The US Department of Agriculture recognized ten major soil groups in the Seventh Approximation. Alfisols are relatively young and acid soils with a clay B horizon. Aridisols are semi-desert and desert soils. Entisols are immature, mainly azonal soils. Histosols are primarily organic in content, developing in marshes or peat bogs. Inceptisols are young soils with weakly developed horizons. Mollisols are characteristic of grassland, high in bases, and with a thick, organically rich A horizon. Oxisols occur in tropical and subtropical areas. They are well weathered and often have a layer of plinthite near the surface. Spodosols have been podzolized. Ultisols develop where summers are wet and winters are dry. They are quite deeply weathered and are often reddish-yellow in colour. Vertisols are clay soils characterized by deep, wide cracks in the dry season.