Detection and Determination of Gypsum in Soils

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Abstract
A rapid, semiquantitative method (I) and a quantitative method (II) are proposed for the determination of gypsum in soils. Method I involves the determination of the degree of gypsum‐unsaturation of an aqueous soil extract nearly saturated with gypsum, i.e., sufficiently dilute to dissolve all gypsum initially present. The unsaturation is determined by electrical conductivity (EC) measurements before and after saturating the extract with reagent CaSO4·2H2O. Increasing the water content of the soil in preparation of the dilute soil extract causes an exchange of adsorbed cations for Ca2+ from gypsum, and the formation of an equivalent amount of sulfate salts of cations other than Ca2+ (“exchange error”). The error is equal to the difference between the observed and calculated contents of salts‐other‐than‐gypsum in the dilute extract. It has been estimated from EC measurements.Method II is a modification of an existing method, where gypsum is precipitated in the dilute soil extract by acetone. The exchange error is avoided by determining gypsum on the basis of sulfate associated with calcium. The method minimizes several potential errors, such as those due to acetone and ions occluded in the gypsum precipitate.For a group of gypsiferous soils, errors due to Ca2+‐exchange and acetone occlusion averaged about −22% and −3%, respectively. Gypsum contents, and recovery of added amounts of gypsum, as determined independently by Methods I and II, compare favorably for the soils studied.

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