EFFECT OF MOISTURE CONTENT ON THE DISSOLVED AND EXCHANGEABLE IONS OF SOILS OF ARID REGIONS

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Abstract
Dissolved ions of 6 soils containing different quantities of soluble salts, alkaline earth carbonates, and gypsum were detd. at field moisture, satn. %, 200%, and 500% moisture. The 3 more dilute suspensions were prepd. from the field moisture sample by simple dilution with boiled dist. water. Soluble Na, K, NH4, CO3-HCO3, and PO4 increased in all soils with increasing moisture content, and SO4 in all but one; Ca and Mg increased in 4 soils, passed through maxima in 1, and decreased continuously in 1. The increases are explained by ion exchanges and the soln. of slightly soluble compounds by additional amts. of water; the decreases in Ca and Mg result from a cation valence dilution effect whereby soluble Ca and Mg replace exchangeable Na, K, and NH4 on dilution. Cl and NO3 were reduced on dilution of all 6 soils by av. values of 28% and 23%, resp.; this may result from unfree soil water or negative adsorption of these monovalent ions. Exchangeable Na of all soils decreased on dilution, up to 58%. The exchangeable Ca:Mg ratio of the 2 gypsiferous soils increased tremendously on dilution. Soluble salts of soils of this nature should be detd. at the moisture content at which the results are to be interpreted. In soils with appreciable contents of mixed salts, the exchangeable base status applies only at the moisture content at which the soluble salts are detd. It is concluded that the mobile soil soln. cannot be homogeneous throughout its volume and generally is less concd. near the surface of the soil colloids.