282 YEARBOOK OF AGRICULTURE 1957 gions of an arid or a scmiarid climate. Under humid conditions, the soluble salts originally present in soil materials and those formed by the weathering of Saline and minerals generally are carried down- ward into the ground water and are transported ultimately by streams to Alkali Soils the oceans. In arid regions, leaching and trans- C. A. Bower and Milton Fireman portation of salts to the oceans is not so complete as in humid regions. Leach- Saline and alkali conditions ing is usually local in nature, and solu- ble salts may not be transported far. lower the productivity and This occurs because there is less rain- value of large areas of agri- fall available to leach and transport the salts and because the high evapo- cultural land in the United ration and plant transpiration rates States—an estimated one- in arid climates tend further to con- centrate the salts in soils and surface fourth of our 29 million acres waters. of irrigated land and less ex- Weathering of primary minerals is the indirect source of nearly all soluble tensive acreages of nonirrigated salts, but there may be a few instances crop and pasture lands. in which enough salts have accumu- lated from this source alone to form a Saline and alkali soils are soils that saline soil. Saline soils usually occur in have been harmed by soluble salts, places that receive salts from other consisting mainly of sodium, calcium, locations; water is the main carrier. magnesium, chloride, and sulfate and secondarily of potassium, bicarbonate, RESTRICTED DRAINAGE usually con- carbonate, nitrate, and boron. tributes to the salinization of soils and Salt-aifccted soils are problem soils may involve low permeability of the that require special remedial measures soil or the presence of a high ground- and management practices. water table. Soluble salts may harm soils by in- High ground-water tables often are creasing the salt concentration of the related to topographic position. The soil solution and by increasing the per- drainage of waters from the higher centage saturation of the soil adsorp- lands of valleys and basins may raise tion complex with sodium. the ground-water level near to the soil The second effect occurs when so- surface on lower lands. Low perme- dium salts predominate. It is more per- ability of the soil causes poor drainage manent than the first because adsorbed by impeding the downward movement sodium usually persists after most of of water. The impedance may be the the soluble salts are removed. result of an unfavorable soil texture or Saline soils contain excessive amounts structure or the presence of hardened of soluble salts only. Alkali soils con- layers, called hardpan. tain excessive adsorbed sodium. Be- Salt-affected soils occur extensively cause leaching may have occurred pre- under natural conditions, but the salt viously, alkali soils do not always con- problem of greatest importance in agri- tain excess soluble salt. They are desig- culture arises when previously produc- nated as nonsaline-alkali or saline- tive soil becomes salt-affected as a re- alkali soils according to their content sult of irrigation. of salts. Irrigated lands are often located in Salt-afifected soils occur mostly in re- valleys near streams; because they can SALINE AND ALKALI SOILS 283 be irrigated easily, the lower and more can interchange freely with those in level soils usually are selected for culti- the soil solution, the proportions of the vation. Such soils may be adequately various adsorbed cations are related to drained and nonsaline under natural their concentrations in the soil solution. conditions, but the drainage facilities Calcium and magnesium are the prin- may not be adequate under irrigation. cipal cations in the soil solution and Irrigation waters may contain from 200 on the particles of normal, productive pounds to as much as 5 tons of salt per soils of arid regions. When normal soils acre-foot, and the annual application come in contact with irrigation or of water may amount to 5 acre-feet or drainage waters containing a high pro- more an acre. Considerable quantities portion of sodium, this cation becomes of soluble salts thus may be added to the dominant one in the soil solution irrigated soils in a short time. and replaces part of the original ad- Farmers who bring new lands under sorbed calcium and magnesium. As a irrigation often have failed to recog- consequence of the adsorption of sodi- nize the need for establishing artificial um, alkali soils are formed. drains to care for the additional water and the leaching required to prevent THE ACCUMULATION of soluble salts the accumulation of soluble salts. As a and adsorbed sodium by soils impairs result, the water table may rise from a their productivity in several ways. considerable depth to within a few feet Because of the presence of consider- of the soil surface in a few