Soils of Namaqualand: Compensations for aridity

The soil mantle in Namaqualand is immature in terms of weathering. Despite or even because of this, the diversity and spatial variability of soil physical and chemical properties is exceptionally high and could be fundamental in governing biodiversity and ecosystem function. In arid landscapes, the way the soil sheds, admits, stores, diverts and transmits water is crucially important. Namaqualand’s soils possess special features, which modify water infiltration, hydraulic conductivity and subsurface storage and provision of water to plants. These include pedodermal crusts; a natural mulch of surface gravel or desert pavement; water-repellent soils with fingered infiltration of rain water; textural discontinuities at depth; subsurface horizons cemented with silica, calcite, fibrous clays or gypsum; soluble salts that affect plant water uptake osmotically; clay minerals having exceptional waterabsorbing capacity and that are unique to arid environments; and a mechanism, in deeper sandy soils, for thermally induced upward transport of water vapour—a kind of nocturnal distillation— that seems to lessen the need for plants to root deeply. Superimposed on this already impressive pedodiversity are the almost ubiquitous heuweltjies— broad, low mounds which add further complexity to fluxes of water, salts, energy and nutrients in the landscape. r 2007 Elsevier Ltd. All rights reserved.