Clay as a constituent of soil is defined as being less than 0.002 mm. in diameter. These particles adhere together to form a sticky mass when wet and hard clods when dry.

Clay particles less than 0.001 mm. in diameter exhibit colloidal properties due to the very small size, these particles also possess a tremendous amount of surface area per unit mass, and also exhibit electromagnetic charges which attract water, and positive and negative ions.

These characteristics along with the specific minerals present in a given clay give rise to most of the chemical and physical activities of soils.