Soil absorption capacity

Last updated: June 16, 2025

Soil absorption capacity — the property of soil to retain substances dissolved in the soil solution and contacting its solid phase. Soil absorption capacity is provided by the soil absorbing complex.

The doctrine of soil absorption capacity was developed by K. K. Gedroits; it was further developed in the works of G. Wiegner and S. Mattson.

Depending on the absorption mechanism, the following types of soil absorption capacity are distinguished:

mechanical;
physical;
exchange (physicochemical);
chemical;
biological.

Mechanical absorption capacity

Mechanical absorption capacity of soils — the ability to mechanically retain in soil pores particles suspended in water passing through the soil profile. In this case, suspensions of aluminosilicate and organic particles in colloidal state are retained.

Mechanical absorption capacity depends on soil porosity, pore size, and dispersion of the substance. Clayey and loamy soils are characterized by higher mechanical absorption capacity than sandy and sandy loamy soils.

Mechanical absorption capacity allows colloidal fractions of poorly soluble fertilizers and ameliorants to be retained in the soil. 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 — the ability to retain mineral and organic substances on the surface of the solid phase under the action of adsorption forces. Physical absorption is due to the presence of a highly dispersed soil phase, “soil + soil solution,” which possesses surface energy; owing to this, soil water and the substances contained in it are attracted and accumulated on the surface of solid soil particles. In coarse-grained, for example sandy, soils, adsorption forces are relatively small, so their physical absorption capacity is low. With an increase in soil dispersion, the amount of silt, and the colloidal fraction, adsorption forces increase.

Physical absorption in soil can also arise as a result of coagulation (sticking together) of colloidal particles. This process stops the leaching of the silt fraction of the soil.

In this type of absorption, the substances contained in the soil solution do not undergo changes.

An increase in the concentration of a dissolved substance in the layer of dispersion medium adjacent to solid phase particles 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; in negative adsorption, it repels them. Hydrates of metal oxides and salts formed by strong bases and weak acids (hydrolytically alkaline) are positively adsorbed. Anions are adsorbed negatively.

Positive adsorption reduces the leaching of dissolved substances into deep soil layers and maintains the concentration of nutrients in the root zone.

Due to the large number of factors capable of influencing physical absorption, this type of absorption is dynamic.

It is difficult to determine physical absorption because it is interrelated with chemical absorption and exchange adsorption.

Physical absorption is important for the rational use of soluble fertilizers. For example, chloride ions in large amounts are toxic to many crops, so chlorine-containing fertilizers are applied in autumn so that during autumn–spring precipitation they are leached from the arable layer. Conversely, for nitrate fertilizers such leaching is undesirable, so they are applied in spring before sowing or in top dressings.

The physical absorption capacity of soils has ecological significance: adsorption of pesticides reduces their penetration into adjacent environments, including plants.

Exchange absorption capacity

Exchange, or physicochemical, absorption capacity of soils is the ability of fine-dispersed colloids of soil particles with a negative charge to absorb cations from solution.

Exchange absorption in soils was first studied by K. K. Gedroits, who formulated the basic law of exchange adsorption of soils: all soils possess the ability to exchange the 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 solution.

During exchange absorption, electrolytes of the soil solution enter into an exchange chemical reaction with molecules on the surface of soil particles. During exchange absorption, the concentration of the solution does not change, but the composition of cations in the solution changes; the concentration of anions remains almost unchanged. When a cation is absorbed from solution, the soil releases into solution an equivalent amount of another cation that was previously in the solid phase:

$$\text{SAC} Ca + 2NH_4^+ \rightleftarrows \text{SAC} (NH_4)_2 + Ca^{2+};$$

$$\text{SAC} Ca + 2Na^+ \rightleftarrows \text{SAC} Na_2 + Ca^{2+};$$

The exchange reaction of a fertilizer with the soil absorbing complex (SAC) can be represented by the following equation:

$$\text{SAC} \begin{array}{c} Ca \\ Mg \\ H \end{array} + 5NH_4NO_3 \rightleftarrows \text{SAC} \begin{array}{c} NH_4 \\ NH_4 \\ NH_4 \\ NH_4 \\ NH_4 \end{array} + Ca(NO_3)_2 + Mg(NO_3)_2 + HNO_3.$$

