Chemical changes in a saline-sodic soil after gypsum application and cropping

Reclamation is needed on three million ha of slowly permeable saline-sodic soils in the Indus Plain of Pakistan. Previous studies measured an increased field-saturated hydraulic conductivity (K-fs) in the soil under study with cropping and gypsum application. This field experiment was conducted on a low permeability, saline-sodic soil (a fine-loamy, mixed thermic Typic Natrustalf) to compare the leaching of sodium and soluble salts and changes in chemical properties after various treatments. Treatments were: (i) perennial alfalfa (Madicago sativa L.), (ii) a rotation of sesbania [Sesbania bispinosa (Jacq.)W.E Wright]-wheat (Triticum aestivum L.)-sesbania, (iii) incorporated wheat straw at 7.5 Mg ha(-1) and (iv) a fallow control. These four treatments were each combined with and without 25 Mg ha(-1) of gypsum and open-ditch drainage, Electrical conductivity (EC), pH, Na+, Ca2+, Mg2+ and C1(-) of tile soil in the saturated paste extract under each treatment were measured in each 20 cm increment to 120 cm after 6 month and 1 yr. Gypsum application increased the souble Na+ in the top 20 cm soil. Poor internal drainage of the soil caused the exchanged Na+ to remain in the soil solution. However, one year after the treatments, the crop rotation with gypsum significantly decreased SAR, EC, pH and C1- in the top 20 cm of soil. Alfalfa decreased these same parameters when compared to fallow in the top 80 cm of soil in gypsum-treated plots, The open-ditch drainage was rot helpful in reclamation of this soil. In general, for surface soil improvement, a combination of added gypsum plus crop rotation was the best. For improvement of the deeper soil profile, gypsum plus crop rotation was the most effective of the treatments used. (C) 1997 Elsevier Science B.V.