years. able dissolved salt and the absence of During the early development of ir- significant amounts of adsorbed sodi- rigation projects, water is frequently um, saline soils generally are flocculat- plentiful, and there is a tendency to ed. Their tillage properties and per- use it in excess. This hastens the rise of meability to water therefore are equal the water table. When the water table to or higher than those of similar non- rises to within 5 or 6 feet of the surface, saline soils. The abnormally high salt ground water containing more or less concentration of the soil solution of dissolved salt moves upward into the saline soils, however, reduces the rate root zone and to the soil surface. at which plants absorb water; conse- Ground water, as well as irrigation quently growth is retarded. The retar- water, then causes the soil to become dation of growth is almost directly re- saline. lated to the total salt concentration of the soil solution and is largely inde- ALKALI SOILS CONTAIN excessive pendent of the kind of salts present. amounts of adsorbed sodium. The salinity status of soils is ap- Because of the presence of negative praised in terms of effects on crop electrical charges at their surfaces, soil growth by measuring the electrical particles adsorb and retain cations, conductivity of the solution extracted such as calcium, magnesium, and sodi- from saturated soil paste. The elec- um. While the adsorbed cations are trical conductivity of a solution is a combined chemically with the soil par- good measure of its total salt concen- ticles, they may be replaced or ex- tration, and the water content of satu- changed by other cations that are rated soil is related to the field-mois- added to the soil solution. Each soil has ture range. Thus the electrical con- a reasonably definite capacity to ad- ductivity of the saturation extract is sorb and exchange cations, and the directly related to the total salt concen- percentage of this capacity that is tration of the soil solution under field taken up by sodium is referred to as conditions. the exchangeable-sodium-percentage. The effects of salinity on growth are The exchangeable-sodium-percentage largely negligible when the electrical of alkali soils is usually 15 or more. conductivity reading (expressed in mil- As cations adsorbed on soil particles limhos per centimeter) is less than 2. 284 YEARBOOK OF AGRICULTURE 1957 At readings in excess of about 16, only leaching with irrigation water to re- a few very salt-tolerant crops yield sat- move excess soluble salts. The improve- isfactorily. The yields of very salt- ment of alkali soils involves (besides sensitive crops may be restricted at drainage and leaching) the replace- readings as low as 2; moderately salt- ment of adsorbed sodium by calcium tolerant crops grow satisfactorily below or magnesium and the use of practices readings of 8; only salt-tolerant crops that develop good soil structure. grow satisfactorily when readings range Adequate drainage is essential for between 8 and 16. the permanent improvement of saline While the primary effect of soil salin- and alkali soils. Leaching operations ity on crops is one of retarding growth and the application of amendments by limiting the uptake of water, certain for the replacement of adsorbed so- salt constituents are specifically toxic to dium will be largely ineffective unless some crops. Boron, for example, when the ground-water table remains deep present in the soil solution at concen- enough to prevent appreciable upward trations of only a few parts per million, movement of water. is highly toxic to many crops. The permissible depth to the water Alkali soils remain flocculated and table in various types of soils under their properties usually are similar to irrigation and drainage requirements those of saline soils as long as consider- and methods are discussed in the chap- able amounts of soluble salts are pres- ter on soil drainage, page 378. ent. If the excess salts are removed by leaching, however, saline-alkali soils SOILS CAN BE LEACHED by applying generally become nonsaline-alkali soils, water to the surface and allowing it to and their physical properties deterio- pass downward through the root zone. rate markedly. Leaching is most efficient when it is As the concentration of the salts in possible to pond water over the entire the soil solution is lowered by leaching, soil surface. the adsorbed sodium present causes Water can be ponded on nearly level undesirable characteristics to develop. land in shallow basins formed by the The soil may become strongly alkaline construction of earthen dikes or bor- (pH readings above 8.5), the particles ders 2 to 4 feet high. The dimensions of may disperse, and the soil may be- individual basins depend on the slope come unfavorable for the entry and of the land. Normally the difference in movement of water and air and for elevation