Effect of Gypsum Fineness and Method of Incorporation on Reclamation of Sodic Soils

A laboratory study was conducted using five different degrees of gypsum fineness (passed through 10,18,30,60 and 120 mesh sieves) and two methods of gypsum application, mixing in whole soil and surface placement. Two sodic soils of pH2 10.10 and 9.80, respectively, were leached with distilled water after application of gypsum equivalent to 50% of laboratory estimated gypsum requirement of the soil. Results showed that surface placement of gypsum, passed through 2 mm sieve openings, maintained higher hydraulic conductivity and improved the soil better as compared to finer grade gypsum material and/or mixing in the whole soil. More precipitation of calcium as insoluble calcium carbonate when gypsum of finer grades is used, and higher replacement of exchangeable sodium in case of surface placement are postulated as the possible reasons for such a behaviour in reclamation of sodic soils containing sodium carbonate. Continuous release of calcium from coarser particles helps in maintaining higher hydraulic conductivity over prolonged periods.