The control of salinity and alkali is accomplished, in general, most easily in coarse-textured soils, which usually are quite permeable and are less susceptible to deterioration of the physical condition upon the accumulation of adsorbed sodium than the soils of finer texture. Medium- and fine-textured soils have the advantage of a greater water-holding capacity and ordinarily present no great problem from the standpoint of salinity control if they have good structure and are underlain by a sand or gravel aquifer which facilitates the removal of drainage water. Prevention of salt accumulation is most difficult in soils consisting of fine-textured, slowly permeable material that extends to a considerable depth.
WHERE SALINITY CANNOT be entirely eliminated, the judicious selection of crops that can produce satisfactory yields under moderately saline conditions may mean the difference between profit and loss.
In selecting crops for saline soils, particular attention should be given to the salt tolerance of the crop during germination, because poor yields frequently result from failure to obtain a satisfactory stand. Some crops that are salt-tolerant during later stages of growth are quite sensitive to salinity during germination.
The tolerances of many crops are listed in Agriculture Handbook No. 60, Diagnosis and Improvement of Saline and Alkali Soils.
Among the highly tolerant crops are barley, sugar beets, cotton, Bermuda-grass, Rhodesgrass, western wheatgrass, birdsfoot trefoil, table beets, kale, asparagus, spinach, and tomato. Crops having low salt tolerance include radish, celery, beans, and White Dutch, alsike, red, and Ladino clovers, and nearly all fruit trees.
CAREFUL LEVELING of land makes possible a more uniform application of water and better salinity control.
Barren or poor areas in otherwise productive fields often are high spots that do not receive enough water for good crop growth or for leaching purposes. Lands that have been irrigated 1 or 2 years after initial leveling often can be improved by replaning to remove the surface unevenness caused by the settling of fill material. Annual crops should be grown after the first leveling, so that replaning can be performed without disturbing the crops.
Soils containing appreciable amounts of adsorbed sodium are especially subject to puddling and crusting. They should be tilled carefully. They should not be tilled when moist. Heavy machinery should not be moved over them. More frequent irrigation, especially during the germination and seedling stages of plants, tends to soften surface crusts on alkali soils and helps to get a better stand.
FAILURE TO OBTAIN a satisfactory stand of furrow-irrigated row crops on moderately saline soils is a serious problem in many places. The failures usually are due to the tendency of soluble salt to accumulate in raised beds that are moistened by irrigation water moving from the furrow. Modifications in irrigation practice and bed shape may alter considerably the tendency of salts to accumulate near the seed. Pre-emergence irrigation in special furrows placed close to the seed often is done to reduce the soluble salt concentration around the seeds and thus permit germination. After the seedlings are established, the special furrows may be abandoned and new furrows made between the rows.
The tendency of salts to accumulate near the seed during irrigation is greatest in single-row, flat-topped planting beds. Sufficient salt to prevent germination may move laterally and concentrate in the seed zone, even if the average salt content of the soil is relatively low. With double-row beds also, most of the salt is carried into the center of the bed, but that leaves the shoulders relatively free of salt and satisfactory for planting, especially if the soil is only slightly saline.
Sloping beds are best on saline soils because seed can be safely planted on the slope below the zone of salt accumulation. The salt is carried away from the soil around the seed instead of accumulating in it. Planting in furrows or basins is satisfactory from the standpoint of salinity control but is often unfavorable for the emergence of many row crops because of crusting or poor aeration.
The method and frequency of irrigation and the amount of irrigation water applied are of prime importance in the control of salinity.
The main ways to apply water are flooding, furrow irrigation, sprinkling, and subirrigation.
Flooding, in which water is applied to the entire surface, is preferable from the standpoint of salinity control if the land is sufficiently level and the crop can be flooded.
Furrow irrigation is well adapted to row crops and is also useful if the land is too steep for flooding. This method allows salts to accumulate in the rows, but plowing and mixing the entire surface soil periodically usually will prevent serious increases in the salt content of the soil. If excess salt does accumulate, a rotation of crops and a change to irrigation by flooding is a possible salinity-control measure.
