A Critical Evaluation on the Reclamation of Complex Sodic Soils Using Gypsum in the Indo‐Gangetic Plain (IGP) of India

The gypsum (CaSO ₄ .2H ₂ O) from diverse sources (mined vs. commercial) is commonly used for the reclamation of sodic soils around the world. The efficiency of soil reclamation differs widely with sodicity levels (slight, moderate, and strong), soil depth (surface vs. subsurface), and texture (coarse to fine) and also with irrigation water quality (good to saline with Sodium Adsorption Ratio [SAR] > 13) or sodic (Residual Sodium Carbonate [RSC] > 2.5 me L ⁻¹ ). Abrupt application of gypsum caused variable results of soil reclamation at the farm level. Based on the comprehensive reviews on the diverse outcome of gypsum‐based soil reclamation for the complex sodic (higher content of alkaline salts), saline‐sodic (rich in both alkaline and neutral salts), and calcareous soils (saturated with alakaline calcium salts) in the IGP, an attempt was made to synthesize a clear guideline for the field application of gypsum following the Gypsum Requirement (G.R.) method. The recommended method includes a thorough mixing of a suitable grade of powdered (mined) gypsum (in tons ha ⁻¹ ) with the topsoil layer (0–30 cm depth), raised field boundary during plowing (to vaoid losses of amendment), and ponding at least for 15 days with irrigation water (preferably ECe < 0.4 dS m ⁻¹ ) to complete the neutralization of carbonate and bicarbonate salts. The experimental data showed the significant reduction in soil pH (from more than 10.0 to less than 8.5), ESP (from 89.8 to less than 15), ECe (from 26.1 to less than 4.0 dS m ⁻¹ ), and the neutralization of Na ₂ and NaHCO ₃₋ to Ca ² +, Mg ² +, and salts and the increase in organic carbon content (due to the higher root biomass), followed by gypsum application in a strongly saline‐sodic soil (pH more than 9.5, ECe less than 15 dS m ⁻¹ ) at surface (0–30 cm) and subsurface (beyond 30 cm) depths, respectively. Spatial analysis data revealed significant areas (56%) of reclaimed sodic soils in Haryana state of the IGP. The block (micro‐)‐level database was also developed to precisely assess sodic areas for further soil reclamation at the farm scale. The reviews also revealed that the supplementary addition of farm and green manures performed well for the reclamation of highly complex calcareous (rich in concretionary and nodular calcium carbonate) and saline‐sodic soils in the IGP, respectively.