Reclamation of Alkali Soils Concept and Principles

Reclamation of alkali soils usually involves a two step process. The first step is replacing exchangeable sodium ions with calcium ions. This calcium may originate from the dissolution of Ca-containing minerals in the soil, such amendments as gypsum and calcium chloride, or irrigation water with calcium ions. A second step involves leaching the resulting sodium salt from the soil. A significant factor in reclaiming alkali soils is the maintenance of hydraulic conductivity by providing a sufficiently high electrolyte concentration in the soil solution to counter the influence of exchangeable sodium. The reason it is necessary to apply a sufficiently high electrolyte concentration is that low salinity water such as rainwater makes clay swell, and swelling clay leads to low impermeability which impedes the leaching salts. Figure 10.2 illustrates the concept of alkali soil amelioration by calcium sulfate. Generally, the higher the electrolyte concentration, the higher the exchangeable sodium fraction at which a relatively high permeability can be maintained.7 The electrolyte concentration affects the hydraulic conductivity less when the content of soil water is low.8 According to our laboratory experiments,9 among the amendments used to bring about exchangeable sodium replacement with calcium, gypsum (CaSO4-nH2O, hydrated calcium sulfate) is far and away the most effective amendment material.

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