The nutrient regime of soils is a set of processes of inflow, accumulation, transformation of nutrients in the soil and their absorption by plants.
It refers to the terrestrial factors of plant life.
- The importance of nutrients in plant life
- Balance of nutrients
- Plant nutrient availability
- Classification of soils by nutrient provisioning
- Influence of nutrient solution concentration on nutrient intake into the plant
- Ratio of macro- and microelements in the nutrient environment
- Influence of soil moisture on the nutrient regime
- Relationship between air and nutrient regime of soils
- Relationship between heat and nutrient regime of soils
- Interrelation of light and nutrient regimes
- Influence of the reaction of the medium on the nutrient regime
- Physiological reaction of salts
- Regulating the nutritional regime of the soil
The importance of nutrients in plant life
The main process responsible for plant nutrition is photosynthesis.
Photosynthesis is the absorption of carbon dioxide and water that occurs in chlorophyll molecules under the influence of sunlight and converts them into glucose and oxygen. However, in order to grow and develop, plants need a complex of minerals, which they absorb mainly from the soil. All plants that use chlorophyll for nutrition are called autotrophic. Plants also absorb some simple nitrogen- and phosphorus-containing organic substances (some amino acids and phytins) from the soil, but their role in nutrition is negligible.
Some plants that do not have a chlorophyll molecule (Cuscuta, Orobanche), as well as fungi and bacteria that feed on ready-made organic compounds are called heterotrophic organisms.
Today, more than 74 chemical elements, 16 of which are vitally important, have been found in the composition of plant matter. They are commonly subdivided into:
- organogenic, that is, from which organic substances are directly synthesized: carbon, oxygen, hydrogen and nitrogen;
- non-organogenic or ash: phosphorus, potassium, calcium, magnesium, iron and sulfur. Their proportion in the plant is counted in percent and tenths of a percent;
- micronutrients: boron, copper, iron, manganese, zinc, molybdenum and cobalt and some others. Micronutrients account for hundredths and thousandths of a percent in the plant;
- ultra-micronutrients: silver, gold, radium, uranium, thorium, actinium, etc. They are found in plants in trace amounts.
These elements are involved in biochemical and enzymatic processes. Other chemical elements are often present in plant parts, but their vital necessity is not fully understood and strictly optional.
Some substances not involved in biochemical processes and absorbed by plants may in some cases act positively on plants, such as sodium on sugar beets or silicon on cereal crops, in other cases negatively, such as chlorine on potatoes, tobacco and other chlorophobic crops.
Nutrients in soil can be in the soil solution, in the organic matter of the soil, and in the solid mineral phase of the soil.
Nitrogen is a component of protein and peptide molecules, chlorophyll, nucleic acids, phosphatides and other organic substances. It is on a par with phosphorus and potassium, the most important element in plant nutrition. Nitrogen participates in synthesis of phytohormones responsible for aging and reproductive processes.
Nitrogen is present mainly in the organic form, so its amount is directly proportional to the content of humus. The rate of decomposition of soil organic matter affects the nitrogen supply of plants.
Nitrogen is assimilated by plants mainly in ammonium and nitrate forms. Due to the mobility of nitrogen-containing ions and the high consumption by plants, requires a constant replenishment of soil reserves of these elements, which is achieved by using mineral and organic fertilizers.
An important source of nitrogen in the soil is the process of nitrogen fixation from the atmosphere.
Phosphorus is in the soil in the form of mineral and organic compounds. In sod-podzolic soils, one third of phosphorus is connected with organic compounds, in chernozems – about half.
Phosphorus is absorbed by plants in a mineral form, so the phosphorus associated with organic compounds becomes available to plants only after mineralization.
Mineral forms of phosphorus are represented by low-soluble and insoluble phosphates of iron, calcium, aluminum, magnesium, potassium and others. The amount of phosphorus available to plants is much less than its gross content in the soil. For example, in gray forest and sod-podzolic soils gross phosphorus content (P2O5) is 1.2-3.6 t / ha, and only 100-200 kg / ha of them is in a plant-available form.
The intake of phosphorus into the plant depends on temperature. An increase in temperature during the dry period leads to an increase in the intake of phosphorus. However, in severe drought conditions, phosphorus outflow from above-ground organs to roots and soil is possible. For example, under short-term exposure to temperatures within 37-41°C and humidity of 16-19%, i.e. under dry weather conditions, the wheat plants grown on the background of phosphorus-potassium fertilizer have higher osmotic pressure and higher water content in tissues, which provides greater plant resistance to dehydration effect of dry weather.
Potassium is contained in soil mainly in the form of salts and absorbed state – available (exchangeable) and inaccessible (non-exchangeable) form. Potassium content in soils is relatively high, in clay soils it can be up to 2%, in sandy soils – less.
The main source of potassium for plants is exchangeable potassium, whose concentration is directly proportional to its gross content, as exchangeable and non-exchangeable forms are in chemical equilibrium with each other.
Various crops such as potatoes, vegetables, root crops, and perennial grasses have higher potassium requirements and are called potassium-loving crops.
Calcium and magnesium
Calcium and magnesium are important in plant nutrition and simultaneously affect nutritional conditions through the soil, regulating the reaction of the environment, the composition of absorbed cations, salt and ionic composition of the soil solution.
Calcium is especially important for plants in acidic soils with low buffering and low base saturation.
Calcium is responsible for the structural and physiological stability of plants. It participates in cell division, cell wall formation and stretching of shoot and root meristem cells. To a small extent it can be replaced by other ions.
Calcium participates in transport of nitrite nitrogen, together with magnesium and manganese activates about 20 enzymatic systems.