at the high and low points of tillage. Adsorbed sodium also may be the basin should not exceed 6 inches. toxic and cause various nutritional dis- It is wise to construct dikes on the con- turbances in plants. tour where possible, especially if the There are two principal aspects of land slopes very much. Overflow gates the salt problem in irrigation agricul- or spillways placed in dikes between ture. One is the improvement (recla- adjacent ponded areas facilitate the mation) of soils that are salt-affected control of water and allow a number under natural conditions or have be- of basins to be kept full simultaneously. come salt-affected because of misman- Either continuous flooding or peri- agement. The other aspect is the man- odic applications of water may be used agement of productive or slightly salt- for leaching. Periodic drying usually affected soils so as to prevent increases helps maintain infiltration rates. When in the soluble salt and adsorbed sodium the topography is such that ponding contents and thus prevent reduction is not practical, moderately effective in crop yields. leaching can be accomplished through frequent applications of excess irriga- SALINE SOILS are improved by estab- tion water in furrows, between border lishing artificial drains if a high ground- strips, or by sprinklers. water table exists and by subsequent Except in climates where the soil SALINE AND ALKALI SOILS 285 freezes, it is often convenient to con- sodium must be replaced with calcium duct leaching operations during the or magnesium and usually soil struc- winter, when water may be more ture must be improved. plentiful and the water table and Some alkali soils contain calcium drainage conditions may be more fa- sulfate in the form of the mineral gyp- vorable than during the regular irri- sum. When such soils are leached, the gation season. gypsum dissolves, and the replacement If the soil is only moderately saline, of adsorbed sodium by calcium takes crops that withstand flooding, such as place concurrently with the removal rice, sesbania, and Bermuda-grass, of excess salts. may be grown while leaching is in When alkali soils do not naturally progress. Care must be exercised in contain gypsum, soluble calcium or hot weather, however, because hold- magnesium must be supplied. Some ing water on some plants too long may waters, when used for irrigation, sup- cause scalding. The permeability of ply appreciable amounts of calcium most soils declines markedly during and magnesium, but it is usually neces- prolonged leaching with water of very sary to apply a chemical amendment low salt content. The decrease in per- to restore alkali soils to productivity in meability is not so marked if the leach- a reasonable length of time. ing water contains a moderate amount of salt and has a calcium plus magne- CHEMICAL AMENDMENTS for the re- sium to sodium ratio of i or more. placement of adsorbed sodium are of three types : Soluble calcium salts (cal- THE AMOUNT of water required to cium chloride and gypsum); calcium leach saline soils depends on the initial salts of low solubility (limestone) ; and salinity level of the soil and the final acids or acid-formers (sulfuric acid, sul- salinity level desired. fur, and iron and aluminum sulfate). When water is ponded, about 50 per- The suitability of the various types cent of the salt in the root zone can be of amendments is governed primarily removed by leaching with 6 inches of by their solubility and by the lime water for each foot of root zone; about content and pH reading of the soil. 80 percent can be removed with i foot Any of the soluble calcium salts or of water per foot of soil; to remove 90 acids or acid-formers may be used on percent of the salt, 2 feet of water must alkali soils containing lime, but the be used for each foot of soil that is addition of limestone will be of no to be leached. value. Acid and acid-forming amend- For example, if the average salinity ments react with the lime in the soil to reading in a 3-foot depth of soil is 40 form gypsum. The addition of acids millimhos per centimeter and it is de- and acid-forming amendments to soils sired to reduce this 80 percent, or to containing no lime tends to make them about 8 millimhos per centimeter, then acid. When the amount of acid or acid- 3 feet of water should be applied. forming amendment needed would If water is not applied to the entire make the soil excessively acid (pH 6 or soil surface, somewhat greater amounts less), the choice of amendment is limit- will be needed to accomplish the same ed to soluble calcium salts, unless degree of leaching. In estimating the limestone also is applied. The applica- amount of water required for leach- tion of limestone alone to lime-free ing, losses of applied water by evapo- alkali soils tends to be beneficial, but ration and transpiration should be its effectiveness is not great unless the