Alfisols

Alfisols are moderately leached soils that have relatively high native fertility. These soils have mainly formed under forest and have a subsurface horizon in which clays have accumulated. Alfisols are primarily found in temperate humid and subhumid regions of the world. The combination of generally favorable climate and high native fertility allows Alfisols to be very productive soils for both agricultural and silvicultural use. They are divided into five suborders: Aqualfs, Cryalfs, Udalfs, Ustalfs and Xeralfs.

Alfisols occupy approximately 10.1% of the global ice-free land area. In the U.S., they account for approximately 13.9% of the land area. They support about 17% of the world's population.

Alfisols - a photo of chalk like grey-white soil.
Alfisols have both: (1) a subsurface zone of clay accumulation — either an argillic, kandic or natric horizon; and (2) base saturation of at least 35% in the subsoil. This second criterion is what distinguishes Alfisols from Ultisols. (USDA-NRCS) 

Aqualfs, Cryalfs

Aqualfs — Alfisols with a water table at or near the surface for much of the year; Cryalfs — Alfisols of cold climates

Ustalfs, Xeralfs

Ustalfs — Alfisols of semiarid and subhumid climates; Xeralfs — temperate Alfisols with very dry summers and moist winters

Udalfs

Alfisols of humid climates