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Irrigation Waler and Saline and Alkali Soils 321 Irrigation tion of in the lower Rio Grande, however, drops because of the lower and Saline and concentration of salts in some of the Alkali Soils tributaries, so that at Rio Grande City, Tex., more than 900 miles down the river from Fort Quitman, the Milton Fireman and H. E. Hayward amount of soluble salts is only about 525 p. p. m. If irrigation water is of good quality, The range in salt concentration in the soils to which it is applied may be ground pumped from wells improved because of the calcium in the may be much greater than that of sur- water and the beneficial effect derived face waters. Ground water from wells from leaching or washing any excess near each other may be different in salts from the soils. But if the quality salt concentration and composition. of water is unsatisfactory, the soil may Even the variations in water from dif- deteriorate until it will no longer pro- ferent depths at a single location may duce satisfactory crops. be large. Analyses of many wells in the Four major characteristics deter- CoachcUa Valley, Calif., indicated a mine quality of water for irrigation: range in soluble salts from 130 p. p. m. The total concentration of soluble to approximately 8,500 p. p. m. Two salts; the concentration of sodium and wells, 565 and 180 feet deep, within a the proportion of sodium to calcium. half-mile of each other had salt con- plus magnesium; the concentration of centrations of approximately 400 and bicarbonate; and the occurrence of the 8,500 p. p. m., respectively. minor elements, such as boron, in The evaporation of moisture from amounts that are toxic. the soil surface docs not remove salt The total concentration of soluble from the soil, and relatively little is salts is an essential consideration in absorbed by plant roots. Consequently waters that are used for irrigation. The the use of saline irrigation water re- salt content of most irrigation waters sults in the accumulation of soluble ranges from o. i to 5 tons of salt to the salts in the soil unless it is prevented by acre-foot of water (approximately 70 leaching and drainage. One can use to 3,500 parts per million). The irrigation water with a moderate de- amount of soluble salts in river waters gree of salinity, however, if drainage is in the Western States varies greatly—■ adequate and enough irrigation water 70 p. p. m. in the Columbia River at is applied so that some of it goes Wenatchee; 1,574 p. p. m. in the through the soil profile and out into Sevier River near Delta, Utah; and the drainageways. In that event, the 2,380 p. p. m. in the Pecos River at concentration of soluble salts in the Carlsbad, N. Mex. The Colorado water will restrict the crops that can be River, an important source of irriga- grown to those with moderate salt tol- tion water in the Southwestern States, erance. If sahne waters are used so contains about i ton of soluble salts sparingly that there is no excess for to an acre-foot of water (740 p. p. m.). drainage, however, there will be an The salt content of a river may increase in the salinity of the soil. change downstream because of return Because of the potential salinity flow from drainage and because of the problem when saline irrigation waters of minerals as water moves are used, leaching and drainage must along the river bed. The soluble salts be provided to remove dissolved salts in the Rio Grande, for example, in- that would otherwise accumulate in crease from about 180 p. p. m. at the root zone or in the subsoil imme- Otowi Bridge, N. Mex., to 780 p. p. m. diately below. at El Paso, Texas, and 1,770 p. p. m. Carl S. Scofield referred to the rela- at Fort Quitman, Tex. The concentra- tionship between the quantity of dis- 325802°—55 22 322 Yearbook of Agriculture 1955 solved salts delivered to an area with moisture stress. Also, it has been shown irrigation water, and the quantity re- that the absorption of water from the moved from the area by the drainage soil by plant roots depends upon the water as the salt balance of the area. soil-moisture stress. Thus, one of the If a favorable salt balance is to be main effects of soil salinity is to limit attained, the output of salts must be the water supply of the plant. This approximately equal to the input. induces modifications in growth that VVhen the salt balance is adverse, the are usually associated with a lack of input of salts exceeds the output. This water in the plant tissue. condition must be avoided in a perma- Wide variations among irrigation nent irrigated agriculture. waters may exist with respect to the kinds of salts present, as well as with THE SALiNizATioN of soil affects the the amount of salts in solution. Salts, grow^th of crop plants in two w^ays. when dissolved in water, dissociate or The first is a reduction in the amount separate into chemically equivalent of water absorbed by the roots. That amounts of positively and negatively occurs because continued irrigation charged particles—ions. The positive with saline water in the absence of a ions are cations. The negative ions are favorable salt balance results in a anions. gradual but progressive increase in the The major ions in irrigation water osmotic pressure of the soil solution. are the cations calcium, magnesium, Osmotic pressure is a measure of the and sodium; and the anions bicar- soluble salts in solution and provides a bonate, sulfate, and