Air drying affects extractable sulfate in soils of variable charge: test of two extraction and two quantification methods

Fertilizer recommendations need to be based on reliable soil sulfate determinations. Airdrying samples changes irreversibly many properties of soils with variable charge and might affect the extractable sulfate. In this study, sulfate extracted from air-dry and field-moist samples was compared. Two extracting solutions [water and 00.1 M Ca(H2PO4)2] and two quantification methods (turbidimetry and ion chromatography) were assayed on A and B horizon samples of five Humic Acrisols from southeast Mexico. Air drying increased water-extractable sulfate in Ah horizons, whereas in Bt horizons, it increased the 00.1 M Ca(H2PO4)2-extractable sulfate. Airdrying increased dissolved organic carbon contents in all samples and increased soil acidity and oxalate extractable iron in 70 and 60% of the samples, respectively. Results showed larger coefficients of variation in air-dried samples. Turbidimetry resulted less sensible than ion chromatography. To enhance sensitivity and reproducibility, particularly organic soil samples should be analyzed field-moist and by ion chromatography.