# 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
The material was tested for seed growth in soil, but no significant differences in root and shoot dry weight were observed compared to the control (clay/talc). Starch-poly(sodium acrylate-co-acrylamide) superabsorbent hydrogel was tested in sandy soils with maize (_Zea mays_) seedlings [132]. The hydrogel improved the soil's ability to retain water, enhanced aeration, promoted faster growth, and minimized the risk of root rot. Additionally, the material increased the cation exchange capacity in the soil by reducing cation leaching from the topsoil, creating a water- and nutrient-rich environment favorable to seed development. [PERSON] et al. (2004) tested the water delivery of starch-g-polyacrylic acid hydrogels during corn seed germination, finding that the hydrogel had excellent water retention capabilities, promoting corn seed growth with a germination rate of 98% [133]. This suggests that the hydrogel has great potential for use in arid and desert regions to support plant growth. The SH modified with acrylic acid was synthesized through a free-radical reaction using KPS as the initiator and MBA as the crosslinking agent [134]. After polymerization, the material was neutralized with NaOH, replacing the H\({}^{+}\) ions of the carboxylic acid groups in the poly(acrylic acid) with Na+ ions, ensuring overall electrical neutrality within the polymeric chain. The concentration of the crosslinker plays a crucial role in the hydrogel's properties, affecting both the swelling behavior and its visual appearance (Figure 7A). As the crosslinker concentration increases, the swelling ability of the hydrogel decreases due to the reduced distance between crosslinked points, leading to a more compact material. The biodegradation of the hydrogel was tested with bacterial strains Pseudomonas aeruginosa and Bacillus subtilis. The starch-based hydrogel degraded into small fragments over 6 weeks of incubation with both bacterial strains. The hydrogel demonstrated a high urea encapsulation efficiency (99.7%) and exhibited a slow-release profile. In the first 5 days, approximately 25% of the encapsulated urea was released, and between 20 and 30 days, nearly all of the urea (99%) was delivered. The release mechanism is attributed to diffusion through the polymeric matrix driven by the osmotic pressure difference between the interior and exterior of the gel. Urea release from the hydrogel significantly enhanced plant growth in chickpeas, increasing shoot length to 2.8 cm, compared to 0.5 cm in untreated soil. Additionally, the hydrogel alone (without urea) created an environment conducive to seed germination and plant growth (Figure 7B). In the previous experiment, the researchers used only 0.05 g of the dose per spot, reducing the use of fertilizer during the experiment. However, the amount of hydrogel was not evaluated in this research, which presents a gap in the literature. This is important because the fertilization dose and the plant's growth stage are key factors, as they need to align the nutrient requirements of the plant with the nutrient release of the material. For instance, during the germination stage, plants require lower nutrient levels, but in the vegetative stage, the plant's macronutrient requirements increase. Thus, the SH modified with acrylic acid becomes a strong alternative for nutrient release in short-cycle crops, as it releases approximately 20% of the urea in the first 5 days, supplying nitrogen during the germination phase. Following this, the release of urea from 20% to 100% supports the plant during the vegetative stage [134]. However, this material could be limited for optimal nitrogen supply in long-cycle crops with a vegetative stage longer than 3 months, requiring adjustment in the fertilization process. To determine the optimal dosage, it is important to characterize the environmental conditions, including temperature, moisture, and microbial activity in the soil, as these parameters directly affect nutrient release.