Soil adsorption capacity: types, mechanisms, and significance in agricultural chemistry

If a solution of certain substances is passed through the soil, they will be retained within it. This ability to retain substances from a solution has been known for over two thousand years. For instance, the works of Aristotle (384–322 BC) indicate that seawater loses its salinity and becomes suitable for drinking after coming into contact with the soil.

Research by J.T. Way, published between 1850 and 1854, played a major role in the study of soil sorption capacity. He formulated the fundamental principles of soil sorption capacity that remain significant to this day. K.K. Gedroyc made a tremendous contribution to the study of this phenomenon. The results of years of research, which he began in 1908, were summarized in his monograph "The Doctrine of Soil Sorption Capacity," published in 1922 and subsequently reprinted many times. He closely linked the study of soil sorption capacity with the development of theoretical and practical issues regarding fertilizer application, plant nutrition, and chemical land reclamation of soils.

Subsequently, many scientists significantly expanded and deepened the knowledge of soil sorption capacity. In the Kuban region, such work was carried out by A.A. Shmuk. However, the foundations of K.K. Gedroyc’s doctrine have not become obsolete even today.

According to one definition, soil sorption capacity is its ability to absorb ions and molecules of various substances from a solution and retain them.

Sorption capacity regulates the nutritional regime of the soil, soil reaction, its buffering capacity, and its hydro-physical properties.

K.K. Gedroyc identified 5 types of sorption capacity: biological, mechanical, physical, chemical, and physico-chemical (exchange). He also noted non-exchangeable cation absorption. Subsequent research in our country and abroad allows for the identification of non-exchangeable absorption as a separate, 6th type of soil sorption capacity.

Biological sorption capacity is the absorption of nutrients from the soil by plants and soil microflora. As a result, these elements are protected from leaching out of the soil.

As K.K. Gedroyc noted, thanks to biological absorption, nutrients accumulate in the upper soil horizons, and to a certain extent, the necessary balance between these elements is created.

A feature of biological absorption is its selectivity, which consists in the fact that plants and microorganisms assimilate from the soil those elements necessary for their vital activities. Microorganisms use the same chemical elements as higher plants – N, P, K, Ca, Mg, Fe, Cu, Mo, Zn, Mn, and others. In this respect, they compete with crop plants for mineral compounds, primarily soil and fertilizer nitrogen.

Studies have shown that 10–20% of nitrate nitrogen and 20–40% of ammonium nitrogen fertilizers are fixed in the soil in an organic form. Biological absorption is especially important for nitrates, as this is practically the only type of absorption to which they are subjected. Nitrates not assimilated by plants are retained mainly due to their consumption by microorganisms.

Immobilization of mineral nutrition elements due to biological absorption is relatively short-lived and is determined by the life cycle of microorganisms. Subsequently, they are released as a result of the mineralization of microbial plasma and can be used by plants or the next generation of microorganisms.

In the event that the soil contains a large amount of organic substances that serve as an energy source for the latter, intensive development of microflora occurs, and the nitrogen consumed by it becomes unavailable to plants for a long time. This must be taken into account when using fertilizers.

For example, it is not recommended to apply strawy manure containing large amounts of fiber and other non-nitrogenous substances, which trigger the reproduction of cellulose-decomposing bacteria that aggressively absorb mineral nitrogen compounds from the soil. And when fertilizing with straw, it is necessary to apply mineral nitrogen fertilizers in accordance with the rates:

The same thing happens when applying only phosphate fertilizers. By absorbing phosphorus in certain ratios with nitrogen, microorganisms use more mineral nitrogen from the soil than they would without phosphate fertilizers. As a result, crop plants find themselves in conditions of nitrogen deficiency. Thus, knowledge of biological sorption capacity makes it possible to regulate the nutritional regime of the soil and increase the efficiency of fertilizers.

The ability of the soil to mechanically retain solid particles from suspensions and colloidal solutions filtering through the soil, which contain a significant amount of nutrients, is called mechanical sorption capacity.

Soil is a porous body. The soil solution, in addition to molecularly dissolved substances, contains colloidal and similarly sized particles in suspension. As the solution passes through pores with a diameter smaller than the suspended particles, the latter will be trapped by the soil. For instance, when muddy water passes through the soil, it becomes almost clear. The mechanical sorption capacity of the soil ensures the preservation of the most valuable colloidal fraction in the soil.

Absorption levels depend on the composition of the soil:

• Soils with a heavy texture—clay and loamy soils—possess the highest absorption capacity.
• Soils with a light texture—sandy loam and sandy soils—possess the lowest absorption capacity.

Certain fertilizers produced in the form of finely ground particles are not leached from the soil due to mechanical absorption.

Physical and chemical absorption: risks of nitrogen loss and phosphorus fixation

The ability of the soil to retain molecules of various substances on the surface of the solid phase is called physical absorption capacity, or molecular sorption. This process occurs in the form of positive or negative adsorption. In positive adsorption, soil particles attract dissolved substances (e.g., organic acids and high-molecular-weight compounds) more strongly than water, concentrating them at their surface. In negative adsorption, water is attracted more strongly, which causes the concentration of the dissolved substance in the free soil solution to increase.

Chemical absorption capacity is based on the formation of insoluble or poorly soluble precipitates as a result of reactions between soluble compounds. For example, the interaction of ammonium hydrogen phosphate with calcium bicarbonate results in the formation of poorly soluble calcium hydrogen phosphate: (NH₄)₂HPO₄ + Ca(HCO₃)₂ = CaHPO₄↓ + 2NH₄HCO₃. The anions of nitric and hydrochloric acids (NO₃^–, Cl^–) do not form poorly soluble salts with soil cations (NH₄⁺, K⁺, Ca²⁺, Mg²⁺, Fe³⁺) and are not chemically absorbed. The anions of carbonic and sulfuric acids (CO₃^2–, SO₄^2–) yield soluble salts with monovalent cations, and poorly soluble ones with divalent cations.