chloride. Other method of expressing the concentration ions that may be present are , of the soil solution on an energy basis. carbonate, nitrate, silica, iron, and Retardation of growth is virtually boron, but they are usually found in linear with increases in the osmotic low concentrations. Occasionally wa- pressure of the soil solution, and is ters may be high in bicarbonates and, largely independent of the kind of salts less frequently, in nitrates. In waters present. In extreme cases, a soil may of low total salt concentration, bicar- become so saline that it will not even bonates frequently exceed the com- support the growth of halophytes (salt- bined sulfate and chloride content, tolerant plants), just as soils may be- but in rivers with higher salt content, come too dry to support the growth of such as the Arkansas, Colorado, Gila, xerophytes (plants adapted to growth Pecos, and Rio Grande, the predomi- under dry conditions). nant salts are sulfates or chlorides. The effect of a given amount of solu- Sodium salts are usually present in ble salts in the soil solution is intensified irrigation waters, and if the proportion as the soil moisture content declines of sodium is high, it may be adsorbed between irrigations—as indicated by on the soil particles and result in an the increase in the osmotic pressure of unfavorable physical condition. Such the soil solution that accompanies dry- soils, w^hen they are wet, tend to run ing out. Furthermore, as a soil gradu- together and impede the movement of ally dries following an irrigation, the water and air. They form hard clods surface attraction of the soil for water, when dry. Irrigation waters with high known as soil-moisture tension, in- sodium percentages therefore may re- creases. Soil-moisture stress is defined quire special management practices. as the osmotic pressure of the soil solu- The use of waters that are low in tion plus the soil-moisture tension. total salts but high in bicarbonate ag- Experiments by G. H. Wadleigh and gravates the sodium problem. Water others in 1946 and 1948 have shown that is relatively low^ in total salts and that, if other factors are not limiting, has a sodium percentage (Na X100/ the vegetative growth of crop plants is total cations) that is within safe limits closely related to the average soil- may be questionable if the amount of Irrigation Water and Saline and Alkali Soils 323 bicarbonate is considerably in excess content of the soil is high, almonds of the calcium and magnesium present. may develop tipburn and avocados a This excess bicarbonate over calcium leaf scorch. Leaf burn in salt-sensitive plus magnesium is referred to as resid- cotton varieties has been correlated ual . The occurrence with high sodium content in the leaves. of residual sodium carbonate is fairly- Other common cations affect specific common in the Western States. Out of crops. An example is guayule, a rub- a group of approximately 450 well and ber-producing shrub native to North stream supplies in the San Joaquin America, which grows poorly in the Valley, slightly more than half con- presence of only moderate amounts of tained residual sodium carbonate. This soluble magnesium. condition also occurs in many other The accumulation of the chloride places—in the White River, South ion in plant tissues frequently results Dakota; the Sevier River, Utah; the in toxic symptoms. Among the crop Humboldt River, Nevada; and the plants that are sensitive to the chloride Nile River in Egypt. ion are peaches and other stone fruits, When an irrigation water containing pecans, some citrus varieties, avocados, residual sodium carbonate evaporates and some grapes. Many species are no in the soil, calcium and magnesium more sensitive to chloride than to carbonates precipitate, and the sodium equal concentrations of sulfate salts. percentage of the soil solution increases. Specific sensitivity to high concentra- Then sodium replaces calcium on the tions of the sulfate ion have been re- soil particles, the exchangeable-sodium ported for tomato, flax, cotton, and percentage of the soil increases, and orchard grass. the physical condition of the soil, es- The bicarbonate ion is toxic because pecially the permeability, may be im- its accumulation in the soil solution paired. In addition, the pH may in- affects mineral nutrition and tends to crease and organic matter may be dis- reduce the availability of iron in many solved, giving the dark color typical plants. Apple orchards in Washington of a so-called black alkali soil. become chlorotic when irrigated with Some salts or ions that arc harmless water high in bicarbonates. Continued in low concentrations may accumulate use of such a water may seriously affect in the soil solution in sufficient amounts the mineral nutrition of the tree. to cause toxic reactions in plants. The Specific toxicity of the bicarbonate ion ions most likely to cause such reactions has also been demonstrated with are sodium, chloride, bicarbonate, and Dallis grass. A reduction in the growth sulfate. Less frequently, crops grown of beans accompanied by chlorosis in soils having excessive amounts of becomes more pronounced with in- calcium and magnesium may show creasing concentrations of bicarbonate. toxic symptoms. Selenium, lithium, On the other hand, garden beets are and fluoride are found in a few waters much more tolerant to bicarbonate. and soils, and may be accumulated in Boron is a minor constituent of prac- plant