Plant-Water 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 important 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 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 tolerance. Sugar beets, milo, Bermuda grass, and Rhodes grass are strongly salt tolerant. Alfalfa, cotton, tomatoes, sorgo, and several rye grasses are regarded as having 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 he. 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.