Very small insoluble particles in soil, with a high surface area relative to mass. Diameter normally falls in the range 1 to 1 000 nm. The particles may be inorganic (e.g., clay mineral particles) or organic (e.g., macromolecular breakdown products of soil organic matter), and are capable of dispersion in liquid or gas. In such a case the particles are referred to as the dispersed phase and the dispersion medium (soil water for example) the continuous phase. The term is also applied to the system of particles plus the dispersion medium. The combination of clay and humus is often referred to as the colloidal fraction of a soil. Because of the high surface area, the colloidal fraction determines the chemical properties of soils. Colloidal particles in suspension are capable of migrating within the profile (eluviation). The presence of ions in the soil solution, above a threshold value, causes the colloidal particles to flocculate.
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