The exchange absorption capacity of soils plays a huge role in plant nutrition and the transformation of fertilizers; it determines the reaction and buffer properties of the solid and liquid phases, as well as the cation–anion composition and concentration of the soil solution. The higher the absorption capacity of the soil, the more economically advantageous and environmentally safe is a single (reserve, periodic) application of high doses of fertilizers and ameliorants. Conversely, the lower the absorption capacity, the more effective is the fractional application of small doses of fertilizers and ameliorants.

Soil absorbing complex (SAC)

Soil absorbing complex (SAC) — a set of mineral, organic, and organomineral fine-dispersed particles (colloids) of the solid phase of the soil. The term was introduced by K. K. Gedroits.

Fine-dispersed particles have a size from 0.25 to 1 µm and are carriers of the exchange absorption capacity of the soil. With increasing size, the ability for exchange absorption decreases.

Mechanism of exchange absorption by organic colloids

Organic colloids are represented mainly by humic substances. The ability of soil organic matter to adsorb cations is determined by the acidic (acidoid) nature of humus substances, owing to which organic colloids have a negative charge from carboxyl groups:

$$RCOOH \to RCOO^- + H^+.$$

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

$$(RCOO)_2Ca + 2K^+ \rightleftarrows 2RCOOK + Ca^{2+}.$$

Mechanism of exchange absorption of cations by mineral colloids

Mineral colloids include crystalline clay minerals of the kaolinite and montmorillonite groups, hydromicas, and amorphous compounds (hydrates of metal oxides, silicic acid, etc.).

The ability for cation exchange in mineral particles of the fine-dispersed fraction is also associated with a negative charge. The appearance of a negative charge in soil minerals is due to isomorphic substitutions in silicate and aluminosilicate structures. A compound of composition (SiO₂))_n is neutral, but if in the silica–oxygen structure part of the silicon atoms in tetrahedral coordination is replaced by aluminum atoms, this leads to the formation of a negative charge:

$$\underset { \text{neutral structure}} {[(SiO_2))_n]}$$

$$\underset { \text{structure with a negative electric charge}} {[(SiO_2)_{n-1}AlO_2]^-}$$

The negative charge in aluminosilicates is balanced by a corresponding amount of cations.

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

$$\lbrack (SiO_2)_{n-1}AlO_2 \rbrack K → \lbrack (SiO_2)_{n-1}AlO_2 \rbrack ^- + K^+.$$

The high absorption capacity of minerals of the montmorillonite group is explained by the structure of the crystal lattice, which consists of flat packets of the following form:

$$\dots O_3Si_2O_2OHAl_2OHO_2Si_2O_3 \dots$$

Such a structure is neutral; the aluminum located in the central part of the packet forms a layer with octahedral coordination, and in this position no negative charge is imparted to the aluminosilicate structure. However, a charge appears if aluminum is partially replaced by a silicon atom in the silica–oxygen layer:

$$\lbrack \dots O_3Si_2O_2OHAl_2OHSiAlO_2 \dots \rbrack ^-.$$

Thus, the influence of aluminum on the charge of the aluminosilicate structure is determined by its position. Aluminum forming an independent aluminum–oxygen or aluminum–hydroxyl layer with octahedral coordination does not possess acidoid properties. Montmorillonite can acquire a negative charge due to isomorphic substitution, for example, partial replacement of aluminum in octahedral coordination by magnesium:

$$\lbrack \dots O_3Si_2O_2OHAlMgOHO_2Si_2O_2 \dots \rbrack ^-.$$

The high exchange capacity of minerals of this type is also due to the exchange capacity of the inner part of the microcrystalline structure, because upon swelling of the mineral the solution penetrates into the interpacket spaces of the crystal lattice. Other minerals of the fine-dispersed fraction, for example kaolinite, have denser packing, as a result of which cation exchange occurs only on the external surface of the microcrystals.

