Though primary production in drylands is constrained by water, it is the soil properties that determine how much of the rainfall will be stored and subsequently become available during the dry periods. The availability of moisture in soil is also an important factor in nutrient cycling, a requisite for primary production. Therefore, soil formation and soil conservation are key supporting services of dryland ecosystems, the failure of which is one of the major drivers of desertification.

The slow process of soil formation, in which plants and micro-organisms are intimately involved is frequently countered by faster soil degradation expressed through erosion and/or salinization. Hence the services of soil formation and conservation jointly determine the rate of soil development and its quality. The rate of soil formation (hundreds to thousands of years, Rust 1983) and its degree of development (depth of soil, infiltration depth and organic content) decline with aridity (Nettleton and Peterson 1983; Sombroek 1990).

In hyperarid areas, surfaces are often capped with mineral crusts which reduce infiltration and help generate soil-eroding flashfloods. In many arid drylands dispersed plant clumps are often embedded in a matrix of apparently bare soil covered by a thin crust of photosynthetic cyanobacteria, with mosses and lichens added in semiarid drylands (Büdel 2001). The crusts reduce water penetration, thus channel runoff, sediments, nutrients and seeds to the plant clumps, which then become active sites of soil formation and organic matter decomposition (Puigdefabregas et al. 1999). These crusts are therefore instrumental in soil development (in and around the clumps) and in soil conservation (in the surrounding matrix, Aguiar and Sala 1999). However, they develop slowly and are sensitive to trampling or air pollution. Dry subhumid soils, on the other hand, are protected from erosion by multi-layered, structurally complex vegetation (Poesen et al. 2003) that permits high water infiltration and storage, as well as water extraction by same vegetation (Puigdefabregas and Mendizabal 1998).