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Solubility and Availability of Sorbed Sulfate in Hawaiian Soils¹

ABSTRACT

The capacity of volcanic ash soils to adsorb sulfate is related to rainfall, a major factor in weathering. Highly weathered and leached soils developed from ash may contain 7,000 µg/g of sulfate S in the subsoil. This sulfate has very low water solubility. Some less weathered ash soils are almost devoid of sulfates. Desorption of sulfate decreases the solubility of adsorbed sulfate remaining. Sulfate sorption-desorption isotherms plotted against log concentration of sulfate in equilibrium solutions give approximately straight lines up to 16 to 30 ppms S in solution. Indications are that "first phase" adsorption maxima were attained by highly weathered soil at about 16 ppm S in solution. For less weathered soils, "first phase" adsorption maxima were attained at about 33 ppm S in solution. Pot studies on soils which sorb much sulfate indicate that the soil solution should contain about 5 ppm SO₄-S for adequate nutrition of kikuyu grass. This corresponds to 60 to 80% of first phase adsorption maxima of the ash derived soils. Soils which have low capacity to adsorb sulfate need higher concentrations of sulfate in solution to compensate for low adsorption capacity. A soil which field experiments had shown to be very S deficient was only 10% sulfate saturated.

¹ Published with the approval of the Director of the Hawaii Agr. Exp. Sta. as Journal Series no. 1165. This work was supported by a grant from the Sulphur Institute. Presented before Div. S-4, Soil Sci. Soc. Amer., Nov. 13, 1969, at Detroit, Mich.

² Graduate Assistant, Professor, and formerly Graduate Assistant, Dept. of Agronomy and Soil Sci., Univ. of Hawaii, respectively. Received April 13, 1970.

Received for publication April 13, 1970. Accepted for publication July 6, 1970.