Magnesium becomes important in light sod-podzolic soils.
Magnesium is part of chlorophyll up to 15-20% of the total amount contained in the plant and takes part in carbon dioxide fixation. It participates in the synthesis of other pigments, in about 300 enzymatic reactions, cellular metabolic processes, and phosphorylation. Its action is due to its ability to form chelates with organic compounds. Magnesium stabilizes cell membranes, along with potassium and calcium ions, affects protoplasmic viscosity and water content.
When calcium is deficient, the physiological equilibrium of the soil solution and the balanced consumption of all other nutrients is disturbed. In plants, calcium participates in photosynthesis and metabolism processes, regulates acid-base balance of cell juice, influences formation of cell membranes, participates in movement of carbohydrates, transformation of nitrogenous substances, in particular, accelerates decomposition of spare proteins of seeds during germination.
In plants, calcium is in the form of carbonates, phosphates, sulfates and salts of pectic and oxalic acids. When determining the calcium content of plants, up to 65% of calcium is extracted with water, the remainder with weak acetic and hydrochloric acids.
Crops consume varying amounts of calcium. For example, removal of CaO from fields by cereal crops at grain yield of 2-3 t/ha is 20-40 kg/ha, by leguminous crops 40-60 kg/ha. Potatoes and sugar beet at yields of 20-30 t/ha remove 60-120 kg СаO/ha, clover, alfalfa at yields of 20-30 t/ha and sunflower (2-3 t/ha) – 120-250 kg СаO/ha, cabbage (50-70 t/ha) – 300-500 kg СаO/ha. The calcium requirement of crops and their resistance to acidity may not coincide. Thus, all cereals are characterized by low calcium consumption, but rye and oats are resistant, while wheat and barley are sensitive to soil acidity. Potatoes and lupine are more tolerant to acidity, but consume significantly more calcium than cereals.
Calcium losses from the soil may be due to leaching by precipitation. Depending on granulometric composition, amount of precipitation, type of vegetation, doses and forms of lime and mineral fertilizers, calcium losses from arable layer vary from 10 kg/ha to 200-400 kg/ha. Calcium accounts for 50-65% of calcium and 30-35% of magnesium equivalents in the total amount of leached substances.
Liming by accelerating the processes of ammonification and nitrification of soil nitrogen, organic and mineral fertilizers, leads to an increase in the concentration of nitrates, and chloride fertilizers – chlorides. These anions are not sorbed by the soil and migrate with calcium and magnesium displaced from the soil absorption complex in equivalent ratios. For this reason, the concentration of calcium and magnesium in the soil solution when high doses of fertilizers are applied can increase dozens of times.
The saturation of seepage water with calcium and magnesium increases with increasing degree of soil cultivation. Leaching of calcium and magnesium decreases with increasing soil depth, and part of the cations washed out of the arable horizon in the dry periods of the year returns with water currents through the capillaries. According to the data of D.N. Pryanishnikov All-Union Institute of Fertilizers and Agrosoil Science experiments with chromatographic columns, 14-35% of calcium and 22-34% of magnesium rose into the arable layer.
The greatest losses of calcium and magnesium are noted in clean fallows, under crops they decrease and reach a minimum under perennial crops of continuous seeding. Other things being equal, washout increases by 1.5-2.0 times in the transition from heavy to light soils. For this reason, on light sandy and sandy loam soils when cultivating cabbage, alfalfa, and clover it may be necessary to apply calcium to improve nutrition of these crops.
Up to 10% of magnesium is a part of chlorophyll molecule, as well as phytin and pectin substances.Magnesium is mainly found in growing organs and seeds; unlike calcium, it can be recycled by plants. Its content in seeds is higher than in leaves, so its lack has a greater effect on reducing the marketable output of crops.
Magnesium participates in movement of phosphorus, activates some enzymes (phosphatases), participates in synthesis of carbohydrates, regulates redox processes by increasing reduction of essential oils and fats, increases ascorbic acid content and reduces peroxidase activity.
Magnesium (MgO) removal from fields by crops at harvest ranges from 10 to 80 kg/ha. The maximum amount of magnesium is taken out by potatoes, sugar and fodder beets, tobacco, legumes and cereals. Hemp, millet and corn are sensitive to magnesium deficiency.
Sulfur is essential for plant growth and development and affects the amount and quality of production. Absorption of sulfur from the soil occurs in the form of sulfate-ion SO42- -sulfuric acid salts, such as CaSO4, MgSO4, K2SO4, (NH4)2SO4. It can be absorbed by leaves from the atmosphere as sulfur dioxide SO2.
The main part of sulfur in plants is contained in organic form in the composition of proteins, amino acids, fats, vitamins, enzymes, a smaller part – in mineral, mainly in the form of CaSO4, form.
Sulfur content in plant organs decreases in the transition from seeds to leaves, stems and roots. Thus, in cereal crops (in percentage of SO2 per dry matter) sulfur content in grain is 0.30-0.45%, in straw – 0.12%; in legume seeds – 0.60-0.80%; in potato tubers – about 0.35%, in tops 0.55%; in roots of sugar beets – about 0.2%, in tops – up to 1.0%.
The maximum sulfur content is observed in legume and cabbage families, considerable content – in lilies and minimum – in cereals. Removal of sulfur from the fields at an average yield of 2 t/ha of grain crops is 7-15 kg/ha, 20-30 kg/ha of leguminous grasses, 30-40 kg/ha of beets, and 50-80 kg/ha of cabbage.
Soils contain sufficient total reserves of sulfur, but 70-90% is concentrated in organic form, inaccessible to plants, becoming available only after mineralization of organic matter. Sulfur bacteria oxidize organic sulfur to sulfuric acid and sulfate ions SO42-.
