# Starch Hydrogels for Slow and Controlled-Release Fertilizers: A Review
## 5 Conclusions, Challenges, and Outlook  
SHs are classified as non-toxic and biocompatible materials, with the ability to absorb and retain water, making them attractive for water supply in arid soils. Additionally, SHs can encapsulate fertilizers to enable a slow release in crops. They can be synthesized through graft copolymerization, as well as chemical and physical crosslinking. However, SHs based on the polymerization of acrylic monomers have shown significant scientific advancements, leading to the development of methodologies that utilize rotating drums and extruder machines for large-scale production. According to in vitro assays (nutrient diffusion from the polymeric matrix into the solution), SHs exhibit a high release rate. Nevertheless, in agricultural applications involving soils and plants, the release rate decreases, extending up to 30-60 days. This means SH-based fertilizers are a viable method for promoting seed and plant growth. Despite the advantages of using starch for controlled nutrient release, there are still some drawbacks and challenges to overcome in this field:  
1. Most studies on SHs have focused on the release of N sources, with limited research on P, K, and micronutrients. In particular, researchers need to focus on developing versatile SHs capable of promoting the simultaneous release of NPK and water to plants. 2. Many researchers have concentrated on SH production at the laboratory scale, with few studies exploring large-scale applications in crop production. As a result, there is limited information on application costs and feasibility, making it difficult to compare SH-based fertilization with conventional methods. 3. The limited information on the application costs of SHs in soils also creates a knowledge gap in areas such as storage, mass production, and standardized protocols. Furthermore, the high biodegradability of SHs presents a challenge; research is needed to develop slow-release nutrient systems with high nutrient loading to reduce application frequency in soil. Additionally, new formulations should aim to improve the resistance of SHs to microbial degradation, which can otherwise lead to a rapid release of nutrients. 4. Although SHs enhance the biodegradability of hydrogels formed with synthetic polymers, they exhibit low water retention under salt stress conditions, which negatively impacts continuous nutrient release in soil applications. Therefore, further research is needed to develop new SHs with improved performance under saline conditions. These materials must also be cost-effective to ensure adoption by farmers. For instance, the use of agricultural residues is emerging as a promising strategy to produce affordable SHs without requiring complex production processes that could increase the final product cost. A.F.C. : writing, review and editing, and formal analysis. M.P. : writing, review, and formal analysis. E.M.C. : conceptualization, writing, review and editing, and formal analysis. All authors have read and agreed to the published version of the manuscript. This research has been funded by Direccion General de Investigaciones of Universidad Santiago de Cali under call no. DGI-01-2025. The authors acknowledge to the National Planning Department of Colombia, specifically the general royalty system (Sistema General de Regalias, SGR) for project BPIN 2020000100261. The authors declare no conflicts of interest.