The Control of Salinity

by H. E. HAYWARD

IN REGIONS where rainfall is sufficient for agriculture, an excess of soluble salts does not ordinarily accumulate in the soil. Rain water is essentially free from salts, and soluble material is leached from the root zone and is carried away in the drainage water. But where there is too little rain for successful farming and land must be irrigated, special care must be taken, or salinity may spoil the productivity of the land, because all irrigation water contains soluble salts.

Of approximately 30 million acres of irrigable land in the 17 Western States, more than two-thirds is under irrigation. Reclamation projects will materially increase the acreage : The Columbia Basin Reclamation Project alone will add an estimated million acres more to the present irrigated total. On much of the area now irrigated in the West, crop yields are reduced by salinity, and economic losses consequently are serious. If irrigation agriculture should fail, dry farming, stock raising, forestry, mining, and manufacturing also would suffer, and the economic stability of the West would be jeopardized.

All soils contain some salt, although the amount present in agricultural soils of the East is low. The successful growing of crops depends upon the availability of the essential nutrients in the soil or soil solution, but plant growth is retarded when salts accumulate in large amounts. If the concentration of salt is too high, seed germination is reduced and the seedling plants may die. In severe cases of salinity, symptoms of injury may be evident, such as burning of leaves and dieback of branches. Under less severe conditions no specific symptoms are detectable, but the plants may be stunted and produce low yields.

Basically, the two major phases of the salt problem are too much total salt in the soil and soil solution, and the presence of too much sodium. Although no absolute rating of water quality is possible, the standards given here, which are approximate upper and lower limits, can be used as a general guide.

1. Excellent to good, suitable for most plants under most conditions.

2. Good to injurious, probably harmful to the more sensitive crops.

3. Injurious to unsatisfactory, probably harmful to most crops and unsatisfactory for all but the most tolerant. Any class 3 water should be considered unsuitable under most conditions. Should the salts present be largely sulfates, the values for salt content in each class can be raised 50 percent. Because soil, crop, climate, drainage, and soil management all influence the suitability of water for irrigation, no simple classification scheme will hold for all cases.

The soil may be saline because of its origin and formation, or soils that are slightly salty may become highly saline because the input of salt exceeds the output. An unfavorable salt balance can result from the use of irrigation water with high salt content, inadequate irrigation, or poor drainage.

To study the problems of salinity in irrigation farming over a wide area, the Regional Salinity Laboratory was established in Riverside, Calif., in 1937. Its work is done in close cooperation with the agricultural experiment stations of 11 Western States and Hawaii and with other agencies of the Government. Its chief functions are to study the relationships of the salinity of irrigation waters and soils to plant growth, investigate the factors that relate to a permanently successful irrigated agriculture, and develop practical applications of the laboratory findings so that field conditions are known and can be controlled. The program involves research in irrigation, drainage, soil chemistry, soil physics, and plant physiology.

The control of salinity requires accurate information on the degree and type of salinity in an area. If reclamation projects are initiated without prior knowledge of saline conditions, serious economic losses may result. To help eliminate this possibility, field methods and procedures are being developed to survey such areas. These include methods for estimating total salts present, the amount and degree of saturation of sodium on the exchange complex, permeability, and other soil-water relationships. The object of a salinity survey is to get an evaluation of the over-all conditions within a given area, including a salinity map, so that recommendations can be made for the improvement or control of saline and alkali soil conditions. Important in a survey are field observations, including the character of the native vegetation, topography and characteristics of the soil profile, drainage conditions, and sources of salinity; soil and water analyses, and soil permeability and the effect on permeability of the quality of the irrigation water that is to be used.