Enhancement of the physiological activity, yield and antioxidant content of tomato using slow-release urea fertilizer based on PVA/PE/HA/CA hydrogel

A biodegradable slow-release urea fertilizer, PVA/PE/HA/CA/U (PPHCU-hydrogel), was developed with the incorporation of urea (U) into a new PPHC-hydrogel made from polyvinyl alcohol (PVA), pectin (PE), humic acid (HA), and citric acid (CA). A comparative slow-release urea fertilizer (PPCU-hydrogel) was also made without humic acid to evaluate the functional role of humic substances. The hydrogels formed ester carbonyl bonds through citric acid crosslinking, confirmed by FTIR analysis. EDX analysis showed that nitrogen content increased from 10.12% in PPHC-hydrogel to 20.15% in PPHCU-hydrogel. Additionally, FESEM analysis revealed a homogeneous, porous structure in PPHC-hydrogel, while the PPHCU-hydrogel displayed a more heterogeneous and crystalline structure, indicating successful urea loading. The PPHCU-hydrogel exhibited pH- and ion-responsive swelling (p < 0.05) and slow release of urea, notable (p < 0.05) water retention properties, and biodegradability. Following 15 days in soil, the PPHCU-hydrogel released 51.34% of its urea, while the PPCU-hydrogel released 63.12%. PPHCU-hydrogel enhanced plant growth and considerably (p < 0.05) enhanced fruit yield (especially fruit number per plant) and nutritional quality (especially vitamin C) compared to other treatments. These results suggest that the synergistic effects of humic acid incorporation and controlled urea release can improve crop performance, highlighting PPHCU-hydrogel's potential as an eco-friendly fertilizer.