Soil colloids are extremely small particles of soil with particle sizes of 2 micrometers in diameter or smaller suspended in a soil with larger particles. Soil colloids are typically found in clay or humus soils.
Soil colloids are an important part of the electrical and chemical properties of a soil. Extremely small particle size results in a high surface area to mass ratio, and the particles tend to have a net negative charge. The net negative charge of the soil colloids attracts and holds positively charged ions. This cation exchange allows the soil to hold positively charged ions like calcium, potassium and sodium. This prevents these nutrients from being lost and allows plants to absorb these necessary nutrients and grow.