In some field and laboratory studies , H2SO4 has shown better reclamation efficiency than gypsum, but the explanation for this has been debated. We tested the hypothesis that significant amounts of HCO might be formed during the reaction of HSO3 with CaCO3 leading to additional Ca2+ in solution and enhanced displacement of exchangeble Na +. To determine if H2SO4 is more efficient than gypsum , we compared four equivalent treatments of H2SO4 and gypsum on three soils and two clay minerals. Gypsum and H2SO4 were reacted with calcareous sodic soils and clay minerals in closed-system batch studies, with CO2 pressures typical of soil root zones. Column studies comparing two equivalent treatments of H2SO4 and gypsum were conducted on one of the soils and produced results comparable to the batch studies. Soluble ions, electrical conductivities (EC), and mineral saturation indices were determined and compared between treatments . Clay dispersion and average dispersed particle size were compared between amendments on bentonite . When H2SO4 was applied to calcite in the absence of exchangeable sodium (soil), the solution was fourfold oversaturated with respect to gypsum. This was not observed in the soils because the exchange phase of the soil was a sink for Ca2+. In the soils, there was significantly more HCO3 in all H2SO4 treatments, with an average of 8 % of the H+ applied producing H3CO3. The concentration of dispersed clay was significantly less in three of the four H2SO4 treatments on bentonite , while the mean particle diameter was significantly larger in two of the four treatments , suggesting better tactoid preservation . Compared to equivalent soil gypsum treat ments, H2SO4 generally caused (1) lower sodium adsorption ratios , (2) more soluble Ca 2+ and Mg2+, (3) lower pH , and (4) greater EC. In the calcareous sodic soils and clays we tested , it appears gypsum supersaturation , HCO3 production , lower pH , and higher EC frequently contributed to better reclamation with H2SO4. Keywords: Gypsum RequirementReclamation EfficiencySodium Adsorption Ratio

J. E. Mace

Comparison of Gypsum and Sulfuric Acid for Sodic Soil Reclamation

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