Use of WATSUIT Model for Predicting the Chemical Composition of the Soil Solution Reclaimed with Mined Gypsum

Proper use of saline and sodic soil and water resources is important for increasing crop production in Iran. For this purpose, chemistry models are very helpful in assessing the impact of saline and sodic water and amendment application on soil and plant. In the present study, effect of gypsum on reclamation of sodic soils was investigated at Dorudzan- Marvdasht plain in Fars Province. Moreover, WATSUIT model was used for assessing chemical composition of a soil irrigated with sodic water,(EC=2.17 and SAR=17.1) after application of mined gypsum. The treatments consisted of four gypsum level (0, 3, 6 and 24 meq/l). The experiment was carried out in three replications in 20-m² plots. Predicted data showed calcium and sulfate concentration increased and soil alkalinity decreased with increasing gypsum rate. Data obtained from the experimental plots indicated that concentration of the ions, salinity(ECe), and sodium adsorption ratio (SAR)  were underestimated by WATSUIT model, mainly due to the low leaching efficiency in the field condition. During reclamation of a sodic soil, salinity threshold for stabilizing soil structure, magnesium to calcium (Mg/Ca) ratio, and the neutralization of alkaline ions to reach a safe value for soil and crop production should be considered. Three mili-equivalents gypsum per liter was sufficient to provide the appropriate values of ECe and SAR to stabilize soil structure, while, based on the model prediction, more gypsum was required for this purpose. The difference is related to the impurity of the applied mined gypsum used in the study. Both the calculated and the measured data show that, even with applying high rates of gypsum, soil alkalinity does not reach a safe range (<1.5 meq/l).  In general, the model predicted that 24meq/l gypsum was the most suitable treatment for reclaiming the soil under the experimental condition.