Mechanism of exchange absorption of anions

Particles of organic and mineral colloids carry a negative charge, except for colloids of aluminum and iron hydroxides. The presence of a negative charge explains the ability of most soils to absorb cations rather than anions.

However, clay minerals of the kaolinite group under acidic reaction conditions can acquire positive charges. The composition and structure of kaolinite packets can be represented as follows:

$$\dots O_3Si_2O_2OHAl_2(OH)_3 \dots$$

Hydroxyl groups bound to aluminum in the octahedral layer located on the external surface of microcrystalline kaolinite particles can be split off under acidic reaction, forming a positive charge on individual areas of the microcrystalline particle. Compared with minerals of the montmorillonite group, kaolinite contains more aluminum and less silicic acid. Aluminum not only does not impart acidoid properties but can exhibit basic (basoid) properties:

$$\lbrack \dots O_3Si_2O_2OHAl_2(OH)_2 \dots \rbrack ^+ + OH^-.$$

To an even greater extent, basic properties are manifested in an acidic medium by free hydroxides of aluminum and iron not bound to silicic acid:

$$\lbrack Fe(OH)_3 \rbrack _n \to \lbrack Fe_n(OH)_{3n-1} \rbrack ^+ + OH^-,$$

$$\lbrack Al(OH)_3 \rbrack _n \to \lbrack Al_n(OH)_{3n-1} \rbrack ^+ + OH^-.$$

Microcrystalline particles with a positive charge are able to 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 the cation exchange capacity and the higher the ability for exchange absorption of anions.

The basic properties of such colloids are manifested in strongly acidic sod-podzolic soils, krasnozems, zheltozems, bog soils, and soils enriched with iron and aluminum hydroxides. An example of exchange absorption of fertilizer anions can be represented by the equation:

$$\lbrack Al(OH)_2 \rbrack HPO_4 + (NH_4)_2SO_4 \leftrightarrows \lbrack Al(OH)_2 \rbrack SO_4 + (NH_4)_2HPO_4.$$

Equilibrium of exchange absorption

The exchange reaction proceeds in equivalent ratios and is reversible. A dynamic equilibrium is established between the solid phase of the soil and the solution. The equilibrium depends on the composition and concentration of the solution, the nature of anions and cations, and the properties of the soil. Fertilizers, ameliorants, the degree and intensity of organic matter mineralization, moisture, ion consumption by plants, and the vital activity of microorganisms influence the equilibrium.

The rate of exchange reactions is high: within 3–5 minutes from the application of water-soluble fertilizers, up to 85% of the cations of the SAC are exchanged with the SAC for an equivalent amount.

Energy of cation absorption

To assess the sorbability of cations, K. K. Gedroits introduced the concept of “energy of cation absorption.” Different cations are characterized by different energies of absorption and retention in the absorbed state. The higher the relative atomic mass and the greater the charge of the cation, the stronger its absorption and the more difficult its displacement from the absorbed state. In the series of monovalent cations, absorption increases in the order:

$$Li^+ < Na^+ < NH_4^+ < K^+ < Rb^+;$$

the relative atomic masses of these cations are respectively 7, 23, 18, 39, 85. Among monovalent cations, due to the smaller sizes of ions in the hydrated state, the exception is ammonium and hydrogen H^{+ or H₃O}+ (hydronium), which in absorption energy exceeds calcium by a factor of 4 and sodium by a factor of 17.

Divalent cations are arranged in the series:

$$Mg^{2+} < Ca^{2+} < Co^{2+};$$

relative atomic mass 24, 40, 59 respectively.

Trivalent:

$$Al^{3+} < Fe^{3+};$$

relative atomic mass 27 and 56 respectively.

In increasing order of absorption ability, cations are arranged in the series:

$$Li^+ < Na^+ < NH_4^+ < K^+ < Rb^+ << Mg^{2+} < Ca^{2+} < Co^{2+} << Al^{3+} < Fe^{3+}.$$

The increase in the energy of cation absorption with increasing atomic mass is explained by a decrease in the degree of hydration. Weakly hydrated cations are more strongly attracted by the colloid surface. Kaolinite-group minerals absorb cations by the external surface of the crystal, since they have a small free space between packets of the crystal lattice (2.8 Å). In montmorillonite-group minerals, the free space between packets is larger and amounts in the dry state to 9.4 Å, and upon swelling up to 21 Å.

