Preparation and characterizations of slow-releasing humic acid fertilizer loaded in starch-based hydrogel

Eco-friendly slow-release humic acid fertilizers (SRF-HAs) with water retention function were fabricated by in situ radiation polymerization through grafting of acrylic acid (AA) or acrylamide (AM) on cornstarch containing surface coated HAs. The balance of properties between water retention and HA loading was optimized on the basis of performance in saline-alkali soil. The SRF-HAs were characterized by FTIR NMR, SEM, Rheometer, and measurement of water retention and release behavior. The quantities of both grafting and conversion ratios were determined based on TGA measurements. The performance of SRF-HAs was evaluated by cotton germination in both normal and saline-alkali soils. Chemical characterizations (FTIR and NRM) confirmed the successful grafting of polyacrylic acid (PAA) and polyacrylamide (PAM) onto starch. The AA and AM graft ratios on the starch achieved approximately 82.57 and 80.74 % respectively. The water retention capability in soil was increased about seven times after adding the hydrogel-based fertilizer for 15 days. The potassium release rate (indicating HA) was significantly slowing (approximately 62.1 %) in SRF-HA. Comparative analysis of fertilizer placement strategies (upper, medium and lower layers in soil) revealed that mid-layer incorporation achieved the best enhancement, which increased germination rates from 9.4 % to 62.5 %. This achievement is principally attributable to the capacity of SRF-HA to keep moisture and equilibrate pH in saline-alkali soils. The fertilizer from the starch grafted AA series consistently demonstrated higher germination rates compared to that grafted with AM series, possibly attributed to abundant carboxyl groups within the PAA chains that effectively mitigate soil alkalinity and stimulate seedling emergence.