# 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
To reduce the urea release time from starch-poly(acrylic acid) hydrogel, and to make it suitable for long-cycle crops, wax and NR have been incorporated to protect the material and inhibit water permeation (BHWCU) [112]. In vitro urea release experiments showed that the BHWCU material released 40% of the nutrient in water within 100 h (approximately 4 days), and the same release percentage was observed in soil environments over 30 days. However, urea release is influenced by factors such as pH and temperature. After 100 h, the material released 30%, 40%, and 70% of urea at pH 4, pH 7, and pH 10, respectively. Additionally, increasing the temperature from 20 \({}^{\circ}\)C to 40 \({}^{\circ}\)C resulted in an increase in cumulative urea release from 30% to 55% at 100 h. This effect is likely due to the increased thermal flexibility of the polymer chains, which expand as the temperature rises, accelerating the release of urea. In plant growth experiments with chili plants, those treated with the BHWCU formulation exhibited healthier growth, with broader, thicker, and darker green leaves compared to plants treated with conventional urea. Thus, the BHWCU formulation effectively supplies urea to the plant, fulfilling its nutrient requirements. Figure 7: (**A**) Crosslinking density and strength of starch-modified poly(acrylic acid) (SAH) at (i) 43.57%, (ii) 50.97%, and (iii) 67.52% of acrylic acid. (**B**) SAH applied in soils: (i) soil with urea-encapsulated SAH, (ii) soil with SAH, and (iii) bare soil. Modified from [134]. \begin{table}
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**Material** & **Nutrient** & **Plant and Seeds** & **Observation** & **Reference** \\ \hline Natural rubber and cassava starch crosslinking with sulfur and GA & Urea & Corn and basil plants & The performance of encapsulated urea beads in corn and basil plantations was significantly higher compared to non-encapsulated urea. The material releases urea through the porous membrane via non-Fickian diffusion. & [135] \\ \hline \multirow{3}{*}{Starch phosphate carbamate hydrogel (SPC-hydrogel)} & \multirow{3}{*}{Urea} & \multirow{3}{*}{Maize seeds} & The hydrogel exhibits a continuous slow-release performance, with 50.3\% of urea released within 15 h and nearly complete release in just over 25 h in water. & [136] \\ \hline \multirow{3}{*}{Cassava starch-g-polyacrylic acid/natural rubber/PVA} & \multirow{3}{*}{Urea} & \multirow{3}{*}{Chili plant} & The formulation demonstrated excellent slow-release N delivery in both water (47.5\% at 168 h) and soil (38.5\% at 30 days). The chili plant growth was effectively enhanced, with a production cost +5 times lower than that of commercial slow-release fertilizers. & [112] \\ \hline \multirow{3}{*}{Succinate-modified potato starch} & \multirow{3}{*}{Water} & \multirow{3}{*}{Corn seed} & At a water supply of 77\% field capacity, the coated seeds exhibited a significantly higher emergence rate compared to uncoated seeds. & [131] \\ \cline{3-5} \cl  
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Note that the experiment was conducted during the vegetative stage for 6 weeks, providing the nutrient requirements for the plants. However, according to the experiments on nutrient release in soils, the release of the nutrient can be modulated by the proportion of NR to polymer. A higher amount of NR (9:1 with the polymer) reduces nutrient release, with only 40% of the urea being released within 30 days. This proportion is suitable for long-cycle crops. On the other hand, a 2:8 ratio of NR to polymer induces a more rapid release, with 80% of the urea being released in the soil, making it an optimal material for short-cycle crops [112]. Thus, the optimal dose can be adjusted according to the plant growth conditions. To develop materials for controlled fertilizer delivery, other monomers, such as acrylamide, were copolymerized with corn starch to encapsulate urea [136].