# Starch Hydrogels for Slow and Controlled-Release Fertilizers: A Review
## 4 Application of SHs to Transport Fertilizers
### In Vitro Evaluation of SHs in Soil and Plant System
Starch phosphate carbamate (SPC) was formed in the presence of urea, NaH\({}_{2}\)PO\({}_{4}\), and Na\({}_{2}\)HPO\({}_{4}\), and then copolymerized with acrylamide using (NH\({}_{4}\))\({}_{2}\)S\({}_{2}\)O\({}_{8}\) as the initiator, resulting in SPCU-hydrogel. The hydrogel exhibited an increase in volume upon absorbing water (Figure 8A). Its SC showed only a slight decrease after six cycles of water absorption and dehydration (Figure 8B). Column leaching experiments were conducted to determine the urea release profile, which was compared with SH (NSU-hydrogel) and neat urea. In sandy loam soil, pure urea released nearly 100% within 24 h. In contrast, the hydrogels exhibited slower release, with NSU-hydrogel releasing 100% of the urea in 25 days, and SPCU-hydrogel in 35 days. This difference is attributed to the inherent properties of esterified SPC, such as high viscosity and hygroscopicity. The ability of SPCU-hydrogel to support maize seedling growth in soil was tested over six weeks (Figure 8C). The SPCU-hydrogel-treated seedlings thrived for at least five weeks, while the control pots (with or without urea) only lasted two weeks without water addition. The SPCU-hydrogel proved to be more favorable for plant growth, potentially contributing to improved crop productivity. Figure 8: (**A**) Dry and swollen SPC hydrogel. (**B**) SC of SPC hydrogel after six hydration and dehydration cycles. (**C**) Effect of neat urea and SPCU-hydrogel on maize seedling growth. Modified from [136]. SH composites incorporating nanozolites (NZs) have been developed to enhance nutrient absorption and regulate nutrient release rates [144]. Hydrogel beads were synthesized using gelatinized sago starch, CS, and NZ, with TPP as a crosslinking agent (SCHNZ-TPP hydrogel). The material was tested for P and urea release behavior in deionized water and in _Philodend_ sp. plants [138]. After 14 days, the hydrogel composite released 64% of P and 41% of urea in water. This behavior is attributed to interactions between NH\({}_{3}\)\({}^{+}\) in urea and Al in NZ, whereas P (PO\({}_{4}\)\({}^{2-}\)) does not interact with NZ, leading to a higher release rate of P compared to urea. The hydrogel containing urea demonstrated a significant effect on _Philodend_ sp. growth, enhancing the growth index and chlorophyll content compared to the control. This confirms its potential as an effective slow-release fertilizer. Other nanomaterials, such as NCNPs incorporated into starch-g-poly(styrene-co-butyl acrylate), have also been explored to optimize urea release from SHs. [PERSON] et al. (2021) evaluated these materials in greenhouse tomato production, testing different NCNP concentrations (0-1% \(w/w\)) [137]. Their findings indicate that NCNPs play a crucial role in improving tomato plant growth while reducing the N release rate. The formulation containing 1% NCNPs resulted in 13.75% greater greenery in tomato plants compared to conventional urea and other tested formulations. This effect was further reflected in an increased number of leaves and higher chlorophyll content.