How secondary salination works
Primary salinity develops naturally in areas where evaporation is high and rainfall low, as seen in salt flats and salt lakes. In contrast, secondary salination most often comes about from a change in land use as is happening in the agricultural part of Western Australia.
Basically farming in Western Australia has seen deep-rooted native trees cleared to make way for shallow-rooted crops. The removed trees used more of the rainfall than the crops that replaced them. Over time, additional groundwater recharge sees watertables rise bringing with it salts previously stored in the soil above the watertable.
Once the now saltier groundwater gets near enough to the surface, capillary action combined with evaporation draws water up and concentrates salts at and near the surface. Initially crop yields decline in the affected area and in many cases the land eventually becomes unusable for agriculture.
Secondary salination of this type is not restricted to Australia. Anywhere is susceptible where there is the combination of extensive tree clearing, high temperatures (high evaporation), in ground salt stores, and relatively low rainfall. This combination is often seen where farming takes place on the margins of hot deserts such as in many places in Africa, the southern parts of North America and the northwest of the Indian subcontinent.