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The Control of Salinity by H The Control of Salinity by H. E. HAYWARD L.N 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 agricul- tural 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. 547 548 YEARBOOK OF AGRICULTURE 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 Salt content Conductivity Water-class (kXlO^at Sodium Boron 25° C.) Total Per acre- foot Parts per Parts peí million Tons Percent million Class 11 0-100 0-700 1 60 0 0-0 5 Class 2 2 100-300 700-2, 000 1-3 60-75 5 2 0 Class 33. over 300 over 2, 000 3 75 over 2 0 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 formationj 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 agri- cultural 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 find- ings so that field conditions arc known and can be controlled. The program involves research in irrigation, drainage, soil chemistry, soil physics, and plant physiology. The control of sahnity 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 pro- cedures 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 THE CONTROL OF SALINITY 549 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 char- acteristics of the soil profile, drainage conditions, and sources of salinity; soil and water analyses, and soil permeability and the effect on permea- bility of the quality of the irrigation water that is to be used. The reclamation of an area that is high in soluble salts and the maintenance of the productivity of potentially saline areas involve the same basic principle. The removal of salts from the root zone of the soil must exceed the quantity of salts deposited by the irrigation water. That can be accomplished by good drainage and proper control of irrigation unless the amount of sodium in the soil or the irrigation water is high. Good drainage is essential. That means that the water table should be at least 5 to 6 feet below the surface. In order to observe variations in the water table throughout the year, an efficient observation well was developed that consists of lengths of ^-inch pipe terminating at différent depths in the soil and a device for measuring the distance to water. With it, the pressure and direction of movement of ground water and information regarding soil permeability can be determined. The drainage engineer can use the information in selecting the most practical methods of drain- age, improving drainage design, and evaluating the effectiveness of drainage systems already in operation. Where artificial drainage is necessary, three methods are used—tile systems, open drains, and pumped wells. In many irrigation projects, deep open drains are installed and tile drains are frequently used as an adjunct to them. If the surface soil is permeable and is underlain with gravel deposits, wells of satisfactory capacity can be developed, and pumping from them may be the most feasible and cheapest method of controlling the water table. The water collected by these methods is usually removed by large outlet drains, which discharge into the river system. In many places drainage water pumped from wells is mixed with irrigation water and used over. Proper management of irrigation water is as important as good drainage. Assuming that water of satisfactory quality is available, the amount of water applied in irrigation is a primary consideration in saline or potentially saline areas. The total amount of water required under non- saline conditions is the quantity of water, expressed in acre-feet per cropped acre per year, used by the crop in the formation of plant tissue or transpired through the leaves, plus the water that evaporates from the soil surface. To this requirement, sometimes called consumptive use, must be added irrigation losses during water conveyance (seepage, leakage, wasteways, and evaporation) and losses during application (surface runoff and deep percolation). In saline areas, besides the requirements just noted, sufficient water 550 YEARBOOK OF AGRICULTURE must be supplied to leach the soil and carry excess salts down below the root zone. Both underirrigation and overirrigation must be avoided. Salts will accumulate if a salty soil is underirrigated, and all the water applied is used by the plant or evaporated. For example, Colorado River water (a class 2 water) contains 1.1 tons of salt per acre-foot. Continued use of water of that degree of salinity in amounts insufficient to cause leach- ing and drainage will eventually make the soil too saline for use. Over- irrigation is also dangerous, especially where drainage is poor, since the excess water passing into the subsoil may cause a rise in the water table and bring about an increased accumulation of salts in the root zone. Alkali Soil 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. Con- versely, adsorbed sodium in soils can be replaced by the addition of a calcium or magnesium salt solution. Such a reaction involving the ex- change of cations (sodium, calcium, or magnesium) is termed cation exchange. It is described here because of its importance in the reclama- tion 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.
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