The soil may be only moderately salty but may be high in sodium. A saline soil with that characteristic is commonly known as black alkali. The particles of clay and organic matter in soils have the property of adsorbing upon their surfaces salt constituents (cations) such as sodium, calcium, and magnesium. In nonsaline soils, the surfaces of the soil particles are largely saturated with calcium and magnesium, the calcium usually predominating. If a nonsaline soil comes in contact with saline irrigation or drainage water that contains a high proportion of sodium salts, an exchange reaction occurs between the sodium in the water and the calcium held by the soil particles. Some of the sodium is adsorbed and an equivalent amount of calcium is released to the solution. Conversely, adsorbed sodium in soils can be replaced by the addition of a calcium or magnesium salt solution. Such a reaction involving the exchange of cations (sodium, calcium, or magnesium) is termed cation exchange. It is described here because of its importance in the reclamation of alkali soils.
The physical properties of soils are greatly influenced by the degree to which the clay and organic matter are saturated with sodium. Soils saturated with calcium and magnesium are usually flocculated and have a good granular structure. Soils containing appreciable amounts of adsorbed or exchangeable sodium ordinarily have poor structure. Studies of saline soils have shown that when sodium makes up 10 percent or more of the total exchangeable cations ( calcium, magnesium, potassium, and sodium) the soil tends to become dispersed. The aggregates of the resulting alkali soil are relatively less stable, and there usually is a change from the granular condition in which the particles are aggregated to a dispersed phase. The soil structure deteriorates, the soil becomes tight or impermeable to water and air, infiltration of irrigation water is retarded, and drainage is difficult. If the irrigation water that is applied to the soil is high in sodium, these unfavorable changes in the physical condition of the soil may take place. No absolute value can be given for the percentage of sodium in irrigation water that will be injurious, but 60 to 75 percent is an approximate value. Other factors that determine the severity of the condition are the texture of the soil, its salt status and content of organic matter, the mineralogical composition of the clay, drainage, and the way soil is managed and cropped.
When high-sodium conditions exist, the essential consideration is to remove the excess sodium. Leaching alone may not do this; it may even aggravate the soil condition. The application of soil amendments to replace the sodium with calcium is a generally recognized practice to improve impermeable alkali soils. The basic principles involved have been worked out by W. P. Kelley, W. T. McGeorge, and others, but more study is needed to determine the most economical methods and means of reclamation.
Gypsum, sulfur, lime, or calcium chloride may be used to supply a source of soluble calcium or to make more soluble the calcium already present in the soil in the form of lime. For example, sulfur is oxidized in the soil to sulfuric acid, which reacts with calcium carbonate to form the more soluble gypsum, a calcium compound. The process requires time, and the sulfur should be worked into the soil and permitted to oxidize for several months before leaching. The selection of a soil amendment may be determined by its availability from local sources and the expense involved.
The use of manure may be effective in improving soil aggregation and permeability. It is thought that the decomposition of the added organic matter liberates carbonic acid, which in turn increases the solubility of the calcium carbonate in the soil. Green-manure crops accomplish the same purpose; in addition, the action of the roots of growing crops improves the soil structure. If the soil has become dispersed or puddled in the process of reclamation, drying is beneficial.
In areas where it is uneconomic to attempt complete reclamation by the application of soil amendments, it is sometimes possible to effect partial reclamation to an extent sufficient to permit the establishment of some vegetative cover. If given time, a good stand of native vegetation or even weeds will bring about a gradual improvement of soil structure and permeability. With careful management and a program of limited application of amendments, such an area may be reclaimed to a point where it is suitable for agricultural use. Thus, successful reclamation may involve a combination of leaching, soil amendments, and good soil and crop management.
Seed germination is often reduced in saline soils, and plants are more sensitive to salt in the seedling stage than when they are more mature. Seedlings of relatively salt-tolerant plants, like alfalfa, sugar beets, and cotton, may be retarded in growth or die if the soil is moderately saline. It is important that the seedbed be carefully prepared and the soil leached before seeding. Where a raised-bed method and furrow irrigation are used, as with lettuce and some other truck crops, soluble salts tend to accumulate toward the peak of the convex beds. If the seed is planted on the shoulders of the bed the danger of salt injury is lessened. The soil should be sufficiently moist to germinate the seed; it should not be allowed to become dry during the seedling stage because salts may accumulate in the row. The use of small furrows along the seed row to keep the soil leached and moist during the early stages of growth has been practiced with some success.