNO3 is applied to the soil, the plant absorbs the anion NO3- and to a lesser extent the cation Na+. When adding ammonium sulfate (NH4)2SO4, the plant absorbs more NH4+ cation and less SO42- anion. Substances that are physiologically necessary for the plant are assimilated (assimilated) through the roots, while substances that are not needed remain in the mineral, easily soluble ionic form in the same form as they were before entering the plant.

After the equilibrium between concentrations in the cellular and soil solutions on the surface of the root hair and on the surface of soil colloidal particles is reached, the processes of ion inflow cease. When the concentration of nutrients on the cellular solution side decreases due to assimilation of these nutrients by the plant and their transfer to other parts of the plant, the absorption process will continue.

The selective property of plants to absorb nutrients is evidenced by the fact that the concentration of salts of a number of nutrients in the cell sap is much higher than in the nutrient solution into which the root system is immersed. For example, the concentration of potassium in the corn graze is 20 times higher, phosphorus 14 times higher, and calcium 4 times higher than in the external nutrient solution.

With the end of the life cycle, nutrient uptake into plants stops and an equilibrium of their concentration on the root hair surface and in the soil solution sets in.

The selectivity of nutrient absorption determines the physiological acidity of fertilizers. If the plant absorbs more cations of mineral fertilizer, the anions remain and accumulate in the soil solution, which causes acidification of the soil solution, and the fertilizer itself is physiologically acidic. Thus, physiologically acidic fertilizer includes ammonium sulphate, ammonium chloride, ammonium nitrate, potassium chloride. Conversely, if the plant absorbs the anion of the mineral fertilizer and the cation is accumulated in the soil solution, the fertilizer is physiologically alkaline, such as sodium nitrate, calcium nitrate.

If there are toxic substances in the soil solution, e.g. heavy metals, pesticides, much of it is retained in the roots as well as in the stems and leaves, and only a small part goes to the seeds.

Nutrient intake periods

There are two periods of nutrition in the life of a plant. The first, or critical, period is during the initial phases of plant growth and development. During this period, plants are most sensitive to a lack or excess of nutrients. In this period the plants are particularly demanding to the conditions of mineral nutrition.

The second period, or the period of maximum intake of nutrients, is characterized by the intake in later phases of development and is determined by the biological characteristics of plants. For example, nutrient intake in cereal plants, with the exception of corn, ends by the end of the earing phase, although by this time only 50-60% of the plant mass of the full crop is formed.

Table. Dynamics of accumulation of nutrients in plants, % of maximum^[3]Agrochemistry. Textbook / V.G. Mineev, V.G. Sychev, G.P. Gamzikov et al; ed. by V.G. Mineev. - M.: Publishing house of the All-Russian Scientific Research Institute named after D.N. Pryanishnikov, … Continue reading

Timing and growth phase	Winter wheat			Barley			Oats
	N	P₂O₅	K₂O	N	P₂O₅	K₂O	N	P₂O₅	K₂O
Autumn and early spring	47	30	43	-	-	-	-	-	-
Beginning of earing	69	65	68	71	56	73	51	36	54
Blossoms	90	93	95	96	74	100	82	71	100
Full ripeness	100	100	82	100	100	64	100	100	83

Winter wheat with good development in the autumn period assimilates up to 43-47% of nitrogen and potassium, while the dry weight of plants is not more than 10% of the total yield. The same is typical for winter rye, which during the autumn period assimilates 50-60% of nitrogen, phosphorus and potassium. Oats and barley absorb 100% of potassium during the flowering phase, and even lose it afterwards (exoosmosis, or excretion). The accumulation of nutrients by corn plants is slower: by the beginning of flowering 30-40% of nitrogen and potassium and 15% of phosphorus of the content of these substances in corn at its maturity occurs.

Such crops as sugar beets, potatoes, cabbage and other vegetable crops assimilate nitrogen, potassium and phosphorus almost throughout the growing season.

Table. Dynamics of the influx of nutrients in the plants of sugar beet (according to the Research Institute of Sugar Beet), % of the maximum^[4]Agrochemistry. Textbook / V.G. Mineev, V.G. Sychev, G.P. Gamzikov et al; ed. by V.G. Mineev. - M.: Publishing house of the All-Russian Scientific Research Institute named after D.N. Pryanishnikov, … Continue reading

Date of observation	Dry matter		Nitrogen	Phosphorus	Potassium
	root	above-ground part
5/VI	0,3	2,3	2,7	2,4	2,6
15/VI	1,1	8,2	9,6	8,5	9,2
1/VII	6,8	35,7	36,0	39,0	35,0
15/VII	16,7	48,0	51,0	46,0	46,0
1/VIII	33,0	64,0	68,0	68,0	69,0
15/VIII	46,0	63,0	77,0	77,0	73,0
1/IX	60,2	73,0	85,0	87,0	85,0
15/IX	85,0	100	100	100	100
1/X	93,0	77,0	89,0	90,0	91,0
15/X	100	58,0	-	-	-

Frequency of mineral nutrition of plants allows to regulate the formation of yield and its quality. In particular, fractional fertilizer application is based on this property of plants.

Application of fertilizers at one time and in one layer of soil does not always allow to achieve full use of the potential yield. For example, doses of readily soluble mineral fertilizers that are sufficient during the critical feeding period will be small for the period of maximum consumption. Conversely, a high dose in the first critical period is harmful to the young roots, sensitive to high salt concentrations. For this reason, the plant fertilization system involves a combination of basic, pre-sowing fertilization and top dressing during plant growth.

Sources

Agrochemistry. Textbook / V.G. Mineev, V.G. Sychev, G.P. Gamzikov et al. – M.: Publishing house of the All-Russian Scientific Research Institute named after D.N. Pryanishnikov, 2017. – 854 с.

Yagodin B.A., Zhukov Y.P., Kobzarenko V.I. Agrochemistry / Edited by B.A. Yagodin. – Moscow: Kolos, 2002. – 584 p.: ill.

Air nutrition of plants (photosynthesis)

22.01.2022

Air nutrition of plants (photosynthesis) is the process of formation of nitrogen-free organic substances (carbohydrates) by plants from atmospheric carbon dioxide and water under the influence of sunlight:

6 CO₂ + 6 H₂O + 674 kcal → C₆H₁₂O₆ + 6 O₂.

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