Plinthosol, one of the 30 soil groups in the classification system of the Food and Agriculture Organization (FAO). Plinthosols form under a variety of climatic and topographic conditions. They are defined by a subsurface layer containing an iron-rich mixture of clay minerals (chiefly kaolinite) and silica that hardens on exposure into ironstone concretions known as plinthite. The impenetrability of the hardened plinthite layer, as well as the fluctuating water table that produces it, restrict the use of these soils to grazing or forestry, although the hardened plinthite has value as subgrade material for roads or even as iron ore (the iron oxide content can be as high as 80 percent by mass). Plinthosols occupy about 0.5 percent of the total continental land area on Earth, mainly in Brazil and West Africa. A related FAO soil group also originating in the tropics is Nitisol.