Soil in the United States
occupied by soils of the twelve orders have been estimated[1] as: Alfisols and Inceptisols occur widely in the US, but the circumstances of their occurrence would be difficult to characterize briefly.Histosols are organic soils lacking permafrost within 100 cm of the surface; they are characteristically formed on wet sites, e.g. bogs, some fens and some muskeg areas.Spodosols are also found in warm, moist environments such as Florida and in fact are the most prominent soil order of the state.Vertisols are not extensive in the US, being confined to areas where there is a great abundance of swelling clays, e.g. montmorillonite, that cause churning of soils as a consequence of wetting and drying cycles.Thus the great diversity of geologic materials, geomorphic processes, climatic conditions, biotic assemblages and land surface ages in the United States is responsible for the presence of an enormous variety of mineral and organic soils.Soil chemical composition reflects not only the original geologic materials (e.g. limestone, granite, basalt), but also soil-forming processes since deposition.In much of the northern US, soil formation commenced either shortly after glacial retreat at the end of the last Ice Age or even more recently.