taken into account. pH reading is 7 or less. The selection of an amendment is THE REMOVAL of excess soluble salts influenced generally by cost considera- is not enough to restore alkali soils to tions and the time required for its productivity. Most of the adsorbed reaction in the soil. The cheaper 286 YEARBOOK OF AGRICULTURE 1957 amendments are slower to react. Be- ately following the application of cause of its high solubility in water, amendments. Leaching dissolves the calcium chloride is probably the most amendment and carries it downward. readily available source of soluble cal- Leaching also removes the soluble so- cium, but it is seldom used because of dium salts that form as the adsorbed its high cost. Sulfuric acid and iron sodium is replaced by calcium. and aluminum sulfates, which decom- When sulfur is applied, 2 or 3 months pose readily in the soil to form sulfuric should be allowed before leaching so acid, also act quickly. Sulfuric acid is that the amendment may oxidize and often cheap enough for field applica- form gypsum. The soil should be kept tion, but the use of iron and aluminum moist, however, as water is essential sulfates is usually too costly. Because for the oxidation of sulfur. of their relatively low cost, gypsum and If the structure of saline-alkali soils is sulfur are most commonly used. good originally, it will tend to remain The rate of reaction of gypsum is good during the removal of excess salt limited only by its solubility, which is and adsorbed sodium if soil disturbance about 0.25 percent. Under field con- is kept at a minimum and adequate ditions, the application of 3 to 4 acre- soluble calcium is supplied. In this case feet of irrigation water is required to tillage should be reduced, and the use dissolve 4 or 5 tons of the high-grade of heavy machinery should be avoided agricultural gypsum. As sulfur must during the improvement process. first be oxidized to sulfuric acid by soil If the alkali soil is relatively free of micro-organisms before it is available salt, its structure usually is poor. Here, for reaction, it is slow acting. the improvement process may be facili- Limestone is a relatively cheap tated by loosening the soil to a consid- amendment, but only occasionally is erable depth with a subsoiler or deep it useful, as most alkali soils already plow before applying amendments and contain lime. Unless the soil is de- leaching. The value of subsoiling and cidedly acid, the reaction of limestone deep plowing alkali soils is question- is slow. Particle size is an important able, however, unless an amendment is factor affecting the rate at which lime- applied. Even after the removal of ad- stone (as well as sulfur and gypsum) sorbed sodium, soil structure may re- reacts in soils. The finer the particle main poor. The rearrangement and size, the faster the reaction. aggregation of soil particles, so as to Because the application of chemical improve structure, is facilitated by al- amendments usually is expensive, ternate wetting and drying, by alter- chemical tests should be made on soil nate freezing and thawing, and by the samples to determine the kind and action of growing plant roots and amount needed. It is worthwhile to get organic matter. the advice and help of a county farm adviser or Soil Conservation Service A SATISFACTORY and sustained crop technician. production on nonsaline irrigated land Chemical amendments, such as gyp- generally requires the use of special soil- sum, sulfur, and limestone, normally management practices to prevent the are applied broadcast and then incor- excessive accumulation of soluble salts porated with the soil by means of a and adsorbed sodium. disk or plow. Thorough incorporation Often it is not practical to reclaim is especially important when sulfur is completely salt-afí'ected soils or even to used to insure a satisfactory rate of re- maintain conditions of very low salinity action. Because of hazards in handling, and adsorbed sodium (alkali) in irri- special equipment is used to spray sul- gated soils. The reasons may be high furic acid on the soil surface. costs of the amendments, difficulty in Except where sulfur is employed, al- providing adequate drainage, use of kali soils should be leached immedi- irrigation water of inferior quality, or SAUNE AND ALKALI SOILS 287 inherently low permeability of the soil. alkali hazard is more difficult to evalu- If the soil cannot be reclaimed com- ate than the salinity hazard. It is de- pletely, farmers can often live with the termined largely by the proportion of salinity or alkali by adopting suitable sodium to calcium plus magnesium management practices. present, together with the total salt content. As the salt content of the wa- MANAGEMENT PRACTICES for the con- ter increases, the permissible ratio of trol of salinity and alkali include: Se- sodium to calcium plus magnesium de- lection of crops or crop