FEASIBILITY OF COAL COMBUSTION FLY ASH ALONE AND IN COMBINATION WITH GYPSUM AND GREEN MANURE FOR RECLAMATION OF DEGRADED SODIC SOILS OF THE INDO‐GANGETIC PLAINS: A MECHANISM EVALUATION

The disposal of coal combustion fly ash (FA) generated from thermal power plants is a worldwide problem. However, its low bulk density and presence of calcium and iron-aluminium sesquioxide make it a prospective ameliorant alternative to gypsum for amending degraded sodic soils. A field experiment consisting of eight treatments with variable doses of FA and gypsum along with green manure (GM) was conducted for evaluating the amelioration mechanism by measuring changes in pH, exchangeable sodium percentage, zeta (zeta) potential, and properties of meso-aggregates (0.5-2.0 mm). The results showed that compared with control, FA significantly reduced zeta-potential, bulk density, and pH. The GM with FA further decreased zeta-potential and increased hydraulic conductivity. The hydrolysis of iron and aluminium sesquioxide, present in FA, functioned to lower soil pH, whereas the reduction in exchangeable sodium percentage is attributed to bioavailable Ca (27.4%). A lower FA application rate produced better crop yields. FA 2.5% (w/w) alone, compared to the FA 5% (w/w), resulted in higher yield of rice (1.96 tonnes ha(-1)), whereas 25% gypsum requirement (25GR) + FA 2.5% (w/w) and 25GR + FA 2.5% (w/w) + GM provided wheat yield on par with 50% gypsum requirement (50GR). The study identifies that where gypsum is scarce, the recommended amendment is 25GR + FA 2.5% or 25GR + FA 2.5% + GM when applied for sodic soil to achieve a similar yield response to standard 50GR. If gypsum is unavailable, a one-time application of FA 2.5% (w/w) alone will improve the productivity of degraded sodic soils corresponding to more exchangeable calcium and organic carbon, which were found to be higher for meso-aggregates (0.5-2.0 mm).