Why are arable soils in the Namib Desert mostly limited to floodplains and river terraces?

Arable soils in the Namib Desert are limited to floodplains and river terraces primarily because much of the desert is otherwise unsuitable for agriculture. Large portions of the region are completely soilless, consisting of exposed bedrock or shifting sand dunes.1

Where soil does exist, it is often highly saline, impregnated with gypsum, or cemented by a layer of calcium carbonate known as calcrete, which forms just below the surface and hinders cultivation. In contrast, floodplains and river terraces benefit from periodic inundation, which provides the necessary moisture and sediment deposition to support more viable soil conditions in an otherwise arid environment.1