varieties that creases. Under some conditions, the will produce satisfactory yields under bicarbonate content of the water as re- moderately saline conditions; use of lated to its content of calcium plus land-preparation and tillage methods magnesium may influence the alkali that aid in the control or removal of hazard. It should also be recognized salinity and alkali; special planting that irrigation waters may contain procedures that minimize salt accumu- toxic amounts of boron. lation around the seed; irrigation so as For more detailed information on to maintain a relatively high soil- water quality, publications such as moisture level and at the same time Circular 969, Classification and Use of allow for periodic leaching of the soil; Irrigation Waters, of the Department of maintenance of water conveyance and Agriculture, should be consulted. drainage systems; and special treat- We know of ño economically feasible ments, such as additions of chemical method for reducing the salt content amendments and organic matter, and and, thus, the salinity hazard of irri- growing sod crops to improve structure. gation waters. The quality of water used for irriga- The alkali hazard of waters can be tion and the soil texture determines to low^ered or nearly eliminated by low- a large degree the kind and extent of ering the ratio of sodium to calcium management practices needed. In plus magnesium through the addition judging the quality of irrigation wa- of gypsum (calcium sulfate). Reducing ters, primary consideration is ordinar- the alkali hazard of waters is most ily given to the salinity and sodium practical when the total salt content is hazards involved in their use. low because smaller amounts of gyp- The total salt content of the water, sum are required and the resulting in- as measured by its electrical conduc- crease in the salinity hazard is less. tivity, is a good index of the salinity Special equipment for adding gypsum hazard. Waters whose electrical con- to water is available. A simple method ductivity is less than 0.25 miliimhos per of application consists of placing a centimeter can be used for the irriga- cloth bag of gypsum with the side slit tion of most crops on most soils with open at a place in the irrigation system little likelihood that soil salinity will where the water has considerable develop, but in only a few locations turbulence. can water having an electrical conduc- The control of salinity and alkali is tivity in excess of 5 miliimhos per cen- accomplished, in general, most easily timeter be successfully employed. The in coarse-textured soils, which usually greater the electrical conductivity of are quite permeable and are less sus- waters, the greater the care that must ceptible to deterioration of the physical be taken to prevent the accumulation condition upon the accumulation of of salts. Moreover, the salinity of the adsorbed sodium than the soils of finer irrigation water may limit the choice texture. Medium- and fine-textured of crops that can be grown. soils have the advantage of a greater The alkali or sodium hazard of an water-holding capacity and ordinarily irrigation water is an index of its tend- present no great problem from the ency to increase the adsorbed sodium standpoint of salinity control if they content of soils upon application. The have good structure and are underlain 288 YEARBOOK OF AGRICULTURE 1957 by a sand or gravel aquifer which fa- should be tilled carefully. They should cilitates the removal of drainage water. not be tilled when moist. Heavy ma- Prevention of salt accumulation is most chinery should not be moved over difficult in soils consisting of fine- them. More frequent irrigation, espe- textured, slowly permeable material cially during the germination and seed- that extends to a considerable depth. ling stages of plants, tends to soften sur- face crusts on alkali soils and helps to WHERE SALINITY CANNOT be entirely get a better stand. eliminated, the judicious selection of crops that can produce satisfactory FAILURE TO OBTAIN a satisfactory yields under moderately saline con- stand of furrow-irrigated row crops on ditions may mean the difference be- moderately saline soils is a serious prob- tween profit and loss. lem in many places. The failures usu- In selecting crops for saline soils, ally are due to the tendency of soluble particular attention should be given salt to accumulate in raised beds that to the salt tolerance of the crop during are moistened by irrigation water mov- germination, because poor yields fre- ing from the furrow. Modifications in quently result from failure to obtain a irrigation practice and bed shape may satisfactory stand. Some crops that are alter considerably the tendency of salts salt-tolerant during later stages of to accumulate near the seed. Pre- growth are quite sensitive to salinity emergence irrigation in special furrows during germination. placed