# 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  
Table 3 presents SHs and composites applied to nutrient delivery in soils and plants, respectively. The nutrient release depends on environmental conditions. [PERSON] et al. (2016) achieved a reduction in N application rates by coating urea granules with starch-g-polyacrylamideamide hydrogels formed using a twin-roll mixer with ceric ammonium nitrate as the initiator and MBA as the crosslinker [128]. The study evaluated the effect of the starch source (maize, potato, and cassava) on urea delivery in soil. After approximately 20 h, all materials delivered more than 40% of the urea encapsulated, with the hydrogel formed from potato starch showing a slow release rate (70% at 96 h). Nutrient release in soils was studied using a PVC column with a diameter of 5 cm and a length of 35 cm (Figure 6A). At the bottom of the column, a controlled tap was installed to collect soil leachate samples at various intervals. Typically, the soil is wetted to 100% of its water retention capacity [129]. The SH formed with borax was evaluated for controlling the delivery of urea in sandy loam soil. Figure 6B illustrates the potential formation mechanism of the hydrogels, with borax acting as a crosslinker. The chemical gelation of borax with the hydroxyl groups of the polysaccharide was observed through the B-O stretching vibration in the FTIR spectra at 1015 cm\({}^{-1}\). The release rate of pure urea (without encapsulation) in soil reached approximately 96% within three days. In contrast, the hydrogel tested delivered about 14% of the urea in 6 days, with the maximum release occurring at 36 days, reaching approximately 94% of the encapsulated urea. This demonstrated that the hydrogel reduced the release time by 89% compared to in vitro release experiments in distilled water. Soil nutrient release experiments can also be performed in plastic beakers, using laterite sandy clay loam soil as a sample for each interval. Starch-g-poly(acrylonitrile) granule hydrogel  
Figure 6: (**A**) PVC column used to evaluate nutrient release in soils. (**B**) Crosslinking mechanism between borax and starch to form a hydrogel. (**C**) Granules of urea coated with starch-g-poly(acrylonitrile) hydrogel. (**D**,**E**) Scanning electron micrographs of hydrogel-coated urea and a cross-section of the coated urea. Modified from (**A**,**B**) [129]; (**C**–**E**) [130]. (Figure 6C) showed lower N release (45%) after 7 days of incubation in laterite sandy clay loam soil compared with the control (79% in 7 days) [130]. The N release was extended up to 108 days, with approximately 70% of the urea released. According to the research, the urea release depends on water penetrating the coating, which dissolves the urea in the core (Figure 6D,E), allowing the urea to diffuse through the coating into the environment. On the other hand, water is essential for plant growth, but it can easily evaporate, decreasing the efficiency of plant growth. SHs with fertilizer can also be used to deliver water, improve soil aeration, reduce evapotranspiration, and prolong water availability for seeds and plants. To efficiently deliver nutrients to seeds, the coating material is usually applied via the dry coating method, which involves mixing the corn seeds with the wet hydrogel [131]. The effect of different concentrations of the material can be studied by mixing it with other components. For instance, bentonite was mixed with SH made from potato starch modified with succinin anhydride (starch-MS) to create a powder mixture with hydrogel concentrations ranging from 10% to 50% (\(w/w\)). Methylcellulose is commonly used as an adhesive in the solution to enhance the hydrogel-seed coating. Increasing the starch-MS concentration in corn seed coatings reduces the coating efficiency from 80% to 60%. This behavior can be attributed to the tendency of the MS to wick the adhesive solution, reducing the seed's surface bonding with the powder material.