Non-exchange absorption of cations

Cation exchange can occur on the external (extramicellar) and internal surface of colloids (intramicellar).

Clay minerals of the montmorillonite group with a three-layer crystal lattice, expanding upon moistening, can absorb cations extramicellarly and intramicellarly. When the soil dries, the interpacket space contracts and locks the absorbed cations in hexagonal cavities. Upon moistening, the cations again become available to plants.

The absorption of potassium and ammonium cations increases with increasing content of montmorillonite-group minerals. The application of ammonia and potassium fertilizers into soil layers with stable moisture (deeper layers) allows reducing non-exchange absorption (fixation) of cations and preserving them in a plant-available form. In chernozem soils, non-exchange absorption is more pronounced than in sod-podzolic soils. It increases with periodic moistening and drying.

Cation absorption capacity of soil

Cation absorption capacity of soil, or cation exchange capacity (CEC), is the maximum amount of cations that can be exchange-absorbed by the soil. The absorption capacity for a particular soil is a constant value.

T = \frac{m}{A},

where

m: — maximum amount of cations that can be exchange-absorbed by the soil, mmol;
A: — atomic mass of the cation.

Absorption capacity is expressed in millimoles (mmol) per 100 g of soil, or the outdated expression mg-eq per 100 g.

Example 1.

If 100 g of soil contains 500 mg of absorbed calcium in the absence of other cations, then the absorption capacity will be equal to:

500 : 20 = 25 mmol-equivalent/100 g soil,

where

20: — equivalent atomic mass of calcium,

and 1 mmol-equivalent of calcium is equal to:

40 : 2 = 20 mg,

where

40 mg/mmol: — atomic mass of calcium,
2: — equivalent of the substance, equal to the valence of the calcium atom.

If all calcium (in the example, 500 mg) is displaced by ammonium cation NH₄⁺, then 450 mg NH₄⁺ will be absorbed, since the equivalent mass of the NH₄⁺ ion is 18 mg. Therefore, upon exchange of Ca²⁺ for NH₄⁺, 18 mg of ammonium is absorbed in place of 20 mg Ca. The absorption capacity remains the same: in millimole-equivalents it will also be equal to 25 mmol/100 g soil, since 450 : 18 = 25.

Example 2.

If 100 g of soil contains in exchange-absorbed state 160 mg calcium, 24 mg magnesium, and 3 mg hydrogen, then CEC (T) will be equal to:

T = 160/20 + 24/12 + 3/1 = 13 mg-eq/100 g,

where

20, 12, 1 mg-eq/mmol: — equivalent atomic masses of calcium, magnesium, and hydrogen, respectively.

Factors influencing absorption capacity

Influence of the content of highly dispersed particles in the soil

Soils with a high content of fine-dispersed particles are characterized by high absorption capacity:

clayey soils have a higher absorption capacity than sandy soils;
soils rich in organic matter have a higher absorption capacity than poor ones.

Influence of the chemical and mineralogical composition of soil colloids

Different groups of soil colloids differ in absorption capacity. Among clay minerals, montmorillonite minerals, characterized by high dispersion, have the greatest cation absorption capacity; their absorption capacity reaches 60–150 mmol/100 g of mineral. Kaolinite is characterized by low dispersion and active surface; its cation absorption capacity is 3–15 mmol/100 g of mineral.

Organic soil colloids have greater absorption capacity compared with mineral ones. The cation exchange capacities of humic acids of different origin are determined by the presence of functional groups: humic acids of podzolic soils have a capacity of about 350 mmol/100 g of substance at pH = 7.0, while humic acids of chernozems and chestnut soils have 400–500 mmol/100 g under the same conditions. Therefore chernozems are distinguished by a higher absorption capacity — 40–60 mmol/100 g soil (according to other data, 20–70 mmol/100 g soil) compared with sod-podzolic soils, whose absorption capacity is 10–15 mmol/100 g (for sandy sod-podzolic soils, 5–10 mmol/100 g). The absorption capacity of the organic matter of the solid phase of the soil is 10–30 times greater than that of the mineral part, and at a humus content of 5–6%, it accounts for more than 50% of the total capacity.

