Particle Size Distribution and Gypsum Content in Gypsic Soils

Gypsic soils are extremely difficult to obtain a true representative particle size analysis, as gypsum induces clay flocculation in aqueous solution. Hesse (1974) recommended the treatment of gypsum crystals with BaCl₂ solution to form a protective coating of barium sulfate which isolates gypsum from further reaction in an alkaline medium thus achieving dispersion of clays. Four different approaches described in the literature using BaCl₂ were evaluated using 10 g samples with varying amounts of gypsum, but none reduced flocculation when performing particle size analysis using the pipette method. Current efforts to develop a procedure for soils with greater than 40% gypsum using laser diffraction particle size analysis will be presented. Criteria used during development include non-aqueous media, and use of particle size distributions that are known by accepted methodology. Also, the common laboratory method of determining gypsum by dissolution in water and precipitation with acetone (procedure 6F1a, Soil Survey Laboratory Methods Manual, 1996) was evaluated. The decanted supernatant was saved for each sample, evaporated, and brought to volume in 100 ml of distilled water. Calcium and SO₄^2- were determined using atomic absorption and ion chromatography, respectively, to determine each loss during the decanting steps. Calcium loss was from 0 to 0.01 mmol(+)/L and SO₄^2- loss was from 0.002 to 0.005 mmol(+)/L. Losses were well within acceptable limits for gypsum analysis.