Sulfate and Gypsum Determination in Saline Soils

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Abstract
Despite advances in instrumentation for direct determination of SO2‐4, many investigators analyzing saline soils continue to estimate SO2‐4 by difference between the sums of major cations and anions and to measure gypsum by methods that avoid direct SO2‐4 determination. The present study compares SO2‐4 in saturation extracts as estimated by difference with SO2‐4 measured by single‐column ion chromatography for saline Aeric Calciaquolls and Typic Haplaquolls in eastern North Dakota. Results should be applicable throughout the Northern Great Plains. For extracts of sulfatic, saline soils, estimation of SO2‐4 by difference compared favorably with SO2‐4 determined directly. Results were more variable for highly chloridic soil extracts because errors from other methods were large relative to the amount of SO2‐4 present. Gypsum was determined by using a sufficiently dilute extract to completely dissolve the gypsum present in the soil sample. Gypsum content was calculated in two ways, based either on Ca2+ or SO2‐4 measured in the dilute extract. For highly chloridic soils, gypsum contents based on Ca2+ determination were essentially equal to the results based on SO2‐4 data. For sulfatic soils, gypsum contents were lower when Ca2+ values were used due to loss of Ca2+ to the exchange complex during the extraction process.