A Carboxymethyl Cellulose Hydrogel Switch Enabling pH-Responsive Fertilizer

The Green Revolution boosted crop yields with synthetic fertilizers, but their high solubility causes environmental pollution and poor synchronization with crop nutrient demands. To address this problem, we developed green intelligent fertilizers (GIF) using a pH-responsive material to enable root-fertilizer feedback. An ultra-sensitive and biodegradable pH-responsive material (pH-B) was synthesized by coordinating carboxymethyl cellulose with Ca2+ and cross-linking, with H+ sensitivity at 10–4 mol L–1. We then constructed a smart release system by embedding this pH-B switch into a 5% polyurethane coating to encapsulate diammonium phosphate (DAP) granules. In this design, the polyurethane matrix impedes the contact between DAP granules and soil water and thus minimizes phosphorus solubilization, leaching, and fixation, while the pH-B switch acts as an acidity-activated gatekeeper whose shrinking enables water penetration through the coating material. In acidic soil, the shoot P content of faba bean increased significantly (by 37–61%) in the treatment with pH-responsive fertilizer compared with DAP. Consequently, in calcareous soils, these fertilizers release nutrients specifically in response to rhizosphere acidification caused by proton exudation from phosphorus-deficient plant roots. This mechanism precisely synchronizes nutrient supply with crop demand. Our work provides a novel strategy for enhancing nutrient-use efficiency and promoting sustainable agriculture.