Amendment-assisted low-grade rock phosphate enhances maize productivity and phosphorus availability in contrasting soil orders

Abstract

Efficient phosphorus (P) management is essential for improving crop productivity while reducing dependence on energy-intensive, highly soluble P fertilisers. Low-grade rock phosphate (RP) is an underutilised, economical indigenous P source, but its agronomic effectiveness is often limited by low solubility and slow P release. This study evaluated the effectiveness of indigenous low-grade Udaipur rock phosphate (URP) amended with elemental sulphur (S) and phosphate-solubilising bacteria (PSB) for improving P availability, maize growth, yield and P-use efficiency in two contrasting soil orders, Alfisol and Vertisol. A pot experiment comprising nine P management treatments, including single super phosphate (SSP), URP, PSB ( Burkholderia cenocepacia ) and S combinations was conducted under maize. Soil available P, plant height, number of leaves, leaf area index, dry matter accumulation, grain yield, P uptake, harvest index (HI), physiological P-use efficiency (PPUE) and P harvest index (PHI) were assessed. Across treatments, agronomic performance and soil P availability were consistently higher in the Vertisol compared to the Alfisol. Amendment-assisted URP improved grain yield by up to 68% over URP alone in Alfisol and 35% in Vertisol. Compared with 100% P through SSP, grain yield increased by 38.7% and 24.8% under URP 100 + S 25 + PSB 5 and by 42.7% and 21.0% under URP 150 + S 10 + PSB 5 in Alfisol and Vertisol, respectively. Both treatments maintained plant growth, dry matter production, grain yield, and P uptake comparable with SSP 150 + PSB 5 . Soil available P increased with P fertilisation, with URP 100 + S 25 + PSB 5 and URP 150 + S 10 + PSB 5 maintaining levels close to SSP 150 + PSB 5 and higher than URP alone. Despite these differences in P acquisition, PPUE, PHI and HI remained statistically similar, indicating that treatments improved overall P supply and biomass production proportionally rather than altering P partitioning efficiency. Overall, combining URP with PSB and S improved the agronomic effectiveness of a low-solubility P source and produced maize productivity comparable with a higher rate of soluble P fertiliser under controlled conditions. Efficient utilisation of indigenous low-grade RP through suitable solubilising amendments offers a promising approach for improving crop yield and sustainable nutrient management, with future scope for field-scale validation.