1. Soil structure refers to the arrangement of soil particles into aggregates, which are separated by planes of weakness. The natural processes that form aggregates include wetting/drying, freezing/thawing, microbial activity, root/animal activity, and adsorbed cations. 2. Aggregates are described by their shape, size, and stability. The main types are granular, blocky, prismatic, columnar, platy, single-grained, and massive. 3. Soil structure can be graded on stability from weak to strong, and classified by aggregate size as fine, medium, or coarse. Tillage and traffic can alter soil structure by breaking down natural aggregates.