Dissolution and Precipitation of Gypsum in the Soil Under Irrigation

Two soils were used in this soil column study. Yolo loam soil a non- c al c areous, non-gypsiferous soil from central California and Vernal soil a calcareous soil from eastern Utah. Initial studies were conducted where the solubilities of pure CaC0 3 and CaS0 4 \u27 2H 2 0 were determined in the presence of Logan river water, a KCl solution (2.8 mmhos/cm) and a K 2 S0 4 solution (Z. 7 mmhos/cm). Gypsum was more soluble in the KCl solution than in the K Z S0 4 solution. The solubility product of both gypsum and lime were determined from the analytical data. The formation of CaS0 4 0 and caco 3 0 ion pairs were cons idered to be the most important complex ions pres ent. A reasonable agreement was observed between the theoretical values of K and the ca lculated K sp sp Essentially the same results were obtained when these waters were used in leaching the columns of Yolo and Vernal soil columns The solubility of gypsUJn was greatest usmg the KCl leaching\u27 solution and least using the K 2504 leaching solution. Logan river water gave intermediate values. Calcium carbonate was most soluble in the K 2504 leaching solution and les s in the KCl solution. It was found that measuring the ci+ concentration in the effluent was a more reliable index to steady state condition than using an electrical conductivity lneasurement. Unsaturated flow removed more salt per unit volume of effluent than saturated flow. The relative area method was used to analyze the effluent under saturated moisture flow. 2+ It was found that a certain amount of Ca derived from gypsum dissolution precipitated in the column as CaC0 3 . This amount 2+ , was greater than 15% of the Ca dlssolved from gypsum