Much of central Australia was at one time a shallow inland sea. This means that in the thin, dry soil there is a layer of saline soil. Prior to 1788, the bush forest above this soil ensured that the soil remained dry, and that the layer of saline soil did not rise. The advent of land clearing for grazing and farming, and the use of irrigation created a much wetter soil environment. Food and industrial crops, while requiring a much larger water usage, also required a much wetter soil environment. This wetness leached down into the saline layers of soil, and the crops then drew the water, and dissolved salts, towards the surface.
Over time this process caused the thin top-soil layers to become irreversably salty, and no longer suited for agriculture. Large amounts of land are now affected by salinity. Where the land is not yet a salt-pan it is occasionally possible for farmers to reduce the speed at which land becomes saline by planting gum trees, which reduce the general wetness of the soil.