Irrigation by sprinkling allows a close control of the amount and distribution of water. Sprinkling often is used in places where the slope is too great for other methods. One tends to apply too little water by this method, and leaching of salts beyond the root zone is not accomplished without special effort.
Subirrigation, in which the water table is maintained close to the soil surface, is not suitable when salinity is a problem. Even under the most favorable circumstances, this method is not suitable for longtime use unless the water table is lowered periodically and leaching is accomplished by rainfall or by surface applications of water.
As soluble salts retard plant growth in almost direct relation to their total concentration in the soil solution, the moisture content of saline soils should be maintained as high as practicable, especially during the stage of vegetative growth. With a given amount of salt in the soil, the salt concentration in the soil solution drops as the moisture content of the soil increases. A high moisture level is maintained by irrigating oftener than would be the practice for similar nonsaline soils.
BECAUSE ALL IRRIGATION waters contain dissolved salts, some water in addition to that required to replenish losses by plant transpiration and evaporation must be applied occasionally to leach out the salt that has accumulated during previous irrigations.
The additional irrigation water required for leaching is called the leaching requirement and is defined as the fraction of the applied irrigation water that must be leached through the root zone to control salinity at any predetermined level. The leaching requirement therefore depends on the salt content of the irrigation water and on the maximum salt concentration permissible in the soil solution. This maximum concentration in turn depends on the salt tolerance of the crop.
For salt-sensitive crops, the maximum concentration of the soil solution in the root zone should be 3 to 4 millimhos per centimeter. For moderately salt-tolerant crops it should not exceed 8 millimhos per centimeter. For highly salt-tolerant crops, it should not exceed 16 millimhos per centimeter.
If there is no rainfall and no removal of salt by the crop and if drainage is adequate and no salt becomes insoluble in the soil, the leaching requirement is simply the ratio of the electrical conductivity of the irrigation water to the electrical conductivity of the drainage water, expressed as a fraction or as a percentage.
For example, where an electrical conductivity of 8 millimhos per centimeter can be tolerated in the soil solution of the root zone and the irrigation water has a conductivity of 2 millimhos per centimeter, the leaching requirement will be 2 divided by 8, or 25 percent. That means that if crop use and evaporation amount to 30 inches of water during the growing season, 10 extra inches should be added a total of 40 inches that enter the soil. Because of the assumptions involved, the 10 extra inches are a maximum value. Care must be exercised in estimating the leaching requirement by this method, especially if leaching due to rainfall has taken place. In any event, the method is useful as a concept of what must occur in the root zone of the growing crop.
Unless the soil is well drained, the application of irrigation water in considerable excess over that required for the crop and for leaching can be as detrimental from the standpoint of salinity control as under-irrigation.
Overirrigation increases the amount of water that the drainage system must convey; if the capacity of the system is exceeded, the water table will rise to an unsafe level. It is apparent therefore that a proper relation between irrigation, leaching, and drainage is of utmost importance in preventing soils from becoming salt affected. The amount of water applied should be sufficient to supply the crop and satisfy the leaching requirement but not enough to overload the system.
EXCESSIVE Loss of irrigation water from canals constructed in permeable soil is a major cause of high water tables and salt accumulation. Seepage losses can be reduced by lining canals with cement, buried asphalt membranes, or more commonly with earth of low permeability. The maintenance of drainage systems is also important and usually involves nothing more than keeping tile lines in repair or open ditches clean and excavated to grade.
A gradual decrease in soil permeability is a common cause of declining productivity in land under irrigation. Without satisfactory soil permeability, crops cannot be kept adequately supplied with water and the leaching of salts is not accomplished. Soil treatments for the maintenance of permeability are the same as those we discussed for improving soil structure.
To meet the demand for agricultural products, it will be necessary to utilize salt-affected soils and irrigation waters of inferior quality more and more fully. Thus it can be assumed that the improvement of salt-affected soils and the management of productive soils so as to prevent the excessive accumulation of soluble salts and adsorbed sodium will grow in importance.