SOIL FERTILITY EVALUATION OF ACID COARSE-TEXTURED ULTISOLS FOLLOWING CO-APPLICATION OF NATURAL AND SYNTHETIC LIME-PHOSPHORUS AMENDMENTS

Abstract

Soil acidity and associated phosphorus (P) fixation in highly leached humid tropical soils are major constraints to their agronomic productivity. They thus require effective acidity-alleviating cum P-supplying fertility management. This study assessed the potential of co-application of wood ash (WA) and calcium oxide (CaO) as natural and synthetic limes with rock phosphate (RP) and single superphosphate (SSP) as natural and synthetic P-fertilizers, respectively for acid coarse-textured Ultisols. The WA and CaO were added at 157 and 5 kg ha–1 equivalents, and RP and SSP at 50 and 333 kg ha–1 equivalents, respectively to 5-kg potted soils. Over 9 weeks, treatment effects in the soil evaluated on soybean growth showed enhancements. Relative to the control, soil pH increased in WA/WA+RP by 107%-121%; the highest relative increases in total N were in WA+RP (2,350%), available P in WA+SSP (520%), and K+/Ca2+/Mg2+ and base saturation in WA (50%-391%), with highest N-P-K uptake from WA+RP (450%-600%). The highest relative increases in above-soil plant growth and dry matter were in WA+RP/WA+SSP/CaO+SSP (63%-102% and 86%, respectively). Treatment residual effects over also 9 weeks showed similar nutrient-uptake/growth trends. Agronomic responses to treatment largely reflected soil pH-regulated differences in soybean uptake of P found to be linearly related to soil available P. Unlike WA, CaO needs P-fertilizer to improve soil productivity. Overall, however, WA+RP/WA+SSP/CaO+SSP is suggested for the soils.