As a result, salts accumulate at the limit of the soil wetting front after each rainfall event. Other sources of salts are atmosphere and ground water. Salts can accumulate, forming a layer of soil concrete called calcrete if formed from calcium salts, silcrete from silica, and gypcrete from gypsum. Over time, erosion often exposes the cemented soil layers that appear as rock. Many areas of exposed calcrete are difficult to distinguish from limestone or marl.
Salt also accumulates in basins or playas through rainfall runoff.
Salty soils of arid lands pose severe obstacles to construction and agricultural production. Oases and playas Water is an especially important resource in arid lands and the soils associated with water sources are of equal value. These wetland and riparian areas contrast sharply with the surrounding landscapes both visually and in biomass production. Competition for grazing, agriculture, and wildlife is most acute in these areas. Playas are often very salty as they can accumulate salt from runoff and groundwater pumping.