close to the seed often is done to The tolerances of many crops are reduce the soluble salt concentration listed in Agriculture Handbook No. around the seeds and thus permit ger- 60, Diagnosis and Improvement of Saline mination. After the seedlings are estab- and Alkali Soils, lished, the special furrows may be Among the highly tolerant crops are abandoned and new furrows made be- barley, sugar beets, cotton, Bermuda- tween the rows. grass, Rhodesgrass, western wheatgrass, The tendency of salts to accumulate birdsfoot trefoil, table beets, kale, as- near the seed during irrigation is great- paragus, spinach, and tomato. Crops est in single-row, flat-topped planting having low salt tolerance include rad- beds. Sufficient salt to prevent germi- ish, celery, beans, and White Dutch, nation may move laterally and con- alsike, red, and Ladino clovers, and centrate in the seed zone, even if the nearly all fruit trees. average salt content of the soil is rela- tively low. With double-row beds also, CAREFUL LEVELING of land makes most of the salt is carried into the cen- possible a more uniform application of ter of the bed, but that leaves the water and better salinity control. shoulders relatively free of salt and sat- Barren or poor areas in otherwise isfactory for planting, especially if the productive fields often are high spots soil is only slightly saline. that do not receive enough water for Sloping beds are best on saline soils good crop growth or for leaching pur- because seed can be safely planted on poses. Lands that have been irrigated the slope below the zone of salt accu- I or 2 years after initial leveling often mulation. The salt is carried away from can be improved by replaning to re- the soil around the seed instead of ac- move the surface unevenness caused by cumulating in it. Planting in furrows the settling of fill material. Annual or basins is satisfactory from the stand- crops should be grown after the first point of salinity control but is often un- leveling, so that replaning can be per- favorable for the emergence of many formed without disturbing the crops. row crops because of crusting or poor Soils containing appreciable amounts aeration. of adsorbed sodium are especially sub- The method and frequency of irriga- ject to puddling and crusting. They tion and the amount of irrigation water SALINE AND ALKALI SOILS 289 applied are of prime importance in the tion to that required to replenish losses control of salinity. by plant transpiration and evaporation The main ways to apply water are must be applied occasionally to leach flooding, furrow irrigation, sprinkling, out the salt that has accumulated dur- and subirrigation. ing previous irrigations. Flooding, in which water is applied The additional irrigation water re- to the entire surface, is preferable from quired for leaching is called the leach- the standpoint of salinity control if the ing requirement and is defined as the land is sufíicicntly level and the crop fraction of the applied irrigation water can be flooded. that must be leached through the root Furrow irrigation is well adapted to zone to control salinity at any prede- row crops and is also useful if the land termined level. The leaching require- is too steep for flooding. This method ment therefore depends on the salt con- allows salts to accumulate in the rows, tent of the irrigation water and on the but plowing and mixing the entire sur- maximum salt concentration permis- face soil periodically usually will pre- sible in the soil solution. This maxi- vent serious increases in the salt content mum concentration in turn depends on of the soil. If excess salt does accumu- the salt tolerance of the crop. late, a rotation of crops and a change For salt-sensitive crops, the maxi- to irrigation by flooding is a possible mum concentration of the soil solution salinity-control measure. in the root zone should be 3 to 4 milli- Irrigation by sprinkling allows a close mhos per centimeter. For moderately control of the amount and distribution salt-tolerant crops it should not exceed of water. Sprinkling often is used in 8 millimhos per centimeter. For highly places where the slope is too great for salt-tolerant crops, it should not exceed other methods. One tends to apply too 16 millimhos per centimeter. little water by this method, and leach- If there is no rainfall and no removal ing of salts beyond the root zone is not of salt by the crop and if drainage is accomplished without special efl'ort. adequate and no salt becomes insolu- Subirrigation, in which the water ble in the soil, the leaching requirement table is maintained close to the soil sur- is simply the ratio of the electrical con- face, is not suitable when salinity is a ductivity of the irrigation water to the problem. Even under the most favor- electrical conductivity of the drainage able circumstances, this method is not water, expressed as a