Salt in soil

Scan of an article by L. A. Richards and C. A. Bower entitled, "Salt in soil," originally published in the 1962 Yearbook of Agriculture, pages 202-208, reprinted as "Yearbook Separate no. 3156"A house plant suffers when salts accumulate in its soil. YEARBOOK SEPARATE NO. 3156 Reprinted from pages 202-208 of the 1962 Yearbook of Agriculture Salt in Soil A P O T T E D plant has a lovely color and is growing vigorously when one brings it h o m e from the florist's shop. In a few weeks or months it may have stopped growing, and its leaves are dropping off. Very likely it is an example of what happens when soluble salts accu­mulate in the soil, a problem mainly in arid regions where irrigation is required to produce crops. Water used for drinking or irrigation contains more or less dissolved salt, such as c o m m o n table salt. Each cup of water supplied to the potted plant adds a certain amount of salt to the soil. The water evaporates from the plant leaves and from the soil surface, but nearly all the salt remains behind. Some of the salt may appear as a white crust on the soil surface. As each cup of water is added, the amount of salt in the soil increases until finally the plant can no longer grow. Soil salinity usually can be corrected. The treatment for a potted plant is the same as for agricultural crops in the field. If water is caused to move through the soil, it will dissolve and leach out the excess soluble salts. A potted plant that has become affected by salt can be placed in a sink with the faucet dripping fast enough to maintain a layer of water over the soil. The water draining from the opening in the bottom of the pot will soon carry away the excess salt, and the plant will recover. Because the leaching will also remove soluble nutrients, recovery will be more rapid if ferti­lizer is added after leaching. The United States Salinity Laboratory at Riverside, Calif., has developed a simplified test for salinity. A sample of soil from the root zone, whether from a potted plant, a greenhouse bench, or an irrigated field, is placed in a cup and mixed to a saturated paste by adding distilled water while stirring. The paste is then placed in a vacuum filter, and the salt content of the solution removed from the paste is measured with an electrical conductivity bridge. T he bridge can also be used to test the salt content of irrigation water. The management of irrigation water for a potted plant is similar in many ways to water management for irrigated farms. The water that drains into the saucer under the pot contains more salt than the irrigation water, but this saline water moves back into the root zone if it is not discarded. Potted plants often are subirrigated by supplying water to the saucer and allowing it to move up into the soil. This same process occurs in fields when a water table exists 202 PURCHASED BY THE UNITED STATES DEPARTMENT OF AOBICULTURE FOR OFFICIAL USE &^7&&^m Cotton on nonsaline soil in California has a uniform stand and growth. Cotton on saline soil has an irregular stand and growth. close to the soil surface. This is hazardous in arid climates because the ground water contains salt and the root zone becomes salinized. The water table should be kept below the root zone in irrigated land. This occurs naturally in some places, but often one has to provide artificial drainage for lowering the water table. A grid of . 203 Testing soil for salinity. deep ditches along the boundaries of fields often is provided. Some­times tile lines are laid in the field to collect ground water and convey it to a collector ditch. Roots absorb the water applied to soil but exclude most of the salt. T o prevent the soil from becoming saline, some of the applied irrigation water has to leach through the root zone and be discarded as drainage water, in which the amount of salt removed should equal the amount added in the irrigation water. The fraction of the irrigation water that must be removed as drainage water is called the leaching requirement. Its value depends on the salt con­tent of the irrigation water and the tolerance of the particular crop to salinity. If soil to be put under irrigation is naturally saline or if irrigated soil becomes saline because of improper water management, the whole field often is leached until it is essentially free of soluble salts. That is done by ponding water on the surface; the ponding is done by building a network of contour dikes. Plants vary in their tolerance to soil salinity. M a n y fruit and vegetable crops suffer if only a small amount of soluble salt is present in the soil. Cer­tain field crops, such as sugarbeets, alfalfa, and cotton, can tolerate larger amounts and still produce profitable yields. The reduction in yield of crops caused by various kinds and amounts of soluble salts is determined by controlled experiments at agricultural research stations. W e grow different kinds of plants under controlled saline condi­tions at the Salinity Laboratory. The roots of the plants are sup­ported in clean, coarse sand. Beneath the sand is a reservoir of solution that contains all the essential mineral nutrients. In addi­tion, the solutions in the various cultures contain different salts that cause soil salinity. Periodically a timeclock operates a pump, 204 " AA7: •••''•07,00}%:. ***** S^AO^:^7\;\7: The tile outlet into a drainage ditch. Leaching of soil by ponding water on the surface. Sand cultures for testing the salt tolerance of plants. Small plots for testing the salt tolerance of plants at the United States Salinity Laboratory, Riverside, Calif 206 An apparatus for studying water retention by soils. Dr. L. A. Richards of the Salinity Laboratory uses psychrometers to measure the relative humidity of water in soil. which floods the solution in the reservoir over the surface of the sand. The excess solution drains back into the reservoir. Thus the salt solution surrounding the plant root is controlled. The tolerances of various species and varieties of plants also are tested in small field plots. W e irrigate the plots with water to which measured amounts of different salts have been added. The tolerance of crops to salinity is usually expressed in terms of the amount of salt in the soil solution that reduces the yield to half of that obtained when the crop is grown in nonsaline soil. Information on the toler­ance of various crops to salinity is given in agricultural handbooks. W e have developed a pressure membrane apparatus for measuring the relation between the water content and the tenacity with which water is retained by soil. This apparatus is used in laboratories around the world for classifying soils and for determining irrigation practices. The purpose of irrigation is to maintain water conditions in the soil that are favorable for plant growth. Because soils are porous and fine grained, the total surface area of the particles is enormous. For example, the total surface area of the particles in a vial of clay soil amounts to i acre. T h e surfaces of soil particles adsorb and retain water as a film. As soil dries, the water films on the surfaces become thinner and are held more tightly. The result is that plant roots, in effect, must exert increasing suction to obtain water from the soil. As the suction required to obtain water increases, the growth of the plant diminishes. W h e n the amount of soil in the vial has its water depleted to the volume represented by the colored liquid in an adjacent vial, crops in this soil wilt and stop growing. This is one of the determinations made with the membrane appara­tus. (L. A. Richards and C. A. Bower) 208 GPO 94022