Development of slow-release humic acid fertilizer using starch-based hydrogel

The use of humic acids (HAs) has gained increasing attention as a response to the decline in agricultural land productivity, which has been exacerbated by the excessive use of synthetic fertilizers. This work presents a novel HAs-based slow-release fertilizer (SRF-HAs) developed through in-situ radiation-synthesizing starch-grafted polyacrylamide (AM) hydrogels encapsulating HAs. To prevent the highly active phenolic groups in HAs from quenching the free radicals generated during in-situ radiation synthesis, the surface of HAs particles was firstly coated with stearic acid or wax. The microstructures and performances of the SRF-HAs were characterized using SEM NMR, FTIR, rheometer, and release rate measurement. Additionally, TGA was utilized to quantify the grafting and conversion rates of AM. It was found that the release rate of HAs strongly depends on the microstructures and strength of the starch-based hydrogel, which is governed by both the surface coating and radiation-synthesizing processing. Both FTIR and NMR confirmed the successful grafting polymerization on starch chains. The results showed that with increasing content of SA-coated HAs from 3 % to 10 %, the grafting efficiency, grafting rate, and monomer conversion of starch grafted acrylamide reaction system decreased slightly from 42.7 % to 40.8 %, 78.3–76.6 % and 86.5–82.08 %, respectively. The release periods of HAs reaching 90 % from the coated samples extended up to 53 and 60 days, respectively. Meanwhile, the initial bust releases were significantly reduced to about 30 %, complying with ISO and ASTM standards. The relationships between the microstructures, properties of the hydrogels, and release rate were discussed and established, providing valuable guidelines for developing SRF-HAs. • SRF-HAs was developed through in-situ radiation-synthesizing starch hydrogels encapsulating HAs. • Coating prevents phenolic groups in HA to weaken initiator radicals during radiation synthesis. • Release rate of HA depends on microstructures and strength of starch hydrogel