Sources of mineral forms of sulfur for plants in soils are usually few, but industrial SO2 emissions enter the soil with precipitation. Organic and sulfur-containing mineral [(NH4)2SO4, K2SO4, Ca(H2PO4)⋅CaSO4] fertilizers compensate for possible sulfur deficiency. Therefore, as a rule, for cultivated crops on most soils there is no sulfur deficiency. The deficit may appear on soils poor in organic matter with a lack of organic and mineral fertilizers, as well as on cultivated soils with the intensification of agriculture.
Micronutrients are involved in many physiological and biochemical processes of plants, are part of a large number of enzymes, vitamins and growth substances. Micronutrients play an important role in the vital activity of soil microflora.
Micronutrients include: manganese, boron, copper, molybdenum, zinc, cobalt, iodine.
Plants consume micronutrients in very small amounts. Deficiency or excess affects the metabolism of plants. For example, the lack of micronutrients in sugar beets is manifested in the form of heart rot, in flax – bacteriosis, in cereal crops on peat soils – hollowness, etc. Lack of micronutrients sharply reduces the yield of plants.
Manganese contributes to the selective absorption of ions from the external environment. With its deficiency the content of other trace elements increases. Manganese affects the movement of phosphorus from aging leaves to young ones.
Cobalt is involved in the regulation of plasmalemma permeability, improves the flow of nitrogen and other elements in plants.
Molybdenum promotes the uptake of phosphorus by plants through its participation in nitrogen metabolism and can increase the provision of plants with this element.
Nitrogen intake is also influenced by copper and boron.
Zinc changes membrane permeability to potassium and magnesium cations. In plants under zinc deficiency, increased concentration of inorganic phosphorus is observed. It participates in structural organization of cells and regulation of transport of ions through cell membranes.
Copper affects the activity of K-N3-ATPase and promotes accumulation of organic phosphorus compounds in plants. Sufficient supply of copper, zinc and boron improves the supply of magnesium to plants.
Balance of nutrients
The balance of nutrients is the total consumption and receipt of nutrients in the soil.
Sources of nutrient inputs are:
- mineral and organic fertilizers;
- nitrogen fixation;
- atmospheric precipitation;
- plant residues;
- influx of substances with surface and ground water.
The expenditure part of the balance includes:
- removal with crops and green matter;
- runoff with surface and downstream water currents;
- losses from water and wind erosion;
- decomposition with the release of gaseous substances;
- alienation with parts of weed plants;
- transition into an inaccessible form for plants.
Plant nutrient availability
The gross content of elements in soils is different. The content of calcium can vary by 1310 times, phosphorus, magnesium, iron, copper, manganese, cobalt, boron – by 100-300 times. Strongly vary depending on soil type and form of compounds soluble in 1 n. hydrochloric acid: manganese – in 70, iron – in 1420 times. The smallest variation in content is noted for nitrogen and potassium – about 10 times.
All soluble and exchange-absorbed forms of nutrients are available to plants. Other compounds are not directly available and are assimilated only after conversion to a more accessible form, for example, as a result of destruction of primary minerals in the process of weathering, mineralization of organic matter. When external conditions change, some of the macro- and microelements can pass into a hard-to-access form, for example, when the environmental reaction changes, the microbiological fixation of nutrients intensifies.
Significant influence on the availability of soil nutrients have plants themselves. A change in the environmental reaction under the influence of substances excreted by plants promotes the transition of inaccessible compounds into an assimilable form.
Nutrient uptake by plants depends on biological features of the crop, soil properties, level of potential fertility, mineralogical and granulometric composition, temperature, humidity, aeration, reaction and concentration of soil solution, light. Thus, at night the rate of potassium, calcium and phosphorus uptake decreases by 1.5-3 times.
Of the total (gross) reserves of nutrients in soils, as a rule, a small part of them (1-10%) is available to plants.
Classification of soils by nutrient provisioning
In Russia, classification of soils according to the degree of provision with nutrients and reaction is used. The classification is used in agrochemical surveys of soils, preparation of agrochemical maps (cartograms), field passports, and for calculations of optimal doses of fertilizers and ameliorants for crops in relation to specific natural and economic conditions.
For individual regions the levels of plant nutrient provision are specified on the basis of data from field trials, species and variety diversity of crops and soil and climatic conditions.
Table. Classification of soils on the provision of nutrients (mg/kg) and acidityYagodin B.A., Zhukov J.P., Kobzarenko V.I. Agrochemistry / Edited by B.A. Yagodin. - Moscow: Kolos, 2002. - 584 p.: ill.
* Average supply for cereals, legumes, annual and perennial grasses.
** Average supply for row crops.
*** Average supply for vegetable crops and phosphorus for technical crops.
Influence of nutrient solution concentration on nutrient intake into the plant
Insufficient concentrations of nutrient solution negatively affect plant growth and development. Thus, the ultimate minimum concentration of phosphate ions, providing normal nutrition, is 0.03-0.1 mg P2O5 per 1 liter of soil solution. At lower concentrations, plants experience a sharp deficit of phosphorus. The upper limit is in the range of 2-3 g for all mineral nutrient salts per 1 L of solution.
The optimum concentration of the nutrient solution, i.e. the one at which the maximum plant productivity is achieved under given conditions, can vary greatly depending on the periods of ontogenesis of a particular plant species and variety.
The root system of plants is capable of assimilating nutrients from highly diluted solutions (0.01-0.05%), especially when this concentration is constant. Under natural conditions, the concentration of soil solution on non-saline soils ranges from 0.02 to 0.2%.
