# Starch Hydrogels for Slow and Controlled-Release Fertilizers: A Review
## 4 Application of SHs to Transport Fertilizers
### Fertilizers Encapsulation and In Vitro Assays
\begin{table}
\begin{tabular}{l l l l} \hline
**Material** & **Nutrient** & **Observation** & **Reference** \\ \hline \multirow{3}{*}{Hydrogel formed by the starch (corn, sweet cassava and bitter cassava)} & \multirow{3}{*}{N, K} & The SC of the oxidized samples follows the trend of Na\({}_{2}\)SO\({}_{4}\)? NaCl? \\  & & KNO\({}_{3}\)? CaCl\({}_{2}\). All starch samples exhibit a similar “5”-shaped release kinetic profiling, consisting of three main stages. A noticeable “burst effect” was observed up to 0.5, particularly for KNO\({}_{3}\). & [122] \\ \hline \multirow{3}{*}{CS/starch hydrogel crosslinking with sodium TPP} & \multirow{3}{*}{P, N} & The fertilizer release ratio exceeded 70\% of the total loading. The hydrogel composition and crosslinking time determined the mechanism governing K release, which was controlled by matrix relaxation. & [110] \\ \cline{1-1} \cline{3-3}  & & The results suggested that the prepared fertilizer exhibited excellent water retention and urea release control. Moreover, the addition of \\ \cline{1-1} \cline{3-3}  & \(\beta\)-cyclodextrinxtrin modified by maleic anhydride, accompanying & N & \\ \cline{1-1} \cline{3-3}  & & \\ \cline{1-1} \cline{3-3}  & & \\ \cline{1-1} \cline{3-3}  & & \\ \cline{1-1} \cline{3-3}  & & \\ \cline{1-1} \cline{3-3}  & & \\ \cline{1-1} \cline{3-3}  & & \\ \hline \end{tabular}
\end{table} 
Table 2: Cont. The use of machinery enables the large-scale production of SHs. One promising method is REX, a continuous, solvent-free process in which polymer reactions occur under high-viscosity conditions within an extruder. This technique is both cost-effective and environmentally friendly [125]. [PERSON] et al. (2022) developed a twin-screw extruder system connected to a single extruder equipped with four injection ports, allowing the separate feeding of monomers, initiator-1 (ceric ammonium nitrate), initiator-2 (KPS), and a saponifying agent (sodium hydroxide) for the graft copolymerization of acrylamide onto corn starch [115]. A schematic representation of this REX system is shown in Figure 5D. The process was conducted at a constant rotation speed of 90 rpm across all reaction zones. The crosslinker was first injected via a metering pump in zone 3, followed by the sequential injection of the initiators in zones 5 and 7, and finally, the saponification agent in zone 9. The reaction mixture underwent thorough mixing and devolatilization through ports located between zones 13 and 15 to remove volatile byproducts. FTIR and NMR analyses confirmed the success of the grafting reaction, with a monomer conversion rate of 97%. This system demonstrates strong potential for the industrial-scale fabrication of SHs designed for controlled fertilizer release. Nanoparticles (Nps) and microparticles have demonstrated great potential as modifiers of hydrogel properties when incorporated into the polymeric matrix. This strategy enhances the interactions between the matrix and the encapsulated nutrient, thereby improving the nutrient release profile. For example, natural charcoal Nps (NCNPs), with particle sizes below 25 nm, were incorporated into a starch-g-poly(acrylic acid-co-acrylamide) [starch-g-poly(AA-co-AAm)] superabsorbent hydrogel to enhance its performance in controlled slow-release applications [126]. FTIR analysis revealed that NCNPs contain oxygenated functional groups, which promote their stabilization within the polymeric network through hydrogen bonding and electrostatic interactions. The inclusion of NCNPs significantly altered the release behavior of urea, reducing the cumulative release from 80% to just 10% over a 21-day period. This sustained release follows a Fickian diffusion mechanism, highlighting the effectiveness of nanoparticle incorporation in controlling nutrient delivery. The NCNPs promote interfacial polymer-filler interactions, which help extend the urea release time. This material has also been produced using a planetary ball-mill machine (NARYA-MPM), incorporating stearic acid crystals and zirconia particles with sizes ranging from 5 to 10 nm [124].