A sustainable pH responsive hydrogel-based phosphorus fertilizer using nano oxalic acid-modified rock phosphate and humic acid.

Phosphorus (P) fixation and precipitation severely limit fertilizer efficiency in alkaline soils, contributing to resource depletion and environmental losses. In this study, a sustainable, pH-responsive phosphorus nano fertilizer was developed by mechanochemically converting Eppawala rock phosphate (ERP) into nano oxalic acid-modified rock phosphate (nano-Ox-ERP) and encapsulating it within a biodegradable sodium alginate-carboxymethyl cellulose (SA-CMC) hydrogel matrix supplemented with humic acid (HA). Nano Ox-ERP particles (60-180 nm) were characterized using PXRD, SEM-EDX, and FTIR. Hydrogel beads with varying nano Ox-ERP (20-80%) and humic acid (1-3%) compositions were evaluated for encapsulation efficiency, swelling behavior, and phosphorus release across pH 3.5-8.5. Formulation B8 (80% nano Ox-ERP + 2% HA) achieved 91.5% phosphate encapsulation and sustained release following Korsmeyer-Peppas kinetics. Pot experiments using Capsicum annuum L. (banana pepper) grown in alkaline soil demonstrated that the optimized formulation significantly enhanced plant growth, yield, and phosphorus uptake compared with conventional phosphorus sources, including triple super phosphate (TSP), untreated rock phosphate, and oxalic acid-treated rock phosphate, while also increasing soil bacterial abundance. The integrated nano-hydrogel-humic acid system effectively improves phosphorus bioavailability, minimizes nutrient losses, and promotes soil microbial activity, offering a scalable and environmentally benign strategy for sustainable phosphorus management in alkaline agroecosystems.