Nutrient ions from moderately saturated solutions are better assimilated, and water is better absorbed by the root system located in the non-saline layer. This should be taken into account when applying fertilizers locally or focally.
Increasing the concentration of salts in the solution leads to an increase in osmotic pressure and makes it very difficult for water and nutrients to reach the plants.
The rate at which an ion is absorbed by a plant depends on its concentration in the outdoor environment. For example, when the concentration of trace elements in the solution is up to 5 µM, the ions are almost completely absorbed by the roots. When concentration is up to 25 μM, the curve describing absorption changes from linear to hyperbolic: the selective nature of divalent cations absorption appears. Thus, the absorption of Mn2+ exceeds by 2 times the amount of absorbed Co2+ and Zn2+ ions, while the manganese content in plants also increases. Up to the concentration of 0.1 mg/l, manganese uptake is directly proportional to the growth of its concentration in the solution. Further increase of manganese concentration by 10 and 100 times leads to increase of its content in the plant only by 4 times.
Detailed consideration of micronutrients absorption processes allows to distinguish 4 phases and to assume participation of different mechanisms in this process:
- Phase I – physical and chemical adsorption;
- Phase II – free space saturation. In this phase metabolic absorption is possible, as the rise of absorption isotherm after two-hour period indicates the active nature of this process;
- Phase III – active absorption;
- Phase IV – a sharp rise in the absorption of ions at their high concentration in the solution (500 μM) during the period from 6 to 12 hours, which is probably associated with a violation of the barrier function of the root.
There is a certain membrane structure in the root cells that changes its properties when reaching certain concentrations. For example, up to a concentration of iron ions of 0.5 mM, absorption increases with increasing concentration; in the range from 0.5 to 1 mM, absorption stabilizes, after that it increases again, reaching a plateau at a concentration of 10 mM.
The concept of multiphase ion absorption shows the relationship between the absorption phase transition points and the intensity of growth processes.
Table. Effect of nutrient solution concentration on cucumber growth and yield (Zhurbitsky)
At a young age, plants are most sensitive to excessive concentrations of nutrient solution. Certain crops cannot tolerate an increase in concentration above a certain limit.
With increasing the concentration of nutrient solution to 15.7-25.9 mM/l, plant development improved: the highest fruit yield was obtained at the concentration of 25.9 mM/l.
Higher concentrations caused a sharply negative effect: plants at 1 month of age during flowering showed drying of middle and lower leaf margins, drying and browning between leaf veins.
Sensitivity to nutrient solution concentration is manifested differently in different plants. Flax, lupine, cucumbers, carrots are the most sensitive to increased concentration. Sensitivity of the same plant may change with age. Differences in sensitivity to concentrations must be taken into account when designing a fertilizer system.
Ratio of macro- and microelements in the nutrient environment
Each plant species requires a certain ratio of nutrients for normal development, which can vary during the growing season. When plants are fed from a soil or water solution consisting of a mixture of nutrients, it is not the concentration but the ratio of elements and their mutual influence that plays an important role.
A change in the level of nutrient supply causes numerous organismal responses. Thus, when there is a sharp excess of any element of mineral nutrition, the protective reaction of plants manifests itself in an increase in the absorption of other elements. A small excess, which does not threaten the life of the plant, of one of the trace elements leads to a decrease in the intake of other mineral elements. The negative effect of excess nutrients can be partially reduced by introducing other elements. For example, in the experiment with lettuce and barley with a small excess of magnesium, nitrogen had a positive effect.
Normal functioning of plant organism is possible with a certain ratio of cations and anions in the external environment. This provision is a theoretical basis for the development and justification of the composition of nutrient mixtures. Along with this, the position on the antagonism of ions at their entry into the living cell is formulated. The latter is explained by the competition between the ions for the exchange entry into the donnan space. A solution that contains an optimal ratio of nutrients is called physiologically balanced. A solution containing only one nutrient cannot meet the needs of the plant even for a short period.
Ions with the same charge mutually inhibit each other, and conversely, ions with opposite charges mutually accelerate their entry into the plant. This phenomenon is called synergism. A harmful excess of a cation or anion can be weakened by the corresponding ion. For example, the entry of the NO3– ion can be accelerated by adding the cation Ca2+, and an excess of Ca2+ weakened by Mg2+, the harmful effect of H+ and Al3+ ions is eliminated by adding Ca2+ and Mg2+.
Different salts in the form of a single-salt solution have different effects on plants.
When studying the patterns of nutrition elements absorption, the duration of the experiment is important, because in the case of long experiments, changes in the physiological state of plants occur, for example, in the absorption capacity of the root system.
Increase of nitrogen concentration in nutrient media leads to increase of phosphorus, potassium, calcium, magnesium, copper, iron, manganese and zinc intake into plants. The intensity of the effect of nitrogen is reversed in its excess and depends on the form of nitrogen compounds.
In the soil solution, cations and anions are simultaneously present, and there is competition between them for places of adsorption on the root surface and in the intercellular free space. Thus, an increase in Ca2+ concentration leads to an increase in its proportion on the root surface by displacing other previously adsorbed cations (K+, Mg2+, NH4+), while an increase in Cl– leads to a decrease in NO3–, H2PO4– and other anions.
Excessive doses of phosphorus reduce the intake of copper, iron, and manganese. Excessive potassium reduces the intake of calcium, magnesium.
