If clay dominates, the texture is called clay loam, or, with more clay, just plain clay. Silty soils that contain little or no sand are called, in order of increasing clay content, silt loams, silty clay loams, or silty clays. Coarse-textured sandy and gravelly soils have the largest pores and the most rapid permeabilities. Fine-textured clayey soils have very tiny pores and very slow permeability rates. Medium-textured loams, silt loams, and clay loams have intermediate rates of soil permeability.
Organic matter helps create and stabilize aggregates of the grains of sand, silt, and clay.
These aggregates, or units of soil structure, have relatively large spaces between them, permitting more rapid water movement. Macropores also are common in very coarse-textured soils and in soils that crack extensively upon drying. Macropores are especially important where they are connected to the soil surface. Heavy rainfall or irrigation events may create temporarily saturated surface soil, which can lead to rapid flow through macropores. If soluble pesticides also are present, they can be carried deep into the soil in a short time.