(colloidal complex), a combination of fine mineral, organic, and organomineral compounds insoluble in water, created during soil formation and partly inherited from the parent material. The mineral part of the soil adsorption complex consists of the compounds SiO2, Al2O3, and Fe2O3 and products of the synthesis of their colloidal hydrates, formed by mutual coagulation. The organic part contains free or almost free humic substances and salts of humic and fulvic acids. Its composition, properties, and stability depend on the conditions of soil formation and development. The organic part possesses greater mobility than the mineral part and is more readily leached out of the soil. The quantity of humic substances and salts is an important criterion in classifying soils. The organomineral part contains chelates and adsorption compounds of humic and fulvic acids with particles of clay minerals.
The soil adsorption complex, especially its colloidal fraction with particles less than 0.1 micron (μ) in diameter, determines the adsorbing capacity of soil. The specific surface and surface energy of soil increase with fineness, thus stimulating chemical activity and exchange capacity. Fractions ranging from 0.1 to 5 μ are of less significance for ion exchange and particles larger than 5 μ do not participate in it.
The soil adsorption complex plays an important part in soil formation and in the evolution and genesis of soils. Many soil properties that determine the level of fertility—its texture, physicochemical properties, and water, air, microbiological, and nutrient regimes—are largely dependent on the nature and composition of the soil adsorption complex. Study of the soil adsorption complex is thus important for developing methods of improving land.
IU. A. POLIAKOV