Increasing the provision of plants with basic nutrients increases the need of plants for micronutrients, which, in turn, increases the efficiency of macronutrients and their intake. For example, in the experiments, nitrogen accumulation in plants decreased when iron, manganese and zinc were lacking and did not depend on copper, boron and chlorine. Under other conditions, nitrogen accumulation in plants increased when copper and boron were added. Molybdenum and cobalt improved the use of fertilizer nitrogen from the soil. Phosphorus uptake increased with copper, zinc, calcium, and molybdenum and decreased with magnesium and iron. Potassium intake decreased under the influence of copper, manganese, nickel, zinc, molybdenum, iron and boron and increased when chlorine was applied.
Plant growth depends on the element in minimum or excess provided that other nutrients or other life factors do not limit growth. Soil acidity affects the adsorption of ions, affecting the absorption of nutrients. For example, the application of lime increases the growth of legume crops because at neutral acidity (pH > 4) the adsorption of MoO42- by soil and aluminum and iron oxides is reduced and molybdenum becomes more available to plants.
Competition between silicates and phosphates as well as phosphates and molybdates is possible in soil. Manganese oxides adsorb cobalt from solution, transferring it into inaccessible form. Therefore, waterlogging leads to increased plant uptake of cobalt due to dissolution of manganese oxides (Mn4+ → Mn2+).
Adsorption of H2PO42- by aluminum and iron oxides and hydroxides increases Zn2+ adsorption, which decreases zinc uptake by roots, with the formation of Zn2+ and H2PO42- complexes on the oxide surface.
Excessive concentrations of NO3– ions inhibit the supply of PO43- and PO3– ions to plants, and vice versa. Calcium in high concentrations inhibits potassium intake, and vice versa.
Alkaline soils are characterized by deficiency of manganese, while acidic soils are characterized by excess. The solubility of manganese oxides is affected by root excretion. Crop varieties sensitive to manganese deficiency have more Mn-oxidizing bacteria in the rhizosphere. While absorption leads to acidification and improved manganese supply, absorption of NO3– leads to alkalinization and manganese deficiency.
Ion antagonism is evident between:
- iron and calcium;
- aluminum and sodium;
- iron and zinc;
- manganese and zinc;
- copper and zinc; copper and zinc;
- zinc and iron;
- manganese, copper and molybdenum;
- potassium and sodium;
- calcium and magnesium;
- potassium and magnesium.
Antagonism is manifested to a greater extent between ions with similar properties, for example, between the anions NO3– and PO3– it is more pronounced than between NO3– and PO43-, between cations K+ and Na+ more than between K+ and Ca2+.
The phenomenon of synergism is established for:
- sulfur and manganese;
- zinc, copper and cobalt;
- boron, zinc, cobalt, molybdenum and manganese;
- molybdenum and manganese;
- molybdenum and copper;
- copper and manganese;
- calcium and cobalt.
Antagonism and synergism depend on the reaction of the environment, the level of mineral nutrients in the environment and in the plant, their ratio, plant species, and temperature. Depending on the conditions, antagonism and synergism can alternate. Reducing temperature and light intensifies the effect of excessive doses of nutrients, while increasing humidity reduces the negative effect of excessive amounts. For example, the increase of nitrates in vegetables when they are grown in greenhouse conditions in winter time with a lack of light.
The increase in nitrate content can be caused by increasing the doses of nitrogen fertilizers. Thus, in experiments with cabbage lettuce, increasing doses of nitrogen fertilizers led to an increase in the weight of the cabbage and simultaneously increased the nitrate content (table).
Table. Influence of nitrogen fertilizer doses on the mass of head salad and its nitrate contentYagodin B.A., Zhukov Y.P., Kobzarenko V.I. Agrochemistry / Edited by B.A. Yagodin. - Moscow: Kolos, 2002. - 584 p.: ill.
Strontium and manganese displace calcium. Calcium and phosphorus absorption is improved with cobalt and manganese. Nickel displaces calcium and phosphorus. Cobalt and manganese are antagonists of strontium and barium. Excessive aluminum leads to imbalance of macro- and microelements, reduces the content of calcium and manganese in plant tissues.
Absorption of elements of mineral nutrition is interconnected: deviation of one element concentration by 30-100% from the optimum leads to changes in absorption of other elements. An increase in the amount of an element at an insufficient concentration promotes the absorption of other elements, i.e. synergism is manifested, and an excess of it hinders the absorption of other elements (antagonism).
At deviations of 100 times or more concentrations from the optimum, the relative content of other elements increases. At the same time, their total intake decreases due to slowing down the growth of plant weight.
In contrast to cations, anions NO3–, PO43-, SO42-, Hal– do not exhibit competitive relations during absorption. Competition is shown by chemically similar anions such as SO42- and SeO42-, or Hal– halides.
Nutrients that are in short supply enter the root first. Root cell vacuoles smooth out fluctuations in nutrient content in the external environment.
The ability of plants to repeatedly use elements of mineral nutrition is important. When assessing the availability of plant nutrients, it should be taken into account that some of them can be reused by the plant. For example, in the outflow from leaves to reproductive organs. Calcium, iron, manganese, boron, copper and zinc are not reutilized; sulfur is partially used in organic compounds, nitrogen, phosphorus, potassium, magnesium – repeatedly.
Deficiency of reusable nutrients is manifested primarily on older leaves. On the old parts of plants, the symptoms of excess of elements unsuitable for reutilization and in excess in the external environment are more pronounced.
Influence of soil moisture on the nutrient regime
The influence of soil moisture on nutrient supply to the plant is determined by the following physiological and physical factors:
- The general physiological condition of the plant. For example, normal tissue water content improves photosynthesis, protein biosynthesis and other metabolic processes.
- The development and location of roots with normal moisture content in the soil and the overall absorptive capacity.
