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1 Inorganic-coated fertilizers: These are primarily made by using binders to attach inorganic coating materials such as clay, calcium magnesium phosphate, magnesium oxide, sulfur, silicates, phosphates, humic acid, peat, etc., to the fertilizer particles. These inorganic materials pose no harm to the soil, while also providing plants with various base ions. Urea with a sulfur coating currently accounts for a large proportion in the market. Studies have found that the fineness of inorganic materials has a significant impact on inorganic coated fertilizers. As the fineness increases, the alignment of the material particles on the surface of the fertilizer improves, the specific surface area decreases, and the number of micropores is reduced. The nutrient release time of coated fertilizers produced from materials with a fineness of 60 μm is more than 10 times longer compared to those using 120 μm materials. However, as the fineness of the material increases, the specific surface area of the particles rises sharply, leading to some undesirable properties. Therefore, how to reduce the specific surface energy of particles and modify inorganic materials is an urgent issue to be addressed in inorganic coated fertilizers. However, the sealing effect of inorganic coatings on fertilizer particles is poor; larger pores tend to form on the surface of the coating layer. The nutrients in the fertilizers dissolve rapidly, and the coating layer can easily peel off during storage or transportation, which affects their slow-release properties. Some manufacturers use organic polymers (thermoplastic plastics or resins) to coat sulfur-coated urea with a thin layer of ordinary polymer film, in order to improve wear resistance and enhance the release properties of the product. 2. Organic-coated fertilizers: Organic substances are primarily used as coating agents, which allows for effective control over the properties of the coating layer. It is possible to produce coated products with good slow-release characteristics. The coating materials that have been used so far include dozens of types such as polymers (silicone polymers, urea-formaldehyde resins, polyamides, epoxy polyesters, polystyrene, polyethylene, polyamides, vinyl chloride-vinyl acetate copolymers, polyvinyl chloride, polyvinyl alcohol, copolymers of dicyclopentadiene and ethylene glycol, polyvinyl acetate), tar, paraffin, asphalt, oils, latex, synthetic rubbers, and others. These coatings are thin and uniform, with good toughness and elasticity; their products are also suitable for mechanized fertilization. To slow down the dissolution rate of chemical fertilizers, pores and cracks are often sealed with paraffin or similar substances. Rosin can also be added to polymer encapsulation materials to produce fertilizers with higher wear resistance and controlled-release properties; such encapsulation materials usually possess a certain degree of biodegradability. Their controlled-release effect is related not only to their inherent properties but also to factors such as the solvents used, pore-forming agents, and surfactants. In its experiments, the Biology Research Laboratory at Huazhong Agricultural University used several biodegradable organic polymer materials (such as urea-formaldehyde resin, polyvinyl alcohol urea phosphate, and polyvinyl alcohol acetal) as coating materials to produce coated urea. The finished product contains 40% N, 4%–5% natural polymers, 2%–3% synthetic polymers, and 4%–6% inorganic substances. In the fertilizer efficacy tests, wheat yield increased by 66%, while corn yield increased by 11–4%. Liu Fuyan used oligolactic acid, which is synthesized from lactic acid, as an encapsulation material to physically coat urea, thereby producing encapsulated urea. The test results show that the higher the molecular weight of polylactic acid, the better its slow-release effect. However, since polymer solutions contain large amounts of solvent, there are strict requirements for equipment sealing and solvent recovery, which constitutes a constraint on the application of organic solvent-based polymer coatings. 3 Polymer emulsion-coated fertilizers: To address the problem of organic solvent pollution, a new method using polymer emulsions as coating agents has been developed. Its use of a water-dispersible system offers significant advantages, enabling controlled release of nutrients in coated products. During the encapsulation process, only water needs to be removed; there is no need for strict sealing of the equipment or solvent recovery, making large-scale production easy to achieve. Tzika et al. used polyvinylidene chloride emulsion as an encapsulant to coat the fertilizer in a spouted fluidized bed, resulting in coated fertilizers with an encapsulation rate of 10% and a dense film layer; the release rate after 300 hours was 80% n. Yao Junjie et al. used acrylic polymer emulsions to coat urea particles multiple times, along with a composite coating of paraffin and sulfur, which effectively prevented the degradation of the membrane’s density caused by the crystallization and dissolution of urea in high-humidity environments, thereby improving the slow-release properties of the coated urea. Currently, biodegradable polymer materials that use inexpensive coating materials that can be degraded by microorganisms and cause no pollution to soil and water sources are attracting increasing attention. Wu Chunhua used starch and polyvinyl alcohol as raw materials, and with the help of a crosslinking agent, prepared an coating solution. This solution was used to coat urea, resulting in coated urea. The results show that this coated urea exhibits good slow-release properties as well as water retention and anti-caking characteristics. The coating material is low in cost, biodegradable, and causes no environmental pollution. However, polymer emulsion encapsulants are still in the early stages of development and have some shortcomings. On the one hand, hydrophilic polymers result in a membrane layer with high water permeability ； On the other hand, during the encapsulation process, a highly humid environment is likely to form on the surface of the particles, which can lead to the dissolution of some of the fertilizers and their recrystallization on the particle surface. The crystallized particles form a composite membrane layer with the polymer; when these particles come into contact with water, the crystallized fertilizers dissolve rapidly, causing the membrane layer to leak and undermining the slow-release properties of the nutrients.