Durisol, one of the 30 soil groups in the classification system of the Food and Agriculture Organization (FAO). Durisols are soils in semiarid environments that have a substantial layer of silica within 1 metre (39 inches) of the land surface. The silica occurs either as weakly cemented nodules or as hardpan and accumulates as a result of downward translocation (migration) when solubilized during weathering of the soil. Durisols are found in the southwestern United States, Chile, South Africa, and especially Australia, where rainfall is low. They usually occur in association with Arenosols, Calcisols, Cambisols, Gypsisols, or Vertisols. Soils in the Aridisol and Vertisol orders of the U.S. Soil Taxonomy that exhibit hardened layers of silica accumulation are closely related to the Durisols.