tissues. Ordinarily selenium and tically all natural waters. Irrigation fluoride do not affect plant growth but waters should be analyzed for it if one may have serious effects on animal suspects its presence at toxic levels. life. A fraction of a part per million of Boron is essential to the growth of lithium in irrigation water produces plants, but it may be toxic at concen- tip and marginal burning and defolia- trations only slightly in excess of those tion of citrus leaves. needed for optimum growth. Besides the indirect effects of sodium 'Toxicity may develop with boron- on plant growth resulting from adverse sensitive crops when the concentration modifications of the physical proper- is as low as one part per million (i p. ties of the soil, there is some evidence p. m.). Water containing i p. p. m. or of its specific toxicity. If the sodium less of boron may be regarded as 324 Yearbook of Agriculture 1955 excellent. For most crops, however, and lack of drainage facilities, or from water that contains i to 2 p. p. m. is some combination of these factors. satisfactory. Water up to 3 p. p. m. Saline and alkali soils contain exces- may be used with the more boron- sive concentrations of either soluble tolerant crops. Water containing more salts or adsorbed sodium (alkali) or than 3 p. p. m. is doubtful or definitely both. The original sources of these salt unsuitable for irrigation purposes. constituents are the primary minerals Boron is responsible for symptoms of found in soils and in the exposed rocks toxicity that have appeared on citrus of the earth's crust. As a result of and walnut trees in southern Cali- chemical decomposition and physical fornia and on those and other crops weathering, the soluble constituents elsewhere. are gradually released from the min- Irrigation waters are classified on erals. These soluble salts in humid the basis of the more important con- areas are carried downward by rain stituents in solution so that the efí'ect into the ground water and ultimately of the water on crops and on soils can arc transported by streams to the be anticipated with some assurance. oceans. Leaching usually is local in Such classifications assume that the nature in arid regions, and the soluble water will be used under average con- salts may not be transported far. This ditions with respect to climate, amount is so because there is less rainfall to of water used, drainage, texture and leach the soluble salts out of the soil permeability, and salt tolerance of the and transport them away and because crop. Under unusual circumstances it the high evaporation rates character- may be possible to use a water that istic of arid climates tend to concen- under average conditions would be trate the salts in ground waters and in considered unsafe. Conversely, under soils. some conditions, it may not be safe to Inadequate drainage is associated use a "good" water. with and contributes to the severity of The United States Salinity Labora- saline and alkali soil conditions. Be- tory has proposed a scheme for classi- cause of the low rainfall in arid regions, fying irrigation waters on the basis of surface drainageways may be poorly two main factors. Waters are divided developed, and consequently drainage into four classes with respect to salt basins may have no outlet to perma- concentration, the salinity hazard, and nent streams. The salt-bearing waters into four other classes with respect to drain from the surrounding high lands the probable extent to which soil will of the basin to the lower lands, and absorb sodium from the water and may temporarily flood the soil surface the length of time required to ad- or form permanent salty lakes. versely afí'ect the soil (the sodium Saline and alkali soil problems most hazard). The characteristics of irriga- often develop as a result of the irriga- tion water based on these two criteria tion of level valley lands, which may are determined by chemical measure- be nonsaline and well drained under ments and are assigned values that natural conditions but may have indicate the overall water quality. drainage facilities inadequate to take care of the additional ground water A PROBLEM of great economic im- resulting from irrigation practices. In portance to the farmer arises when that event, the ground-water level may salinity or alkali conditions develop in be raised from a considerable depth good farmlands. This can occur as a to within a few feet of the soil surface result of natural causes, such as salty in a relatively short time. When that ground water and poor drainage, or occurs, the water moves upward to the from manmade causes, such as the ap- soil surface as a result of evaporation plication of irrigation water of poor and plant use. This increases the salt quality, improper soil management, content of the surface soils and of the Irrigation Water and Saline and Alkali Soils 325 soil water in the root zone, forming centration of the soluble salts, rather problem soils varying from a few acres than their chemical nature, is mainly to hundreds of square miles in area. responsible for the harmful effects of Soil deterioration frequently results saline soils on crop growth. from the application of either good or Often it is not economically feasible poor irrigation water to soils with im- to maintain a condition of low salinity paired drainage, since in this case the in a soil. The reason may be the ex- accumulation of soluble salts cannot treme salinity of the