SOIL SOLUTION CONCENTRATIONS AT THE WILTING POINT AND THEIR CORRELATION WITH PLANT GROWTH

Abstract

Despite a half-century of research on “soil alkali,” a need exists for a semiquantitative index of the degree of plant injury to be expected from various ranges of soil salinity. It is realized that salinity stresses may be modified or masked by other factors of soil, plant, and climate. Nevertheless, if an approximate quantitative relationship could be established between soil solution concentration and plant growth, it would be of value in the diagnosis of salinity effects. The purpose of this paper is to show the ranges of soil solution concentration and composition that occur in irrigated soils and their correlation with plant growth. These relationships are compared to those that have been obtained with plants grown in sand and water cultures of controlled salinity. A collection at this laboratory of stock soil samples from important agricultural areas of the West provided an excellent opportunity for making the type of investigation reported here. Plants growing in soils necessarily respond to the actual soil solution concentration and composition and not to the values calculated from an extraction of salts at artificial moisture contents, which may result in extremely erroneous interpretations. This study, therefore, has involved the securing of the soil solutions actually existent at field moisture contents. It is recognized that the soil solution may not represent the true culture medium of the plant, particularly with respect to nutrients that are slowly released by soils, such as phosphorus and potassium. As regards concentrations of salt, and their effect on plant growth, the soil solution does represent conditions to which plant roots are subjected. The wilting percentage2 represents the lowest moisture content at which a plant can grow. For a constant amount of salts, the concentration found in the soil solution at the wilting percentage is the maximum to which the plant will be subjected, because of the general inverse relationship between concentration and moisture content. Consequently, all soil solution concentrations discussed here have been referred to the wilting percentage by appropriate calculations. Since most of the soil solutions were obtained at soil moisture contents near the wilting range, changes made in the concentrations by these calculations are relatively small.