# 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 c c c} \hline \hline
**Material** & **Nutrient** & **Observation** & **Reference** \\ \hline Hydrogel: Sulfonated corn starch/poly(acrylic acid) synthesized by polymerization & p & P exhibited a sustained release over 72 h; however, 40\% of the & [116] \\ \hline Hydrogel: PVA-modified starch biopolymer crosslinked with CA & N & The transport of nutrients through the swollen network of & \\  & & macromolecule polymer chains follows a non-Fickian or anomalous & [117] \\ \hline CM starch-graft-polyacrylamideamide (PCMS-g-PAM) & P & The phosphate release mechanism from P-CM5-g-PAM was analyzed using the Korsmeyer-Peppas model. & [118] \\ \hline  & & The results showed that a higher gel strength was achieved with lower & \\ \begin{tabular}{l} Starch acrylamide, N,N’-methylene-bisacrylamide (crosslinker) by hot \\ compression vulcanizer \\ \end{tabular} & N & The density in starch-based hydrogels. Additionally, the hydrogel network exhibited zero-order, or at least time-independent, urea release kinetics during the intermediate stable stage. & [113] \\ \hline  & & The time required to reach the maximum concentration of N release & \\ \begin{tabular}{l} Starch acetate (SA)/PVA/glycerol (GLY) \\ \end{tabular} & N, P & was 24 and 3.2 times longer than that of uncoated DAP when DAP was coated with CF-3 (70\% SA) in a single layer and double layer, & [109] \\ \hline  & & The hydrogel showed a slow release of the nutrient to P, releasing up to & \\ \begin{tabular}{l} Cassava SHs crosslinked with CA \\ \end{tabular} & N, K, P & \begin{tabular}{l} 30\% in 150 h; however, N and K showed a burst release effect in the first \\ \(15\), releasing until 80\% of the nutrient was wrapped in the polymeric \\ matrix. Additionally, the nutrient release followed a Fickian diffusion \\ from the hydrogel to the bulk of the diffusion medium. \\ \end{tabular} & \begin{tabular}{l} 119,120] \\ \end{tabular} \\ \hline \begin{tabular}{l} Maize starch (AS) was modified by graft copolymerization with sodium \\ acid maleate and disodium maleate and covalently crosslinked with \\ N,N’-bismethylene acrylamide \\ \end{tabular} & K, P & \begin{tabular}{l} The fertilizer loading percentages for KNO\({}_{3}\) ranged from 95\% to 57\%, \\ while for KIF\({}_{3}\)PO\({}_{4}\), they ranged from 90\% to 55\%. The release \\ percentages were between 92\% and 95\% for KNO\({}_{3}\) and between 89\% \\ and 61\% for KIF\({}_{3}\)PO\({}_{4}\), respectively. \\ \end{tabular} & [4] \\ \hline \begin{tabular}{l} Starch-g-poly (acrylic acid-acrylamide)/\(\backslash\) zeolite hydrogel composite \\ \end{tabular} & N, P, K & \begin{tabular}{l} Measuring the concentrations of N, P, and K released in buffer solutions \\ at pH 5, pH 17, e\({}^{-}\), and in water over a period of 0 to 100 h revealed a gradual release pattern. \\ \end{tabular} & [121] \\ \hline \
\begin{tabular}{l} Hydrogel formed by starch oxidation with KMO\({}_{4}\) and NaHSO\({}_{4}\) \\ \end{tabular} & N, K &
\begin{tabular}{l} The loading and release of the fertilizer depended on the initial \\ fertilizer concentration in the medium, as well as the nature, structure, and morphology of the starch used. \\ \end{tabular} & [111] \\ \hline \end{tabular}
\end{table} 
Table 2: Starch material applied to deliver nutrients (in vitro assays).