irrigation water, be prevented. Saline and alkali prob- the cost of providing adequate drain- lems also arise if drainage facilities are age, or the inherently low permeability adequate but insufficient irrigation of the soil. Then the farmer has to learn water is applied to provide for the nec- to live with the salt. essary leaching of excess salts. He can adopt management practices that minimize the effects of salinity, A SALINE SOIL contains enough solu- and he can make a judicious selection ble salts so distributed in the soil that of crops or crop varieties that will pro- they interfere with the growth of most duce satisfactory yields under moder- crop plants. Ordinarily the soil is only ately saline conditions. In selecting slightly alkaline in reaction (pH 7.0 to crops for saline soils, particular atten- 8.5) and contains very little adsorbed tion should be given to the salt toler- sodium. Saline soils are often recog- ance of the crop during germination. nized by the presence of white salt Poor crops frequently result from a crusts; by damp, oily-looking surfaces failure to obtain a satisfactory stand. devoid of vegetation; by stunted It is possible to modify planting prac- growth of crop plants, with consider- tices to minimize the accumulation of able variability in size and with a deep salt around the seed and to improve blue-green foliage; and sometimes by the stand of crops under saline con- tipburn and firing of the margins of ditions. Recommended management leaves. Chemical and electrical-con- practices and the salt tolerance of many ductivity measurements rather than species and varieties of crop plants are observations, however, are commonly listed in the Department of Agriculture used for assessing soil salinity. The de- Handbook 60, Diagnosis and Improve- termination of salinity status in terms ment of Saline and Alkali Soils. of plant response should take into ac- The chemical characteristics of saline count the moisture-holding capacity of soils are determined chieñy by the the soil in addition to its salt content. kinds and amounts of salts present. The water required for the growth Soil particles, as a consequence of processes of plants is absorbed by the electrical charges on their surfaces, roots from the soil solution. Many crop adsorb and retain cations such as plants absorb and transpire or evapo- sodium, calcium, and magnesium. The rate 500 pounds of water in a season for adsorbed ions are combined with the each pound of dry matter produced. soil particle, but they can interchange The soil moisture in arid regions is re- freely with other ions in the soil solu- plenished by irrigation with water con- tion. This reaction is called cation ex- taining appreciable amounts of soluble change. The proportion of the various salts, or by upward movement of more cations on the exchange complex is or less saline ground waters. In either related to their concentration in the instance, soluble salts are added to the soil solution. Calcium and magnesium soil with each application of water. are less easily exchangeable than so- Also, the concentration is increased be- dium. Since sodium salts seldom make tween each irrigation through loss of up more than half of the soluble con- water by evaporation from the soil, as stituents of saline soils, very little well as by plant transpiration. And, sodium is adsorbed by the clay par- as we mentioned earlier, the total con- ticles. Therefore the adsorbed ions in 326 Yearbook of Agriculture 1955 saline soils are principally calcium and kind and the amount depending upon magnesium. Such clays are stable in the soil characteristics, the desired rate water, are easily worked into granules of replacement, and economic con- and crumbs, and help produce a de- siderations. Chemical tests are used to sirable environment for seed germina- obtain estimates of the amounts of tion and plant growth. chemical amendments needed to re- Because of the presence of excess duce the exchangeable sodium to a salts and the absence of significant given level. Sodium is relatively easy amounts of adsorbed sodium, saline to replace, so the amount of calcium soils generally are flocculated; and, that must be added to insure replace- as a consequence, their permeability ment is only slightly in excess of the is equal to or higher than that of simi- sodium present. The sodium released lar nonsaline soils. Consequently, if must be removed by leaching with adequate drainage is provided, the water to insure completion of the re- excess soluble salts may be removed action. by leaching with ordinary irrigation. The choice of an amendment may be influenced by the time required for its AN ALKALI (or sodium) soil contains reaction in the soil. In general, the sufficient adsorbed (exchangeable) so- cheaper amendments are slower to dium to interfere with the growth of react. Consequently, if immediate re- most crop plants. The soil may be placement of exchangeable sodium is highly alkaline in reaction but docs desired, one of the quicker acting, but not contain excessive amounts of sol- more expensive, amendments will be uble salts. These soils correspond to needed. Because of its high **black alkali" soils and frequently in water, calcium chloride is probably occur in small irregular areas called the most readily available source of ''slick spots." Sodium usually becomes soluble calcium, but it is seldom used the dominant cation in alkali soils because of its cost. and either