Alfisol forms under deciduous forests in humid, temperate regions. Alfisols have extensive O-horizons, dark rich A horizons that support excellent plant growth and subsoil that includes a clay layer. Their C-horizons are glacial, alluvial or loess deposits. Spodosol forms under evergreen forests. Decomposed pine needles acidify "soil solution" (water in the soil), so minerals are leached from upper horizons and lower horizons contain oxides of iron and aluminum.
Oxisol develops in tropical forests with 100 inches of annual precipitation. Although rainforests are complex and abundant ecosystems, soils beneath them are infertile. The plant cover rapidly uses nutrients, and when that cover is removed, the remaining soil is easily eroded down to the oxidized bricklike subsoil. Utisols are Oxisols that have developed a clay layer in their B-horizon.
Mollisols are dark brown, mineral-rich, fertile soils that develop under grasslands, such as the Russian steppe and American Midwest. Abundant humus (decomposed plants) allows the upper horizons to retain water in the root zone. Under mats of grass, Mollisols have no O-horizon and an extremely deep A-horizon. Dry mollisols are friable (crumble between fingers). Andisols are at least half volcanic ash and mineral rich from the layers of their igneous parent materials.
Stable desert soils are Aridisols that weathered in place over time and are arable when irrigated. Aridisols of sand and rock develop under scant rainfall and support scant vegetation, with limited potential for organic horizons. Sodium (salt) accumulates in the root zone, and hard pans of caliche (lime, calcium carbonate) develop in the B-horizon. Deep irrigation will leach salt from the root zone, but caliche must be dug out or broken apart by machine.
Histosol forms worldwide where organic matter decomposes slowly, either because of freezing conditions in the Arctic or acidic soil solution that acts as a preservative in bogs, peat bogs or swamps. On Arctic tundra, Gelisol develops above permafrost. Gelisols were once classified as Histosols, because annual freezing and thawing churned layers together, but detailed field work revealed that Gelisols have organic horizons that extend into permafrost. Vertisols contain clay minerals that expand and contract and crack. Organic matter falls into the cracks and is churned by expansion and contraction. The organic horizon develops at depth, rather than on the surface.
Rocky land, shifting sand and ice or glacier are too unstable for soil development. Entisols are found where vegetation is beginning to stabilize soils in stream valleys or on steep slopes or stream deltas. Inceptisols are one step further in development, as horizons form on stable Entisols.