How does excess irrigation water leach salts out of soil?

Excess irrigation water leaches salts out of soil through a process of flushing, which is a critical component of land reclamation in salt-affected areas.1

When soil contains high levels of soluble salts—often accumulated due to evaporation or the use of saline irrigation water—these salts can inhibit plant growth by preventing roots from absorbing water. To remove these salts, the soil must first have adequate drainage to ensure that water can move downward through the soil profile rather than stagnating or moving upward.21

Once proper drainage is established, nonsaline water is applied to the surface through irrigation. As this fresh water moves downward through the soil, it dissolves the accumulated salts and carries them away in the drainage water. This downward movement effectively "washes" or leaches the excess salts out of the root zone, restoring the soil's productivity. In cases involving sodic soils, this process may also require chemical amendments, such as gypsum, to replace adsorbed sodium before the salts can be successfully leached.1