Sustainable mechanochemical activation of rock phosphate with oxalic acid for high-efficiency phosphorus fertilizers.

Inefficient phosphorus fertilizer uses and increasing resource scarcity highlight the need for sustainable alternatives to conventional soluble fertilizers. This study presents a green mechanochemical strategy using biodegradable oxalic acid to activate Sri Lanka's Eppawala rock phosphate (ERP) and produce an efficient phosphorus fertilizer. The optimized formulation (OA-0.9) achieved 96-99% water-soluble phosphorus through the conversion of hydroxyapatite into monetite via calcium chelation and lattice disruption. Soil incubation studies showed sustained nutrient release, with 51.6% phosphorus released over 50 days compared with 13.6% from triple superphosphate (TSP), indicating reduced fixation in tropical soils. Agronomic trials demonstrated superior performance, producing 92.24 g fresh chili pods per plant, approximately 90% higher than TSP, with a germination index of 209.76%, suggesting phytostimulatory effects. Notably, a reduced application rate (50 kg ha-1) achieved 79% of the maximum yield while improving phosphorus use efficiency by 58%. This environmentally benign approach valorizes the domestic phosphate resources, reduces fertilizer input requirements, and aligns nutrient release with crop demand. The findings demonstrate the potential of mechanochemical oxalic acid modification as a scalable strategy for producing sustainable, controlled-release phosphorus fertilizers that enhance agricultural productivity and resource efficiency.