With optimal soil moisture there is an increase in the total intake of macro- and micronutrients into the plant. When there is a deficit of moisture, the absorption of nutrients is hindered.
Excessive soil moisture has a negative effect on the absorption of nutrients and manifests itself in a one-way increase in the availability of some ions, particularly iron and manganese oxides, an excess of which in the plant is toxic.
On a hot day, a plant can evaporate about as much water in 1 h as it contains in the whole plant. Even the huge suction surface of the root system with insufficient soil moisture cannot provide enough water for transpiration under such conditions. A “midday depression of photosynthesis” occurs, with the stomata closing, the plants ceasing to evaporate water, the flow of carbon dioxide stops.
The water consumption required to create a unit mass of dry matter decreases under conditions of sufficient supply of plants with elements of mineral nutrition.
D.N. Pryanishnikov in experiments showed that water consumption per unit of dry matter on the background of fertilizer decreases by 36.5% at low soil moisture and by 20% at high.
Table. Water consumption for the creation of a unit of dry matter, l
Water transpiration by plants decreases at high relative humidity, while growth rate and nutrient uptake increase.
Table. Effect of air humidity on water evaporation by sunflower plants, yield and nutrient uptake (water crops)
The water reserve in the soil is significantly affected by the tillage system. For example, the water reserve in the field, repeatedly treated with herbicides, the water reserve is 4 times greater than in the mechanical treatment of the fallow field.
The limiting effect of soil moisture on crop yield depends on the provision of nutritional elements. Even in a water crop at high concentration of nutrient solution physiological dryness is manifested.
Optimal plant nutrient supply optimizes water intake, in turn, sufficient moisture increases fertilizer efficiency.
Relationship between air and nutrient regime of soils
The relationship between the air and nutrient regimes of soils was first shown by Hoagland.
Under partially anaerobic conditions, the supply of oxygen to root cells deteriorates and the carbon dioxide content increases. The relationship of root uptake capacity under aerobic conditions determines the nature of the uptake dependence on oxygen supply. Thus, one of the basic requirements in growing plants in aquatic culture is to purge the nutrient solution with air.
Different conditions of the air regime affect the absorption of different ions.
For the root system of whole plants and when growing a culture with isolated roots, the maximum absorption of elements occurs at the oxygen content of 2-3%. A further increase in oxygen concentration up to 100% does not affect the absorption rate. Phosphate uptake by barley roots does not depend on the partial pressure of oxygen in the range from 3 to 100%, provided the total gas pressure is equal to atmospheric pressure.
Conditions of aeration, temperature of air and nutrient medium affect the inflow of mineral nutrition elements to plants.
Table. Effect of temperature and aeration on tomato yield and nutrition
The optimum concentration of oxygen (about 3%) can vary depending on the type of crop. The oxygen content of the soil determines the redox potential of the substance system in the soil. Thus, metal cations with low valence (Fe2+, Mn2+, etc.) are more soluble and mobile.
Excessive content of carbon dioxide negatively affects the absorption of nitrates, phosphates and ammonium by the root system.
Relationship between heat and nutrient regime of soils
The roots of a plant cannot develop and absorb nutrients at low temperatures. Therefore, plants develop slowly in cold soils, which is not due to a lack of nutrients, but to the fact that the root system is unable to absorb water and nutrient ions in a cold environment. Temperature regime determines the accumulation of mobile forms of nutrients in the soil due to the mobilization of potential fertility and their absorption by plants from the soil and fertilizers applied.
Thus, with an increase in temperature from 10 to 25 °C, the mobilization of soil nutrients increases, and below 10 °C, these processes are suppressed. On all cold soils, the greatest effect on yield is observed when phosphorus fertilizers are applied, which is explained by the delayed intake of phosphorus under conditions of low temperature and lack of moisture.
At low temperatures, the metabolic absorption occurring in the active zone is suppressed, but absorption by diffusion occurs regardless of temperature. Increasing the concentration of the solution increases the absorption of nutrients by plants, so the absorption of nutrients at low temperatures can be enhanced by applying higher doses of fertilizers.
Low temperatures at the beginning of plant growth affect nitrogen and phosphorus nutrition, which is explained by poor mobilization and insufficient use of nitrogen and phosphorus reserves of seeds, their less intensive absorption from the external environment and delayed development of seedlings.
For most crops, the optimum temperature for nitrogen and phosphorus intake is 23-25 °C. However, the protein content of wheat grain increases when the temperature rises to 35 °C. It is likely that under conditions of sufficient moisture (60% of the smallest moisture capacity) and an increase in soil temperature, nitrogen mobilization increases, which is reflected in the protein content. Protein content in wheat grown in southeastern Russia is higher than in the northwest. Protein content correlates with an arid climate and elevated temperatures. Bristlegrass, soybeans, beans, and other southern crops also absorb nutrients better at 30-35 °C.
Absorption of ammonium nitrogen is possible at lower temperatures than for nitrate nitrogen.
At 5-7 °C, wheat seedlings had almost no decrease in potassium intake, but the absorption of nitrogen, phosphorus, calcium, and sulfur decreased markedly.
Each plant species and variety is characterized by certain temperatures corresponding to the most intensive absorption of mineral nutrients.
At temperatures below 10-11 °C, the inflow of phosphorus to plants is hindered. The intake of nitrate nitrogen worsens at temperatures below 5-6 °C. Lower temperatures also have a negative effect on potassium intake. Under optimal mineral nutrition, temperatures below 5-6 °C (according to other sources below 10 °C) is critical for the intake of major mineral nutrients. Lower temperatures inhibit the involvement of mineral nitrogen compounds in synthetic processes.
The absorption rate of mineral nutrient elements increases when the temperature increases to a certain limit, different for different plants.
