How Do Excess Salts and Water Degrade Soils?
Some good news is that the approximately 17% of the world’s cropland that is irrigated produces almost 40% of the world’s food. Some bad news is that irrigation has a downside. Most irrigation water is a dilute solution of various salts, picked up as the water flows over or through soil and rocks. Small quantities of these salts are essential nutrients for plants, but they are toxic in large amounts. Irrigation water not absorbed into the soil evaporates, leaving behind a thin crust of dissolved salts (such as sodium chloride) in the topsoil. The accumulation of these salts is called salinization (Figure 9-8), which (1) stunts crop growth, (2) lowers crop yields, and (3) eventually kills plants and ruins the land. Figure 9-6 Desertification of arid and semiarid lands. (Data from UN Environmental Programme and Harold E. Drengue) According to a 1995 study, severe salinization has reduced yields on 21% of the world’s irrigated cropland, and another 30% has been moderately salinized. In th
