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. 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 aggre- gated to a dispersed phase. The soil structure deteriorates, the soil be- comes 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 THE CONTROL OF SALINITY 551 given for the percentage of sodium in irrigation water that will be in- jurious, 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 composi- tion of the clay, drainage, and the way soil is managed and cropped.

Use of Soil Amendments

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 amend- ment 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 solu- bility of the calcium carbonate in the soil. Green-manure crops accom- plish 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 apphcation 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 vegeta- tion or even weeds will bring about a gradual improvement of soil struc- ture 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 recla- mation may involve a combination of leaching, soil amendments, and good soil and crop management. Seed germination is often reduced in sahne soils, and plants are more sensitive to salt in the seedling stage than when they are more mature. 552 YEARBOOK OF AGRICULTURE

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 allow^ed 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.

Plant'Wat er Relations in Saline Soils

Recent research has clarified considerably the problem of availability of water to plants in saline soil. In nonsaline soil, plants stop growing when the supply of available water is exhausted, that is, when the soil moisture approaches the wilting point. The condition of the soil moisture or the tenacity with which soil holds moisture can be expressed in terms of the moisture stress, or soil- moisture tension. That property of the soil water can now be measured over the whole range of soil moisture that will permit the growth of plants, and progress has been made in relating the growth of plants to it. In saline soils, an additional stress is set up at the plant roots be- cause of the osmotic effect of salts in the soil solution. Preliminary results of the research indicate that, when expressed in atmospheres, the effects of soil-moisture tension and osmotic pressure are additive in inhibiting the growth of plants. If further work substantiates this principle, it will represent an im- portant step in understanding and overcoming problems of salinity. It will establish a quantitative basis for expressing salt tolerance and will help in selecting methods and measurements for surveying unreclaimed saline areas and in appraising damage due to salinity. The studies already made indicate that for good plant growth the soil must be kept wetter when salts are present. If soil salinity can be reduced to moderate levels, the land can be farmed successfully under proper management. It is important to select crops that are well suited to the prevailing climatic conditions and that are sufficiently salt-tolerant. Crops do not behave alike in their response to the combined effect of climate and salt. In general, a species of plant will tolerate more salt when grown under the climatic conditions best suited to it than when it is poorly adapted to its environment. If a plant is climatically adapted to its environment, the factors that THE CONTROL OF SALINITY 553

are important in determining its salt tolerance are the total concentration of salts in the soil solution and the toxic effect of specific salts or ions. When there are large amounts of soluble salts in the soil solution, the osmotic concentration will be high and the intake of water by the plant will be reduced. The kind of salt present in the soil solution must also be considered. In general, chloride salts are more toxic than sulfate salts when considered on the basis of chemical equivalents. Magnesium toxicity has been reported for wheat, beans, and guayule. Several crops have been tentatively classified on the basis of salt toler- ance. Sugar beets, milo, Bermuda grass, and Rhodes grass are strongly salt tolerant. Alfalfa, cotton, tomatoes, sorgo, and several rye grasses are regarded as haying good tolerance. Onions, squash, rice, barley, wheat, and flax are moderately salt tolerant. Wax beans, navy beans, field peas, and Elberta peaches exhibit weak tolerance. Within a given species, certain varieties or strains may be more salt- tolerant than others. Trials are being conducted with varieties of alfalfa and cotton ; by careful testing, it is believed that selections can be made that are better adapted to saline conditions than those now in use. As additional information is obtained, recommendations on crop tolerance will be made available to the farmer.

THE AUTHOR H. E. Hayward is director of the U. S. Regional Salinity Laboratory, Riverside, Calif. He has published a number of papers on the anatomical and physiological responses of agricultural crops to saline conditions, among them flax, tomatoes, peaches, and oranges. He is the author of The Structure of Economic Plants. Dr. 'Hayward is a graduate of the University of Minnesota and received his doctorate from the University of Chicago, where, before joining the staff of the Salinity Laboratory, he was professor of botany.

FOR FURTHER READING Fireman, Milton, and Magistad, O. C. : Permeability of Five Western Soils as Affected by the Percentage of Sodium of the Irrigation Water, Transactions of American Geophysical Union^ volume 26, pages 91-94, 1945. Gardner, Robert: Some Soil Properties Related to the Sodium Salt Problem in Irrigated Soils, U. S. D. A. Technical Bulletin 902, 1945. Hayward, H. E., and Magistad, O. C: The Salt Problem in Irrigation Agriculture, U. S. D. A. Miscellaneous Publication 607, 1946. Hayward, H. E., and Spurr, Winifred B. : The Tolerance of Flax to Saline Condi- tions, American Society of Agronomy, Journal, volume 36, pages 287-300, 1944. Magistad, O. C, and Christiansen, J. E. : Saline Soils, Their Nature and Manage- ment, U. S. D. A. Circular 707, 1944. Richards, L. A., and Weaver, L. R.: Moisture Retention by Some Irrigated Soils as Related to Soil Moisture Tension, Journal of Agricultural Research, volume 69, pages 215-235, 1944. Wadleigh, C. H., and Ayers, A, D. : Growth and Biochemical Composition of Bean Plants as Conditioned by Soil Moisture Tension and Salt Concentration, Plant Physiology, volume 20, pages 106-132, 1945.