The effect of cation composition and exchange capacity on soil fertility

How the cation composition affects plant nutrition and soil properties

The reaction of soil solution and the physical state of arable land directly depend on the composition of exchangeable cations. In most fertile soils, calcium and magnesium form the basis of the absorbing complex. When soluble mineral fertilizers (e.g., NH₄NO₃ or KCl) are applied, the cations that previously predominated in the soil are displaced into the soil solution. In chernozems, this is harmless calcium, whereas in acidic sod-podzolic soils, hydrogen and aluminum are displaced, which leads to dangerous soil acidification.

Exchangeable cations also determine the physical soil structure by causing the coagulation of fine colloidal particles. The strength of this process directly depends on the valence of the cation and increases in an acidic environment (it weakens under an alkaline reaction). The ability of cations to bind soil particles increases in the following order: NH₄⁺ < Na⁺ < K⁺ < Mg²⁺ < Ca²⁺ < H⁺ < Fe³⁺ < Al³⁺.

If calcium predominates in the soil, colloids coagulate stably, forming a favorable fine-crumb structure. Conversely, a high concentration of sodium destroys the water stability of soil aggregates. As a result, the physical properties of the soil, as well as its water and air regimes, deteriorate.

• Ca and Mg in chernozems — 80–90 % of exchange capacity
• H and Al in krasnozems and podzols — 50 % or more
• CEC unit of measurement — mmol-eq/kg of soil

Factors influencing cation exchange capacity

Cation exchange capacity (CEC) is the maximum amount of cations that a soil can hold in an exchangeable state. This indicator characterizes the absorption capacity of arable land and determines how well the soil retains nutrients against leaching. The value of CEC depends on four key factors:

• Particle size distribution: the higher the content of clay and silt particles in the soil, the higher the absorption capacity. Clay and loamy soils hold cations significantly better than loamy sand and sandy soils.
• Mineralogical composition of colloids: a predominance of montmorillonite group minerals and hydromicas increases CEC. Kaolinite group minerals, as well as iron and aluminum hydroxides, conversely, decrease it.
• Humus content: organic matter drastically increases exchange capacity, which is why it is substantially higher in chernozems than in sod-podzolic soils.
• Environmental acidity: an acidic reaction reduces the negative charge of soil colloids, which in turn causes the cation exchange capacity itself to drop.

Data on the content of exchangeable cations in various soil types at different depths (in mmol-eq/kg) are provided below:

How excess hydrogen destroys soil and how to regulate cation composition

The accumulation of absorbed hydrogen gradually destroys the minerals of the soil absorbing complex. As a result of these processes, the physical structure of the soil in the field deteriorates. Additionally, the exchange capacity drops, which reduces the ability of the soil to retain nutrients.

The composition and ratio of exchangeable cations in the soil must be regulated artificially. To this end, chemical reclamation is used in agronomic practice. The choice of a specific ameliorant depends on the soil type and the nature of the problem:

• For acidic sod-podzolic soils, lime is applied;
• For solonets soils, gypsum is applied.