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Gypsum is spreaded in soils of arid and semi-arid regions and controls the soil physiochemical properties when it exists as the main soil component. Due to the relatively high solubility, gypsum has unfavor effects in agriculture and plant growth. Therefore, the accurate determination of gypsum concentration is very important for soil classification and land use management. In this study, the different parameters affecting the gypsum determination by the acetone method, namely gypsum dissolution kinetic, extractant to soil ratio, and feasibility of using salts solutions as the extractant in gypsum determination, were investigated. Results revealed that the gypsum dissolution reaction was fast and most of the gypsum was dissolved in 0.5 h. extracting at lower extractant to soil ratios (1:200) underestimated the gypsum. The gypsum dissolution increased by increasing the contact time, which eliminated the underestimation in gypsum determination. Accordingly, the measured gypsum in soils with 10, 20, 30, 40, 50, 60, 70, 80, and 100 % gypsum increased from 10.1, 19.3, 27.1, 35.6, 38.7, 42.4, 42.3, 42.0, and 42.2 % at 0.5 h extraction to 11.9, 23.1, 33.5, 44.4, 51.8, 72.6, 70.0, 78.6, and 81.6 % at 8 h extraction, respectively. Increasing extractant to soil ratio to 1:400 overestimated the gypsum in the samples containing less than 50 % gypsum. NaCl, KCl and MgCl2 solutions increased the gypsum solubility due to the uncommon ion effect and eliminated the underestimation in gypsum determination. The overestimation was increased at the higher extractant to soil ratios (1:400) when the salt solutions were employed. Key words; Contact time, Extractant, Gypsum dissolution, Salt solutions.
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