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Sodicity Levels of Soils Equilibrated with Wastewaters¹

ABSTRACT

To determine the effect of NH⁺₄, K, and Na exchange reactions concurrent with carbonate precipitation on the sodicity levels of soil effluents, samples of sandy, loessial, and clay soils were treated with two wastewaters, a secondary sewage and a liquid sludge. More calcium carbonate (CaCO₃) precipitated following the liquid sludge treatment than after the secondary sewage treatment. However, the sodicity levels of the liquid sludge leachates of the three soils were almost unchanged, as compared with that of the wastewater, while those of the secondary sewage increased slightly. It was assumed that this difference was due to the higher concentrations of NH⁺₄ and K found in the liquid sludge as compared with those in the secondary sewage. To verify this assumption, the preference for NH⁺₄, K, and Na of the three soils was evaluated from cation exchange batch experiments; the preference for NH⁺₄ and K was an order of magnitude higher than that for Na. At a given total salt concentration, the sodicity levels decreased as more NH⁺₄ and K substituted for Na. The ionic activity product of CaCO₃ which precipitated during the treatments with both waste-waters had the same value and was higher than that of soil CaCO₃. This difference was attributed to the inhibiting effect of the organic residues. The sodicity levels of soils equilibrated with wastewaters were affected both by NH⁺₄ and K exchange and by the higher solubility of the precipitated CaCO₃.

¹ Contribution no. 1182-E, 1984 series, from the Agricultural Research Organization, The Volcani Center, Bet Dagan 50-250, Israel.

² Soil Scientist, Graduate Student, and Soil Scientist, respectively.

Received for publication August 6, 1984. Accepted for publication June 24, 1985.