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Effects of Saline Water on the Chemical Properties of Some Tropical Soils¹

ABSTRACT

Surface and subsurface samples of soils representing the Tropical Red Earths and Tropical Red Deserts were equilibrated with unbuffered salt solutions which had the composition of sea water and ranged in concentration between 0.61N and 0.0061N. The effects of these treatments on soil pH, the concentration of soil solution, and exchangeable sodium percentages were subsequently characterized. The original acidities of these soils were highly resistant to change. Neither soil showed a tendency to accumulate salts by extraction from percolating solutions. Studies of exchangeable sodium percentages at the various concentration levels supported the valance-dilution principle. At high salt concentrations, however, the observed affinities of these soils for divalent cations were higher than those predicted by the commonly used US Salinity Laboratory regression equation.

To eliminate the possible interference by CaCO₃ in observed ionic equilibria, Na-Ca exchange studies were conducted on carbonate-free soil samples. Isoconcentration isotherms were constructed at electrolyte concentrations of 0.01, 0.1, and 1.0N, compared with isotherms calculated from the US Salinity Laboratory's regression equation. Large discrepancies were observed at 1.0N concentration, particularly for the lowest soil horizons. For that concentration, it was found that predictions of ionic distribution based on the theory of the diffuse double layer can closely describe Na-Ca equilibrium when large values of surface charge densities are assumed.

¹ Journal Series no. 989 of the Hawaii Agr. Exp. Sta. and Contribution no. 309 of the Hawaii Institute of Geophysics. Paper presented before Div. S-4, Soil Sci. Soc. Amer., Nov. 9, 1967, at Washington, D.C. Work supported in part by Contract no. 14-01-001-673 from the Office of Saline Water, USDI.

² Assistant Soil Scientist and Soil Scientist.

Received for publication May 14, 1969. Accepted for publication October 27, 1969.