Reduction of salt absorption at 40-50 °С is caused by inactivation of enzyme systems involved in the processes of ion assimilation.
Interrelation of light and nutrient regimes
The relationship between light and nutrient regimes manifests itself in the fact that plants, when absorbing nutrients, expend energy that they receive in the process of photosynthesis, which directly depends on the light regime.
Plants start to absorb nutrients intensively at the first rays of the sun. When plants are shaded, both photosynthesis and nutrient uptake decrease. Prolonged shading leads to a complete cessation of mineral nutrients, which is explained by the accumulation of organic substances used in respiration during photosynthesis. When plants are shaded, respiration gradually decreases.
Influence of the reaction of the medium on the nutrient regime
Changing the pH value of the soil due to liming leads to the replacement of hydrogen ions by calcium, which increases the availability of mineral nutrients to plants. Calcium inhibits the flow of hydrogen ions to plants, so with increased calcium content plants are able to tolerate a more acidic environment.
Table. Effect of calcium chloride on wheat root growth at different solution acidity
The reaction of the medium has an indirect and direct effect on plants. With a direct effect, the reaction of the soil solution affects the concentration of ions H+, HCO3–, OH– on the surface of root hairs, and, as a consequence, the concentration of these ions in the cell sap. As a result, the nature of nutrient supply changes. A shift in acidity toward acidic or alkaline reaction disturbs the physiological equilibrium of the ions, worsening the nutritional regime, leading to disruption of carbohydrate, protein and phosphorus metabolism.
Indirect effect is an increase in the concentration of hydrogen ions, accompanied by an increase in the content of mobile forms of aluminum, manganese and iron, which have a toxic effect on plants.
Table. Influence of solution pH on absorption of mineral nutrition elements by plants
Increasing the acidity of the solution improves the inflow of anions, and alkalinization promotes the absorption of cations. But in soil crops this pattern is not always manifested, since the inflow of nutrients depends on their mobility.
The concentration of hydrogen ions plays an important role in the absorption of phosphates by plants: the alkalinization of solutions changes the forms of phosphates from dihydroorthophosphates (H2PO4)– to hydroorthophosphates (HPO4)2- and orthophosphates (PO4)3-. The solubility of these forms also decreases, making phosphorus unavailable to plants.
Soil acidification reduces the availability of molybdenum, phosphorus, and calcium and increases the availability of boron. The percentage and removal of these substances with the harvest are reduced, plant metabolism and protein synthesis are disturbed, and the processes of transformation of monosaccharides into disaccharides and more complex organic compounds are slowed down. Increased acidity disturbs activity of root system enzymes: catalase, peroxidase activity increases, hydrolytic activity of proteolytic enzymes increases, which was established in experiments with different cultures by N.S. Avdonin.
In experiments D.N. Pryanishnikov found that the ammonium nutrition of sugar beet is optimal at pH 7.0, while the nitrate nutrition is optimal at pH 5.5. In the analysis a reduced calcium content in the leaves of plants was noted, because the excess of hydrogen and ammonium ions in the solution prevents calcium entry. The negative effect of hydrogen ions is stronger when the content of other cations is low. Proceeding from this provision, D.N. Pryanishnikov enhanced the development of beet plants at pH 5.5 on ammonium nutrition by adding increased amounts of calcium to the nutrient solution. Magnesium and potassium have a similar effect, but it appears less than when calcium is added.
Especially sensitive to the reaction of the environment plants in the initial period of growth. A shift in the reaction of the environment to an acidic pH of 3.5 during the period of 40-60 days after sprouting had no noticeable effect on the barley yield, but a shift in the reaction in the first 20 days and throughout the growing season dramatically reduced the yield.
Table. Effect of media reaction on barley yield, g per vessel
|pH 7.0 throughout the growing season|
|рН 3,5 throughout the growing season|
|pH 3.5 for the first 20 days, then pH 7.0|
|pH 3.5 from 40 to 60 days after sprouting, the rest of the time pH 7.0|
The indicative pH values given in the table are relative for crops, since many factors influence this value, such as the calcium content in the soil solution, which can reduce the negative effect of an acid reaction due to the antagonism of Ca2+ and H+ ions.
Table. Optimal or permissible reaction of soil solution for major crops
The effect of the environmental reaction on the plant depends on the concentration of the soil solution: increasing the concentration weakens the harmful effects of an acid reaction. The effect of the reaction of the medium depends on the forms of nitrogen fertilizers. Thus, in the background of ammonia forms acid reaction increases the harmful effect than in the background of nitrate forms. Chlorine also increases the negative effect of hydrogen ions. Phosphate fertilizers, on the contrary, weaken the negative effect.
High acidity has a negative effect on plants in low light due to reduced photosynthesis and lack of assimilates involved in metabolic processes. Under the influence of excessive acidity and under insufficient light, laying of generative organs and the process of fertilization are disturbed, the productive coefficient of tillering decreases, the number of spikelets and grains in the ear decreases, and grain filling is impaired.
The effect of high acidity of the environment is amplified by other negative factors. Thus, in experiments of the Department of Agrochemistry of Moscow State University, it was shown that acid reaction and excessive moisture decreases yield, probably due to insufficient aeration of the soil. At excessive acidity and moisture increases the negative effect on the formation of generative organs, the process of fertilization and grain filling. In the experiments, grain yield at acid reaction decreased at optimum moisture by 47.5% and at excessive moisture by 70.9%.
The influence of acidity on the absorption of nutrients by plants is determined by soil properties. For example, a decrease in pH with high content of iron, aluminum and manganese leads to an increase in their mobility and accumulation in plants. Iron and aluminum ions form compounds with phosphorus and molybdenum that are insoluble and inaccessible to plants.