through the accumulation of iron and aluminum sulfates that hydro- sodium salts or as a result of the pre- lyze readily in the soil to form sulfuric cipitation of calcium and magnesium acid are also quick-acting and rela- salts. tively expensive amendments. Lime- As the proportion of exchangeable sulfur, sulfur, sulfur-containing gases, sodium increases, soils tend to become and other acids are useful but generally dispersed and impermeable to water are too expensive. These acids and and air. Because the partially sodium acid-forming amendments should be saturated clay is highly dispersed, it used only on calcareous soils because may be transported downward through they react with limestone (calcium the soil and accumulate at lower carbonate) to release soluble calcium, levels, where the soil may develop into which replaces the adsorbed sodium. a dense layer with a columnar struc- Because of its comparatively low cost, ture, having a low permeability. It gypsum is the most common amend- becomes increasingly diflBcult to re- ment used for reclamation. The rate of plenish the water supply of the root reaction of gypsum is limited only by zone by irrigation, and also more diffi- its relatively low solubility in water. cult to establish a condition of surface Except in places where sulfur is used, tilth favorable for seed germination alkali soils should be leached immedi- and seedling growth. ately following the application of the Alkali soils may be improved or re- amendments. Leaching dissolves and claimed by the replacement of the carries the amendment downward and harmful exchangeable sodium by bene- removes the soluble sodium replaced ficial calcium and magnesium. That by calcium-bearing amendments. is generally accomplished by the ad- The reclamation of alkali soils in- dition of chemical amendments, the volves more than replacement of the Irrigation Water and Saline and Alkali Soils 327 adsorbed sodium, however. A good social conditions, and a few to lack of physical condition must be restored. adequate engineering, most failures That involves the rearrangement and have been due to unfavorable condi- aggregation of soil particles to form tions of water, soil, and drainage. soil granules which produce good tilth. Good soil structure is promoted by alternate wetting and drying, and freezing and thawing, and by the ac- tion of plant roots and organic matter.

A SALINE-ALKALI SOIL contains exces- sive quantities of both soluble salts and adsorbed sodium so distributed that ;>:?:?S^z the growth of most crop plants is re- '^=1JÍ!Í-'-T^-~^L>>V- r duced. The soil is seldom highly alka- ^-^^'^^•^ line (pH above 8.5) in reaction. The Many people have learned through soils form as a result of the combined experience that the productivity of processes of salinization and the ad- some irrigated land may be relatively sorption of sodium. As long as excess short-liv^ed. On the other hand, many salts are present, the appearance and irrigated areas are very successful and properties of these soils usually are continue to be highly productive for a similar to those of saline soils. If the long time. It seems clear that long- excess soluble salts are leached out, the continued irrigation farming can be soil properties may change markedly practicable where conditions are fa- and become similar to those of alkali vorable. Furthermore, if the causes of soils. They become strongly alkaline, failure are ascertained and clearly the particles disperse, and the soil be- understood, methods of avoiding fail- comes unfavorable for the entry and ure may be devised and used. movement of water and gases and for tillage. MILTON FIREMAN is a member of the The management of saline-alkali Agricultural Extension Service of the Uni- soils is then similar to that of alkali versity of ^ in charge of problems soils. That is, the soluble salts and relating to saline and alkali soils and water exchangeable sodium must be removed quality in California. He has published by the addition of amendments fol- papers on soil analysis^ cation exchange, soil lowed by leaching. In theory, it is permeability, and on the characteristics and economical to leach out m.ost of the reclamation of saline and alkali problem soluble salts before the application of areas. Dr. Fireman is a graduate of the amendments. That . is not recom- University of Arizona and received his doc- mended, however, because the per- torate from the University of California, meability of saline-alkali soils declines Berkeley. markedly upon leaching and the rate H. E. H AY WARD is director of the U. S. of reclamation is retarded. Saline- Salinity Laboratory, Riverside, Calif., Agri- alkali soils often contain gypsum. cultural Research Service. He has published When such soils are leached, the gyp- a number of papers on the anatomical and sum dissolves and the replacement of physiological responses of agricultural crops exchangeable sodium by calcium takes to saline conditions, among them flax, toma- place concurrently with the removal toes, peaches, and oranges. He is the author of the excess salts. of The Structure of Economic Plants. Dr. Hayward is a graduate of the Univer- THE HISTORY of irrigation develop- sity of Minnesota and received his doctorate ment in this country and elsewhere from the University of Chicago, where, shows that, while some failures may be before joining the staff of the Salinity Labora- assigned to unfavorable economic or tory, he was professor of botany.