# Starch Hydrogels for Slow and Controlled-Release Fertilizers: A Review
## 4 Application of SHs to Transport Fertilizers
### Fertilizers Encapsulation and In Vitro Assays  
SHs (starch-based hydrogels) have been applied for the encapsulation of essential macronutrients such as N, P, and K. The nutrient release profiles from these hydrogels have been evaluated in aqueous media, with release times and kinetic modeling results as summarized in Table 2. Hydrogels represent an innovative platform for developing slow-release fertilizers, where nutrient encapsulation typically occurs during the hydrogel formation process. For instance, [PERSON] et al. (2021) encapsulated P and N from diammonium phosphate fertilizer into starch acetate/PVA/glycerol hydrogels [109]. The hydrogel was prepared by mixing the polymers and glycerol in an aqueous solution at 90 \({}^{\circ}\)C, followed by the addition of the fertilizer under continuous stirring for 30 min. The resulting mixture was then poured into a Teflon mold and dried at room temperature. Complete release (100%) of the encapsulated P and N occurred within the first 100 min. Alternatively, nutrient encapsulation can be performed post-synthesis via adsorption. For example, CS/SHs hydrogels crosslinked with TPP have been used for the controlled release of KNO3 [110]. The hydrogels were formed using different mass ratios of CS and potato starch by dispersing the polymers in lactic acid, followed by crosslinking with TPP. Hydrogel beads were then produced using a dripping technique. Nutrient loading was achieved by immersing the dried beads in a 20% (\(w/v\)) KNO3 solution for 4 h. These hydrogels exhibited a slow-release profile, releasing 25-35% of the nutrient within 5 h, depending on polymer composition, and reaching approximately 94% release after 16 days. A similar methodology was reported by [111] [PERSON] et al. (2019), who encapsulated urea, KNO3, and ammonium sulfate ((NH4)2 SO4) in hydrogels prepared from starch oxidized with KMnO4/NaHSO3. Various starch sources were evaluated, including corn, sweet cassava, and bitter cassava [111]. The water uptake capacity of the hydrogels increasedunder basic pH conditions due to the ionization of carboxylic groups formed during starch oxidation, enhancing swelling and nutrient diffusion [111]. Hydrogels immersed in nutrient solutions exhibited varying encapsulation efficiencies: urea and KNO3 showed relatively low values (30% and 12%, respectively), whereas ((NH4)2 SO4) demonstrated a high encapsulation capacity of 90%. According to the release profiles, all hydrogels released more than 50% of the encapsulated nutrients within the first 4 h. This rapid release is attributed to the quick dissolution of the nutrients during the initial swelling phase. Therefore, achieving effective controlled nutrient release remains a significant challenge. To address this, hydrogels based on high-amylose (AM) maize starch oxidized with KMnO4/NaHSO3 were grafted with maleic acid in the presence of sorbitan monooleate and methylene bisacrylamide (MBA) to improve the absorption and controlled release of K-based fertilizers [4]. The oxidation of starch hydroxyl groups at the C-2, C-3, and C-6 positions leads to the formation of aldehyde and carboxyl functionalities. Grafting of disodium maleate, monosodium maleate, and MBA onto the macroradicals produced results in self-crosslinking of the starch chains and the introduction of various functional groups. The hydrogel showed an adsorption capacity of 50% to 90% for KH2 PO4, a common source of K+ and PO43-. Regarding nutrient release, between 20% and 50% was released within the first 8 h in water, followed by a gradual release over time, reaching approximately 60% after 300 h (12.5 days). Notably, less than 70% of the total nutrient content was released, indicating that a portion remained entrapped within the polymeric matrix. Machines are employed to coat nutrients with hydrogels, enabling the large-scale production of controlled-release fertilizer systems.