The quest for sustainable agriculture demands nutrient delivery systems that align productivity with environmental responsibility. This review critically evaluates stimuli-responsive starch-based biopolymer coatings for controlled-release fertilizers (CRFs), highlighting their structure, functionality, and agronomic relevance. Starch, an abundant and biodegradable polysaccharide, offers intrinsic advantages such as modifiability, film-forming ability, and compatibility with green chemistry. The paper discusses starch’s physicochemical characteristics, its functionalization to achieve responsiveness to environmental triggers (pH, moisture, temperature, ionic strength), and coating strategies like in situ polymerization, grafting, and nanocomposite integration. A comprehensive analysis of release kinetics, swelling behavior, biodegradability, and water retention is provided, followed by evaluations under simulated field conditions, encompassing various soil types, environmental stressors, and crop responses. Comparative insights with other smart biopolymers such as chitosan, alginate, and cellulose underscore starch’s unique position in CRF technology. Despite promising developments, the review identifies critical research gaps, including limitations in scalability, coordination of multi-stimuli responses, and the need for extensive field validation. This work serves as a consolidated platform for researchers, policy makers, and agro-industrial stakeholders aiming to design smart, eco-friendly fertilizers that address global food security while minimizing ecological footprints.

The rising demand for sustainable agriculture and circular resource management has intensified interest in converting wastewater sludge into value-added products. This review explores the transformation of sewage sludge into slow- and controlled-release fertilizers (CRFs), with a focus on biochar production and encapsulation technologies. Sewage sludge is rich in essential macronutrients (N, P, K), micronutrients, and organic matter, making it a promising feedstock for agricultural applications. However, its use is constrained by challenges including compositional variability, presence of heavy metals, pathogens, and emerging contaminants such as microplastics and PFAS (Per- and Polyfluoroalkyl Substances). The manuscript discusses a range of stabilization and conversion techniques, such as composting, anaerobic digestion, pyrolysis, hydrothermal carbonization, and nutrient recovery from incinerated sludge ash. Special emphasis is placed on coating and encapsulation technologies that regulate nutrient release, improve fertilizer efficiency, and reduce environmental losses. The role of natural, synthetic, and biodegradable polymers in enhancing release mechanisms is analyzed in the context of agricultural performance and soil health. While these technologies offer environmental and agronomic benefits, large-scale adoption is hindered by technical, economic, and regulatory barriers. The review highlights key challenges and outlines future perspectives, including the need for advanced coating materials, improved contaminant mitigation strategies, harmonized regulations, and field-scale validation of CRFs. Overall, the valorisation of sewage sludge into CRFs presents a viable strategy for nutrient recovery, waste minimization, and sustainable food production. With continued innovation and policy support, sludge-based fertilizers can become a critical component of the green transition in agriculture.

Sustainable fertilizer technologies are essential to address nutrient losses, environmental pollution, and inefficiencies associated with conventional urea application. In this study, humic acid–functionalized starch (St–HA) gel coatings were developed and optimized via a Wurster fluidized-bed system to produce controlled-release urea granules, with an additional carnauba wax outer layer to further extend nutrient release duration. The coating formulation was synthesized through in situ crosslinking of tapioca starch with humic acid using N,N′-methylenebisacrylamide and potassium persulfate, yielding a cohesive film. A central composite rotatable design (CCRD) was employed to investigate the influence of atomizing air pressure, fluidizing air flow rate, fluidized-bed temperature, and spray rate on coating performance. Comprehensive characterization; including FTIR, XRD, rheological analysis, thermogravimetric studies, water retention, biodegradability, and surface abrasion, confirmed chemical crosslinking, structural stability, and mechanical robustness of the coatings. Nitrogen release analysis in both water and soil demonstrated a substantial extension of release longevity from less than 2 days (uncoated) to 18–20 days for St–HA-coated urea, and up to 28 days with the additional wax coating. Coated granules exhibited low abrasion (8–24%), high water-retention capacity, and 68% biodegradation in 60 days, ensuring environmental compatibility. The findings establish St–HA/wax hybrid coatings as a viable, eco-friendly strategy for controlled-release fertilizers, integrating renewable feedstocks with scalable industrial processing for precision nutrient management.

Polysaccharide hydrogel-based fertilizer carriers have emerged as promising alternatives to conventional synthetic systems due to their biodegradability, tunable physicochemical properties, and ability to regulate nutrient release through structure–transport interactions. However, their performance is still predominantly evaluated using simplified aqueous testing methods that fail to capture the complexity of real soil environments. This review provides an engineering-oriented analysis of nutrient release behavior from polysaccharide-based hydrogel systems, emphasizing the limitations of conventional aqueous evaluation and their implications for predicting field performance. The discussion integrates material design, transport phenomena, and environmental interactions to establish structure–property–release relationships governing nutrient delivery. Conventional aqueous testing methods are critically examined in terms of experimental configuration, performance metrics, and kinetic modeling approaches, highlighting their tendency to overestimate swelling, neglect ionic and biological interactions, and ignore external transport resistances. The influence of soil-dependent factors, including moisture dynamics, pH, ionic strength, microbial activity, and soil structure, is systematically analyzed to demonstrate their coupled effects on swelling, diffusion, and degradation-controlled release mechanisms. Comparative evidence reveals a consistent laboratory–soil mismatch, where aqueous systems predict faster release rates and shorter durations compared to soil conditions. Based on these insights, key gaps in current evaluation practices are identified, particularly the lack of soil-representative testing protocols and the limited applicability of models derived from aqueous systems. Finally, an engineering framework is proposed for soil-relevant evaluation and improved predictive modeling, aimed at supporting the rational design and scalable implementation of next-generation hydrogel-based fertilizer carriers.