The absorption capacity of amorphous mineral colloids depends on the ratio in their composition SiO₂ : R₂O₃. The more silicic acid and the less aluminum and iron oxides, the higher the absorption capacity. This explains the low absorption capacity of sod-podzolic soils.

Influence of the soil reaction

Soil colloids exhibit acidoid properties to a greater extent under neutral or slightly alkaline conditions. Under acidic reaction, the charge of soil colloids decreases, reducing the absorption capacity.

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 chernozem, chestnut soils, and sierozems, calcium and magnesium predominate in the composition of absorbed cations, with a small share of sodium; hydrogen is absent. In solonetzes and solonchaks, the exchange-absorbed state contains calcium, magnesium, and sodium. In krasnozems, zheltozems, podzolic, and sod-podzolic soils, the absorbed cations include calcium, magnesium, aluminum, hydrogen, and iron. Potassium and ammonium cations in the exchange-absorbed state are present in small amounts in all soils.

A content in the SAC of the alkaline cations potassium and sodium of more than 3–5% of the CEC leads to dispersion of the colloidal fraction, sharply worsening the properties of alkaline soils. Upon interaction with fertilizers, ameliorants, and mineral salts of the soil solution, absorbed sodium passes into solution and forms hydrolytically alkaline salts:

$$\text{SAC} Na_2 + Ca(HCO_3)_2 \leftrightarrows \text{SAC} Ca + 2NaHCO_3;$$

$$NaHCO_3 + H_2O \leftrightarrows NaOH + H_2O + CO_2.$$

A high content in the SAC of exchange-absorbed cations of hydrogen, aluminum, iron, and manganese, which is characteristic of podzolic and bog soils, zheltozems, and krasnozems, worsens the properties of acidic soils. The hydrogen cation gradually destroys the mineral colloids of the SAC, reducing the exchange capacity and worsening soil structure.

Optimal cation exchange capacity

The higher the cation absorption capacity of the soil, the greater the amount of various cations it is able to retain from leaching, thereby providing better conditions for plant nutrition, growth, and development. In addition, an increase in cation exchange capacity positively affects the resistance of soils to anthropogenic impact, in particular to chemical pollution.

Management of cation exchange capacity

The ratios and composition of exchange-absorbed cations of different soils are regulated by the application of fertilizers and ameliorants. In acidic soils, hydrogen, aluminum, iron, and manganese cations are displaced from the SAC by the application of lime ameliorants; in alkaline soils, sodium cations are displaced by the application of gypsum-containing ameliorants. In both cases, cations that negatively affect crops are replaced by calcium.

Chemical absorption capacity

Chemical absorption capacity of soils — the ability of soil to convert anions and cations of the soil solution into sparingly soluble substances.

Nitrates and chlorides (NO₃^- and Cl^-) do not participate in chemical absorption, since they do not form sparingly soluble substances with any cation of the soil solution. Carbonates and sulfates (CO₃^2-, SO₄^2-) are chemically absorbed at a high content in soils of calcium Ca²⁺ and magnesium Mg²⁺. Phosphates quite easily form sparingly soluble compounds with all divalent and trivalent cations (Ca²⁺, Mg²⁺, Fe³⁺, Al³⁺).

In neutral soils or soils with a slightly alkaline reaction, containing exchange-absorbed calcium, calcium or magnesium bicarbonates in the soil solution, chemical absorption of phosphate ions occurs as a result of the formation of sparingly soluble calcium and magnesium phosphates. Thus, when water-soluble phosphorus fertilizers are applied to soil with a high calcium content, the following reactions occur:

$$Ca(H_2PO_4)_2 + Ca(HCO_3)_2 \to 2CaHPO_4 + 2H^+ + CO_3^{2-},$$

$$Ca(H_2PO_4)_2 + 2Ca(HCO_3)_2 \to Ca_3(PO_4)_2 + 8H^+ + 4CO_3^{2-}.$$

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

Along with chemical absorption (retrogradation) of phosphates in soils, the reverse process is possible — mobilization of phosphorus: its conversion from unavailable forms into available ones. Mobilization occurs upon acidification of soils:

$$Ca_3(PO_4)_2 + 2H^+ = 2CaHPO_4 + Ca^{2+};$$

$$2CaHPO_4 + 2H^+ = Ca(H_2PO_4)_2 + Ca^{2+}.$$

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 intensively this process occurs, not inferior to the absorption of monosubstituted phosphates.

