tiny particles of soil, from 0.2 to 0.001 micrometers. In soil water they form colloidal solutions (sols), and in this form they are carried from one soil layer to another and settle, forming horizons that are compacted to various degrees (for example solonets horizon in the soils of the arid steppes and forest-steppes and illuvial horizon in the podzolic soils of the northern taiga). In different soils the colloidal fraction ranges from 2 percent (light soil) to 30–50 percent (heavy soils).
Soil colloids are very important in agronomy: they absorb ammonium, potassium, calcium, magnesium, and phosphate ions from soil solutions, prevent the ions from leaching out, and promote the formation of soil structures. Soil colloids consist of three groups of compounds—organic, mineral, and organomineral (complex). Most of the organic matter in the soil is in dispersed colloidal form. Mineral colloids consist primarily of clayey minerals. The composition of the colloidal mineral fraction is different in different types of soil. The organomineral soil colloids are represented primarily by compounds of humus substances with clayey and other secondary minerals. Under natural conditions colloidal particles form in two ways—condensation and dispersion.
I. N. ANTIPOV-KARATAEV