Refinement of the differential water loss method for gypsum determination in soils

Determining the gypsum content in soil is lengthy and cumbersome using methods based on SO₄ determination. Moreover, as these methods do not strictly titrate gypsum, inaccuracies can be produced by the presence of sulfate minerals other than gypsum. The thermogravimetric properties of gypsum, however, allow determination of its content in a rapid and easy way sufficiently accurate for many pedologic purposes. Our objective was to expand the lower limit of gypsum detection to make the test useful for soil classification and management. We have refined the differential water loss method by estimating the gypsum percentage from the loss of water in the soil sample between 70 and 90°C. Our results, compared with gravimetric determinations of precipitated BaSO₄, found coefficients of determination of 0.98 for gypsum contents ranging from 2 to 50%, and 0.99 for contents >50%. The method is valid for gypsum contents >2% and thus improves previous estimation procedures.