In an acidic medium, sparingly soluble phosphates of iron and aluminum can form:

$$Fe(OH)_3 + H_3PO_4 \rightarrow FePO_4 + 3H_2O,$$

$$Al(OH)_3 + H_3PO_4 \rightarrow AlPO_4 + 3H_2O.$$

Unlike calcium phosphates, aluminum and iron phosphates are insoluble in soil solution with an acidic reaction. Therefore, chemical absorption of phosphates occurs differently in different soils. In soils with an acidic reaction, insoluble phosphates of iron, aluminum, and manganese are formed, while in neutral and alkaline reactions, sparingly soluble calcium phosphates are formed.

Freshly precipitated trisubstituted phosphates of iron, aluminum, and calcium can be absorbed by plants owing to root exudates; however, as the precipitates age, their availability to plants decreases. Disubstituted calcium phosphate (CaHPO₄) is soluble in weak acids and therefore is assimilated by plants due to acidic root exudates. During the period from seed germination to the appearance of developed roots, plants consume only water-soluble monosubstituted phosphates. However, it is these that are rapidly chemically bound in all soil types both with soluble and with absorbed ions of calcium, magnesium, iron, and aluminum.

In increasing order of the ability for chemical absorption of phosphates, soils are arranged in the sequence: chernozems < sierozems < sod-podzolic soils < krasnozems.

Intensive absorption of phosphates necessitates the application and incorporation of phosphorus fertilizers to a certain depth near the location of roots, since they cannot migrate. To reduce the chemical absorption of available phosphorus forms of fertilizers, the total contact surface with the soil is reduced by granulating powdered forms and localizing them during pre-sowing application.

Biological absorption

Biological absorption — absorption of nutrients, fertilizers, and air by plants and soil microflora. In the process of the vital activity of plants and soil microorganisms, organic matter accumulates, including ash elements and nitrogen. Biological absorption includes the fixation of free nitrogen by nitrogen-fixing microorganisms.

Part of the nutrients applied to the soil with fertilizers is consumed by microorganisms. Thus, in cultivated sod-podzolic soils, the plasma of microbial cells contains about 125 kg of nitrogen, approximately 40 kg of P₂O₅, and about 25 kg of K₂O per 1 ha.

A feature of the biological absorption process is selectivity: plant roots and microorganisms assimilate from the soil those elements they need.

Biological absorption is the only way of long-term retention of nitrates and chlorides in the soil. According to studies conducted by the radioisotope method using the ¹⁵N isotope, 10–20% of nitrogen in nitrate form and 20–40% in ammonium form of the applied nitrogen fertilizers is fixed in the soil in organic form.

The intensity of biological absorption is influenced by:

the air regime of soils;
the content of soil organic matter, plant residues, and organic fertilizers;
the thermal regime of soils;
the reaction of the medium.

With a strongly pronounced biological absorption process, a negative influence on plant nutrition is possible.

Regulation of biological absorption is achieved by the use of organic, mineral, and bacterial fertilizers, liming, crop rotation, the timing and methods of soil tillage, and other agrotechnical practices.

References

1. Mineev V. G., Sychev V. G., Gamzikov G. P., et al. Agrochemistry. Textbook. / Ed. by Mineev V. G. Moscow: Publishing House of the All-Russian Research Institute of Agrochemistry named after D. N. Pryanishnikov. 2017.

2. Yagodin B. A., Zhukov Yu. P., Kobzarenko V. I. Agrochemistry. / Ed. by B. A. Yagodin. Moscow: Kolos. 2002.