Most of the world's prime agricultural soils are loam. Soil separates and organic matter are often found together in natural soil structures called soil peds, or aggregates. Cycles of wetting and drying and freezing and thawing promote ped formation. For this reason farmers often leave their fields exposed in winter to mellow the soil. A well-developed soil structure can provide large pores and cracks that enhance water and air movement and root growth.
Soil structure is classified by the shape and size of soil peds.
Granular structure is common under sod and refers to small balls of soil that easily separate. Blocky structure is common in older clay soil in humid regions. Well-developed prisms and columns can be found in some clayey subsoils. The basic chemistry of soil includes colloidal structures, minerals, and macronutrients and micronutrients essential to plant life. Clay and organic matter are finely divided soil particles called colloids, which provide the site for most of the chemical reactions in soils.