Alfisols are moderately leached forest soils that have relatively high native fertility. These soils are well developed and contain a subsurface horizon in which clays have accumulated. "Alf" refers to aluminium (Al) and iron (Fe). These soils having significantly more clay in the B-horizon than in the A-horizon and high base status.
Alfisols are mostly found in temperate humid and subhumid regions of the world. In the US, where soil temperature regime is mesic or cooler, the Aflisols tend to for a belt between the Mollisols of the grasslands and the Spodosols and Inceptisols of the more humid climates. In regions of mesic and frigid soil temperature regimes, Alfisols are mostly on late-Pleistocene deposits or older surfaces if conditions are right.
Alfisols occupy ~9.7% of the global ice-free land area. In the US, they account for ~13.9% of the land area. Alfisols support about 17% of the world's population.
Alfisols represent one of the more important soil orders. The combination of generally favorable climate and high native fertility allows Alfisols to be very productive soils for both agricultural and silvicultural use. Alfisols represent the youngest forest soil order.
Aqualfs - Alfisols with a water table at or near the surface for much of the year. Aqualfs occur in many parts of the world, mostly in late-Pleistoncene deposits.
Cryalfs - Alfisols of cold climates. More or less freely drained Alfisols of cold region. Cryalfs are not extensive, forming in N. America, Eastern Europe and Asia above the 49 N. latitude and in some high mountains south of that latitude. Most Cryalfs are or have been under a coniferous forest. Cryalfs in the USA generally formed in Pleistocend deposits mostly of Wisconsinan age.
Ustalfs - Alfisols of semiarid and subhumid climates. Many of these soils have or have had a savanna vegetation and some were grasslands. Tend to form a belt between the Aridisols of arid regions and the Udalfs, Oxisols, and Inceptisols of humid regions. May be in areas of erosional surfaces or deposits of late-Wisconsinan age. Many are on old surfaces and the minerals may be strongly weathered.
Xeralfs - temperate Alfisols with very dry summers and moist winters. Most of these soil formations border the Mediterranean Sea or lie to the east of an ocean at midlatitudes. Found in Mediterranean, parts of South Africa, Chile, western and southern Australia and in the western USA. As a whole, Xeralfs are not globally extensive, but in regions where they occur, they are extensive or moderatly extensive.
Formed on surfaces that are of different ages. Some formed on erosional surfaces or in deposits of late Wisconsinan age, or in Australia, formed on old surfaces that may reflect an environment greatly different than the present one.
Udalfs - Alfisols of humid climates. Principally in areas of late-Pleistocene deposits and on erosinal surfaces of around the same age. Very extensive in the USA and in western Europe. All of them are believed to have supported forest vegetation at some time during development.