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Desert environment induces strong physical soil dispersion and a special type of weathering caused by hyper-acidic clay surfaces. Due to shallow moisture penetration these phenomena are mostly active in the top soil. Intense soil dispersion causes the formation of surface crusts, either vesicular or takyric, of a platy structure in the topsoil and the mobilisation and eluviation of clay and, to a lesser extent, of silt. Weathering induces in the top soil a relative accumulation of secondary Fe and Mn and a relative concentration of secondary Mn in relation to secondary Fe. The infiltration of aeolian dust in the top soil results in the shallow accumulation of clay and carbonates. It is shown that the pedogenic development of desert topsoils proceeds in three phases which are the stabilisation, the horizonation and the maturation phases. This development leads to the formation of various surface horizons assemblages depending on environmetal conditions. Diagnostic properties and typical surface horizons assemblages (epipedons) were defined in conformity with the morphogenetic approach. They were used to define for the FAO legend specific third level units that correspond to different desert soil surface conditions. These taxa have a good environmental and agronomic interpretative value. The use of the proposed diagnostic criteria also substitutes uneasy soil climate measurements with field observation and simple laboratory determinations. The Idimah area in Saudi Arabia served as a case study to illustrate the above mentioned concepts.

Clay soils --- Clay soils --- Deserts --- Deserts --- Arid soils --- Arid soils --- Calcium carbonate --- Calcium carbonate --- Wind erosion --- Wind erosion --- Geomorphology --- Geomorphology --- Soil degradation --- Soil degradation --- Soil classification --- Soil classification --- Soil water regimes --- Soil water regimes --- Soil profiles --- Soil profiles --- Saudi Arabia --- Saudi Arabia