Optimal for growth and productivity of most crops is slightly acidic environment – pH about 6.5.
In natural conditions, the reaction of the soil environment varies from pH 2.5-3 in sphagnum peats to pH 9-10 in saline soils.
In the experiments of G.Y. Rinkis it was shown that with a decrease in acidity the absorption of manganese, cobalt and zinc, and to a lesser extent of potassium and magnesium is inhibited. The author ranked the elements depending on the reduction of their intake into plants during acidification in the following series: Mn, Co, Zn, Cu, P, Fe, B, Mg, K, N, Mo.
Deficiency of manganese and zinc is more often observed in carbonate soils.
Plants more easily tolerate adverse reaction in soils with high absorption capacity and buffering.
The pH value of the soil solution affects the activity of soil microorganisms. Excessive acidity suppresses the activity of beneficial soil microflora (ammonificators, nitrifiers, nitrobacteria, etc.) and favors the development of pathogenic bacteria and fungi.
Physiological reaction of salts
All mineral salts used as fertilizer can be divided by chemical hydrolytic properties into:
The intensity of absorption of cations and anions in the process of plant nutrition is different, resulting in ions remaining in the nutrient solution causing acidification or alkalinization of the environment.
Physiological acidity of fertilizer – the property of fertilizer to acidify the reaction of the soil solution, associated with the predominant absorption of cations from the composition of the mineral salt by plants. Physiological alkalinity of fertilizer – the property of fertilizer to alkalize the reaction of the soil solution, associated with the predominant absorption of anions from the composition of the mineral salt.
Thus, the change in the reaction of the environment is influenced not only by the reaction of salts, but also by their physiological reaction.
Physiological reaction of salts is stronger in aqueous and sandy crops, i.e. in media with low buffering, so it should be taken into account when applying high doses of fertilizers. When physiologically acid salts are used, it is necessary to carry out advanced liming. Of the nitrogen-containing salts nitrogen is absorbed by plants in the first place. Therefore, ammonium salts are physiologically acidic, and nitrates are physiologically alkaline.
For example, sodium nitrate dissociates into ions Na+ and NO3–, the anion NO3– is consumed by plants in larger quantities than Na+ cation; as a result, hydrolytic alkaline salt NaHCO3 accumulates in solution. A similar reaction occurs when adding KNO3, Mg(NO3)2 and Ca(NO3)2.
Acidification of the solution is due to a more intense intake of ammonium into the plants, formed by the dissociation of NH4Cl and (NH4)2SO4. Application of these fertilizers requires neutralization of the formed acids by preceding liming. Physiologically acid reaction of these salts was established by D.N. Pryanishnikov.
The manifestation of the physiological reaction of ammonium nitrate depends on a number of factors determining the nitrate and ammonium nutrition of plants. Usually the physiological acidity of ammonium nitrate is much weaker than that of other ammonium salts.
The physiological acidity of potassium fertilizers is even weaker than that of ammonium fertilizers. For crops with little potassium requirement, such as oats and barley, the potassium salts were physiologically neutral, whereas for beets, sunflowers, and corn, which consume large amounts of potassium, the potassium salts were physiologically acid. Potassium-loving crops include potatoes, tobacco, and flax.
The resistance to maintaining the reaction of a nutrient solution depends on its composition. For example, a solution containing large amounts of calcium bicarbonate Ca(HCO3)2, formed by the interaction of CaCO3 with carbon dioxide in water, eliminates excess acidity by neutralization with calcium bicarbonate:
Ca(HCO3)2 + H2SO4 = CaSO4 + 2H2O + 2CO2.
Such solution is characterized by certain buffering capacity, and there are no noticeable changes in reaction of solution. Buffer capacity of soils depends on absorption capacity and composition of absorbed cations.
Regulating the nutritional regime of the soil
Regulation of the nutritional regime consists in human influence on the components of the balance of nutrients. Replenishment is possible, both at the expense of human activity, and at the expense of natural processes.
Tasks of regulation of a nutritious regime of soils is maintenance without deficiency, and, ideally, positive balance of nutrients, and also maintenance by them to plants in each phase of growth in sufficient quantities.
The objectives of regulation are achieved by influencing the sources of nutrient inputs and the expenditure side of the balance. This is accomplished by:
- Mineral and organic fertilizers are the most effective way to regulate nutrient supply;
- Improvement of air, heat and water regimes of soils;
- Use of rational system of soil processing, adapted to the conditions of a particular locality;
- Properly planned crop rotation;
- Control of weed vegetation.
Agrotechnical methods provide translation of inaccessible elements into forms easily accessible for plants as well as activation of processes of decomposition of organic matter and their mineralization.
Normalization of the reaction of soil solution significantly affects the availability of nutrients. To this end, lime treatment of acidic soils or gypsum treatment of saline soils is carried out.
Ensuring optimal soil moisture increases the efficiency of plants’ use of nutrients. Therefore, regulation of the water regime is closely related to the regulation of the nutrient regime. Under conditions of sufficient moisture the efficiency of fertilizers is maximum. Excessive moisture leads to an increased consumption of nutrients, not aimed at creating yields. Humidity also plays an important role in the activity of soil microorganisms and biota.
It is possible to regulate the nutrient regime by including in crop rotation crops with deep root system for better use of nutrient reserves of deep soil layers, as well as crops capable of transforming unavailable forms of substances into assimilable ones. In particular, mustard, buttercup, sweet clover, lupine and buckwheat transform inaccessible forms of phosphorus into accessible ones.
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