Dissolution of “sparingly soluble phosphates” in soil by polyacrylic super absorbent polymers

Phosphorus (P) is an essential nutrient for sustaining agricultural productivity and ensuring global food security, however, its availability in soils is often limited by the formation of sparingly soluble phosphate compounds. A substantial proportion of phosphate ions derived from fertilisers become immobilised outside the rhizosphere by binding with Ca, Al, Fe, and their oxides, with Al- and Fe-bound forms constituting sparingly soluble phosphates. This study aimed to evaluate the ability of polyacrylic acid‑based superabsorbent polymers (SAPs), which are rich in carboxyl groups, to solubilise phosphorus from these sparingly soluble phosphate forms. Bioassays, dissolution experiments, and mechanistic investigations were performed to assess this potential. In lettuce cultivation, the application of a 0.08% SAP solution significantly enhanced plant growth and increased P uptake by 1.6-fold in high-P soil and 1.4-fold in low-P soil ( p < 0.05), demonstrating its efficacy across soils with differing P fertility. Dissolution experiments further revealed that SAPs solubilised phosphate ions from agricultural soils and Al/Fe-crystallised- phosphates (variscite and vivianite). Mechanistic analysis using an aluminium phosphate reagent with SAP indicated that P dissolution proceeded primarily via diffusion-driven-processes. These findings indicate that the abundant carboxyl groups of SAPs both compete with phosphate for adsorption sites and chelate metal ions, effectively transforming sparingly soluble phosphates into plant‑available phosphate ion forms. The use of SAPs represents a promising approach for reducing the environmental impacts associated with P loss and promoting sustainable P recycling.