Absorption capacity of soils is the property of soil to retain substances dissolved in the soil solution and in contact with its solid phase. Absorption capacity is provided by soil absorbing complex. Substances may be retained in dissolved state, in the form of colloidal particles and suspensions.
Depending on a way of absorption types of absorption capacity of soils are distinguished:
- mechanical,
- physical,
- physical-chemical (exchange),
- chemical,
- biological.
Soil absorbing complex (SAC) – a set of mineral, organic and organomineral particles of the soil solid phase.
The doctrine on absorbing capacity of soils for the first time was created by K.K. Giedroytz, it was further developed in works of G. Wigner and S. Mattson.
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 (СO32-, SO42-) 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+ + СО32-,
Са(Н2РO4)2 + 2Са(НСO3)2 → Са3(РO4)2 + 8Н+ + 4СО32-.
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…]–.
[…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 → [Fen(OH)3n-1]+ + OH–,
[Al(OH)3]n → [Aln(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.