# Starch Hydrogels for Slow and Controlled-Release Fertilizers: A Review
## 4 Application of SHs to Transport Fertilizers
### Fertilizers Encapsulation and In Vitro Assays
For instance, a rotating drum machine equipped with a hot-air gun (Figure 5A) was used to coat urea prills with a cassava starch-g-polyacrylic acid/NR/PVA (CSt-g-PAA/NR/PVA) hydrogel composite [112]. The hydrogel dispersion was slowly sprayed onto the surface of the rotating urea prills (at 35 rpm), resulting in a uniform biodegradable hydrogel coating. Subsequently, the coated prills were covered with a wax solution (Figure 5B) to enhance water resistance and further control nutrient release. The hydrogel-encapsulated urea exhibited a sustained release profile, delivering approximately 80% of the nutrient over the first 80 h. After this period, the cumulative release plateaued and remained constant for the duration of the experiment (160 h). To mathematically describe and predict the release behavior of nutrients from such systems, several kinetic models are commonly employed, including zero-order, first-order, Higuchi, and Korsmeyer-Peppas models. The urea release from the CSt-g-PAA/NR/PVA hydrogel followed the Korsmeyer-Peppas model, indicating that the release mechanism is governed by Fickian diffusion. Another machine employed in the production of SHs is the hot-compression vulcanizer, which was used to synthesize slab-shaped, urea-embedded grafted SHs. In this process, corn starch, acrylamide, water, and urea were combined in a Haake mixer. The protocol involved gelatinizing the starch at 80 \({}^{\circ}\)C for 10 min, followed by a cooldown to 30 \({}^{\circ}\)C to prevent premature chemical reactions [113]. The urea release profile of these SHs in water exhibited an initial burst, with approximately 50% of the nutrient released within the first 24 h. This was followed by a cumulative release reaching 78% over a 40-day period. Notably, increasing the starch concentration reduced the hydrogel's ability to sustain urea release, as higher starch content diminished the initial burst. This release behavior could be adjusted by modifying the acrylamide-to-starch ratio; increasing the corn starch proportion effectively decreased the burst effect. The overall release kinetics confirmed to a zero-order model, suggesting a dual mechanism involving Fickian diffusion and matrix erosion resulting from polymer degradation. A HAAKE Rheocord Polylab RC500p system, equipped with a HAAKE Rhemix 600p twin-rotor mixer (Figure 5C), was employed to produce starch/acrylamide-basedhydrogels by crosslinking with N,N\({}^{\prime}\)-methylenebisacrylamide for urea encapsulation [114]. The chemical modification process involved adding the reagents to the mixer at 80 \({}^{\circ}\)C, after which the temperature was reduced to 65 \({}^{\circ}\)C by air and N injection. The crosslinking reaction was initiated in the presence of ceric ammonium nitrate as an oxidizing agent. Urea was subsequently added to the reaction mixture and blended for 40 min at 80 \({}^{\circ}\)C and 80 rpm to form the SHs. The resulting material demonstrated high efficiency for slow urea release in static water, with only 15% of the nutrient released within the first 5 days. Between days 5 and 20, more than 50% was released, reaching over 80% by day 40. This approach presents a simple and cost-effective technique for the controlled delivery of nutrients using functional polymeric matrices  
Figure 5: (**A**) Rotating drum machine equipped with a hot-air gun used for CSt-g-PAA/NR/PVA coating. (**B**) Sequence of coating urea with CSt-g-PAA/NR/PVA hydrogel and wax. (**C**) HAAKE Rhemix 600p twin-rotor mixer used for starch/acrylamide hydrogel production. (**D**) Twin-screw extruder connected to a single extruder with four injection ports for starch/acrylamide hydrogel production. Modified from (**A**,**B**) [112]; (**C**) [114]; (**D**) [115].