Gypsisols are soils with substantial secondary accumulation of gypsum (CaSO4.2H2O). They are found in the driest parts of the arid climate zone, which explains why leading soil classification systems labeled them `Desert soils' (USSR), Aridisols (USDA Soil Taxonomy), Yermosols or Xerosols (FAO, 1974).

Gypsisols are exclusive to arid regions; their world-wide extent is probably of the order of 100 million hectares. Major occurrences are in and around Mesopotamia, in desert areas in the Middle East and adjacent central Asian republics, in the Libyan and Namib deserts, in southeast and central Australia and in the southwestern USA.

Most Gypsisols formed when gypsum, dissolved from gypsiferous parent materials, moved through the soil with the soil moisture and precipitated in an accumulation layer. Where soil moisture moves predominantly upward (i.e. where a net evaporation surplus exists for an extended period each year), a gypsic or petrogypsic horizon occurs at shallower depth than a layer with lime accumulation (if present). Gypsum is leached from the surface soil in wet winter seasons. In arid regions with hot, dry summers, gypsum (CaSO4.2H2O) dehydrates to loose, powdery hemihydrate (CaSO4.0.5H2O), which reverts to gypsum during the moist winter. The so-formed (highly irregular) gypsum crystals may cluster together to compact layers or surface crusts that can become tens of centimeters thick. Gypsum precipitates in the soil body as fine, white, powdery crystals in former root channels (`gypsum pseudomycelium') or in pockets, or as coarse crystalline `gypsum sand', or in strongly cemented petrogypsic horizons. In places it forms pendants below pebbles and stones or rosettes (`desert roses').