fraction or as a suitable for longtime use unless the wa- percentage. ter table is lowered periodically and For example, where an electrical leaching is accomplished by rainfall or conductivity of 8 millimhos per centi- by surface applications of water. meter can be tolerated in the soil solu- As soluble salts retard plant growth tion of the root zone and the irrigation in almost direct relation to their total water has a conductivity of 2 millimhos concentration in the soil solution, the per centimeter, the leaching require- moisture content of saline soils should ment will be 2 divided by 8, or 25 be maintained as high as practicable, percent. That means that if crop use especially during the stage of vegeta- and evaporation amount to 30 inches tive growth. With a given amount of of water during the growing season, 10 salt in the soil, the salt concentration extra inches should be added—a total in the soil solution drops as the mois- of 40 inches that enter the soil. Because ture content of the soil increases. A of the assumptions involved, the 10 high moisture level is maintained by extra inches are a maximum value. irrigating oftener than would be the Care must be exercised in estimating practice for similar nonsaline soils. the leaching requirement by this method, especially if leaching due to BECAUSE ALL IRRIGATION waters con- rainfall has taken place. In any event, tain dissolved salts, some water in addi- the method is useful as a concept of 400157°—57- -20 290 YEARBOOK OF AGRICULTURE 1957 what must occur in the root zone of the growing crop. Unless the soil is well drained, the application of irrigation water in con- siderable excess over that required for Erosion on the crop and for leaching can be as detrimental from the standpoint of salinity control as underirrigation. Cultivated Land Overirrigation increases the amount of water that the drainage system must B. D. Blakely, J. J. Coyle, convey; if the capacity of the system is and J. G. Steele exceeded, the water table will rise to an unsafe level. It is apparent therefore To protect soil from erosion and that a proper relation between irriga- tion, leaching, and drainage is of ut- to hold as much of the rain as most importance in preventing soils possible in a place where crops from becoming salt affected. The amount of water applied should be can use it are a big part of sufficient to supply the crop and satisfy modern soil management. the leaching requirement but not enough to overload the system. We cannot avoid all risks of erosion when we lay a soil bare by cultivating EXCESSIVE LOSS of irrigation water it. Neither can we hold all the rain from canals constructed in permeable where it falls in humid or subhumid soil is a major cause of high water areas. But we need to know the risks tables and salt accumulation. Seepage and control them the best we can. losses can be reduced by lining canals Erosion is slow wherever the soils are with cement, buried asphalt mem- covered by trees or grass. Near Zanes- branes, or more commonly with earth ville, Ohio, scarcely any loss of soil of low permeability. The maintenance could be measured in 9 years from a of drainage systems is also important woodland watershed of 2 acres. A and usually involves nothing more than nearby pasture lost soil during the keeping tile lines in repair or open same period at an average rate of one- ditches clean and excavated to grade. tenth ton an acre a year, or about i A gradual decrease in soil permea- inch in 1,500 years. A similar water- bility is a common cause of declining shed cropped to a 3-year rotation of productivity in land under irrigation. corn, wheat, and hay lost an inch of Without satisfactory soil permeability, soil in the 9 years. crops cannot be kept adequately sup- The rate of erosion in any storm de- plied with water and the leaching of pends on the force with which rain- salts is not accomplished. Soil treat- drops stir up soil and the amount and ments for the maintenance of permea- speed of the runoff water. Other fac- bility are the same as those we dis- tors affecting erosion include kind and cussed for improving soil structure. amount of cover, kind of soil, and To meet the demand for agricultural steepness and length of slope. products, it will be necessary to utilize To judge the erosion hazards in a salt-affected soils and irrigation waters particular situation, we need to look of inferior quality more and more fully. at more than one rain and consider Thus it can be assum.ed that the im- the pattern of rainfall for a whole year provement of salt-afifected soils and the or for several years. management of productive soils so as We need to study the location by to prevent the excessive accumulation looking uphill to see how much water of soluble salts and adsorbed sodium is likely to cross the field and by look- will grow in importance. ing downhill to see where the runoff