Chemical absorption binds phosphorus most intensively. The anions of orthophosphoric acid (H₂PO₄^–, HPO₄^2–) form soluble salts only with monovalent cations (KH₂PO₄, Na₂HPO₄, NH₄H₂PO₄). Their compounds with divalent cations vary in solubility: calcium dihydrogen phosphate Ca(H₂PO₄)₂ is highly soluble, calcium hydrogen phosphate CaHPO₄ is poorly soluble, and calcium phosphate Ca₃(PO₄)₂ is practically insoluble. Compounds of phosphorus with aluminum and trivalent iron (AlPO₄, FePO₄) are also classified as poorly soluble, which sharply reduces the availability of the element for plants.

• Chernozems — lowest intensity of phosphorus fixation
• Serozems — increased intensity of phosphorus fixation
• Sod-podzolic soils — high intensity of phosphorus fixation
• Krasnozems (red soils) — maximum intensity of phosphorus fixation

In acidic sod-podzolic soils, krasnozems, and brown forest soils, which contain many aluminum and iron hydroxides, aluminum and iron phosphates are actively formed. Freshly precipitated forms can still be assimilated by plants, but over time they compact, crystallize, and become unavailable, which is especially characteristic of iron phosphates. In neutral or slightly alkaline soils, phosphorus fixation occurs through interaction with calcium bicarbonate: Ca(H₂PO₄)₂ + Ca(HCO₃)₂ = 2CaHPO₄↓ + 2H₂CO₃ or Ca(H₂PO₄)₂ + 2Ca(HCO₃)₂ = Ca₃(PO₄)₂↓ + 4H₂CO₃. The resulting calcium hydrogen phosphate is soluble in weak acids and available to crops, whereas calcium phosphate is only partially assimilated by roots until the moment of its crystallization.

Water-soluble salts of orthophosphoric acid can also be chemically absorbed through interaction with exchangeable calcium of the soil: (soil) Ca + Ca(H₂PO₄)₂ → (soil) 2H + 2CaHPO₄↓. Any chemical absorption reduces the concentration of the soil solution due to the precipitation of salts. In practical agriculture, this process causes low mobility of phosphates and reduces the efficiency of applied phosphorus fertilizers.

Physicochemical absorption capacity: cation exchange

Physicochemical, or exchange, absorption capacity is the ability of the soil to absorb cations and anions from a solution with the simultaneous release into the solution of an equivalent quantity of previously absorbed ions. This process takes place at the interface of the solid and liquid phases of the soil. If soil saturated with calcium is treated with a potassium chloride solution, potassium cations will be fixed in the soil absorption complex. At the same time, an equivalent quantity of calcium cations will move into the solution, and instead of KCl, CaCl₂ will appear in the liquid phase.

Nutrient reserve: how the soil regulates the composition of the soil solution

Cations in the soil are in a state of dynamic equilibrium between the soil solution and the absorption complex. When potassium or ammonium fertilizers are applied, the solution concentration temporarily increases, and a portion of the cations transitions into an absorbed state. When plants absorb nutrients from the solution, their deficit is replenished by the release of previously absorbed ions.

The situation with anions is different—their exchange absorption directly depends on the acidity of the medium. This process occurs primarily in acidic soils (sod-podzolic, red earth, and yellow earth soils) through exchange with hydroxyl ions (OH^-). In neutral and alkaline soils, anions are practically not absorbed. When planning nutrition, it is important to consider the behavioral characteristics of specific anion groups:

• Nitrates (NO₃^-) and chlorides (Cl^-) are practically not retained by the soil. They move freely with moisture and are easily leached from the root zone. Their insignificant absorption is possible only in acidic red earth soils.
• Sulfates (SO₄^2-) are weakly retained in sod-podzolic soils, but are firmly bound in acidic red earth and yellow earth soils.
• Phosphates (anions of orthophosphoric acid) are absorbed in sod-podzolic soils and partially in chernozems, while remaining available to the plant root system.

Non-exchange fixation: why potassium and ammonium are blocked

Non-exchange absorption (fixation) is a process in which the soil firmly secures cations within the crystal lattice of clay minerals (hydromica and montmorillonite). Unlike exchange absorption, these elements are not displaced by neutral salts and transition into a form that is difficult for plants to access. Potassium (K⁺) and ammonium (NH₄⁺) are most susceptible to this type of fixation, followed to a lesser extent by calcium (Ca²⁺), magnesium (Mg²⁺), and hydrogen (H⁺).

The intensity of fixation depends on the type and soil texture. In chernozems, non-exchange absorption of potassium and ammonium is more pronounced than in sod-podzolic soils. The heavier the soil mechanical composition (the more clay particles), the higher its fixing capacity. Also, alternating wetting and drying of the soil promotes the fixation of elements, although the process also occurs in a moist state.

Blocked ammonium is not subjected to nitrification processes. However, the strength of its bond depends on accompanying elements in the solution. Fixed ammonium cannot be displaced by cations that compress the crystal lattice of minerals (potassium K⁺, rubidium Rb⁺, cesium Cs⁺). At the same time, it can be replaced by cations that cause lattice swelling: sodium (Na⁺), lithium (Li⁺), magnesium (Mg²⁺), and calcium (Ca²⁺).