Salt in soil and water
All over the world, the irrigation of crops in arid and semi-arid regions leads eventually to an accumulation and concentration of mineral salts in the soil and in the water that drains from agricultural land. In what is known as the Fertile Crescent in ancient Mesopotamia, farmers were gradually forced to switch from growing wheat to growing only more salt-tolerant barley over a period of 2,000. More recently, in Egypt, farmers saw crop deterioration as a result of salt buildup in only five years.
The concentration of salts in the soil and in agricultural runoff occurs when there is insufficient drainage. Salt that is naturally present in the soil or in the irrigation water gets left behind on the surface of the ground or in the root zone when water evaporates or is transpired by plants. To avoid the buildup of salts, farmers must be able to supply extra water to flush the salts away. The salts generally end up in the groundwater (which may also create a problem).