# Starch Hydrogels for Slow and Controlled-Release Fertilizers: A Review
## 1 Introduction  
Fertilizers play a crucial role in agriculture by promoting efficient crop growth. Their use has become increasingly necessary to meet the rising global food demand, which has grown by approximately three billion people over the past 40 years. The world population is projected to reach 9.7 billion by 2050 [1]. However, the continuous use of fertilizers has resulted in environmental degradation and economic losses, primarily due to high nutrient losses (40-70%), which contribute to serious ecological problems and hinder plant and crop development [2, 3]. Controlled-release hydrogels and composites have emerged as promising alternatives to mitigate these limitations, primarily due to the presence of hydrophilic groups that can be readily modified to introduce new functionalities. These chemical groups also facilitate chemical and physical crosslinking reactions, forming a three-dimensional structure capable of absorbing and retaining large amounts of water. Thiscrosslinked structure is a key feature, enabling a high nutrient retention capacity [4]. Most hydrogels and composites used in agriculture are derived from synthetic polymers such as polyacrylamide. However, these materials are not biodegradable and can negatively affect soil properties. To address these issues, biopolymers such as cellulose, chitosan (CS), alginate, collagen, and starch have been explored as sustainable alternatives. These biopolymers are biocompatible, biodegradable, and non-toxic, making them highly suitable for agricultural applications. Hydrogels are attractive materials for nutrient delivery due to their high water absorption capacity, excellent stability after swelling and during storage, non-toxicity, and high gel fraction after crosslinking. These characteristics are essential for achieving controlled fertilizer release. Furthermore, hydrogels present a more cost-effective alternative compared to nanomaterials commonly used in agricultural applications [5]. SHs serve as both soil conditioners and yield enhancers by retaining water and fertilizers and gradually releasing them throughout crop development [5; 6]. Among the biopolymers previously mentioned, starch (C\({}_{6}\)H\({}_{10}\)O\({}_{5}\))\({}_{n}\) stands out as one of the most abundant polysaccharides in nature, alongside cellulose and chitin. It is derived from various food sources such as cassava, rice, banana, and corn. Structurally, starch is a copolymer composed of amylose (AM) and amylopectin (AP) (Figure 1A), typically consisting of approximately 20-30% AM and 70-80% AP [7]. AM is a lightly branched polymer composed of a limited number of long glucan chains (ranging from 5 to 20), which tend to form helical structures. In contrast, AP is highly branched, consisting of numerous clusters of short chains [10]. The molecular weight (M\({}_{\text{w}}\)) of AP typically ranges from 10\({}^{7}\) to 10\({}^{8}\) g/mol [11]. However, the high M\({}_{\text{w}}\) values reported in the literature are often attributed to aggregate formation in aqueous solutions. When dissolved in a good solvent such as dimethyl sulfoxide (DMSO), AP displays a lower M\({}_{\text{w}}\), typically around 10\({}^{6}\) g/mol [12]. These D-glucose polymers are organized into a  
Figure 1: (**A**) Common starchy foods and the structures of AM and AP. (**B**) Model illustrating the distribution of amorphous and crystalline regions within a starch granule. Starch–lipid–protein complex is shown inside, which affects granule swelling. Modified from (**A**) [8]; (**B**) [9]. complex hierarchical structure that gives rise to starch granules, which generally consist of alternating semicrystalline and amorphous growth rings (Figure 1B). The size of starch granules varies between 2 and 150 \(\upmu\)m, depending on the botanical source. For instance, cereals tend to produce smaller granules compared to starches derived from roots and tubers [13].