What is Slow-Release Fertilizer Coating?
Slow-release fertilizer coating refers to coating the surface of granular fertilizer with a material that controls release. By adjusting the thickness, porosity, and degradation rate of the coating, the release rate of fertilizer nutrients (mainly nitrogen) in the soil is regulated. Compared to ordinary fast-acting fertilizers, slow-release fertilizers can extend the fertilizer effect period from 15-30 days to 60-120 days, significantly improving nutrient utilization and reducing leaching losses. The choice of coating material determines the shape of the release curve, cost, and the product’s environmental attributes.
I. Polymer Coating: Precise Control of the Release Curve
Polymer coating is currently the most precise type of material for release control, suitable for high-end cash crops and rice, where precise nitrogen supply is required. Polyolefin resin coatings (such as polyethylene and polypropylene) are applied to the surface of the granules using a hot-melt spraying process, forming a continuous and dense film. Its release cycle can be designed within the range of 1 to 12 months by adjusting the coating thickness. The release curve exhibits a typical “S” shape—initial release is less than 10%, followed by a stable release of 70% to 80%, and a final decline. The coating rate is 3% to 8% of the particle mass, and the raw material cost is approximately US$800 to US$2000 per ton. The disadvantage of this approach is that the membrane material degrades slowly in soil, and long-term application carries the risk of microplastic accumulation.
Polyurethane coating is an upgraded alternative to polyolefins, forming a film through the in-situ reaction of isocyanate and polyol on the particle surface. Its controlled-release performance is comparable, but the membrane material is more flexible and less prone to damage during storage and transportation. Polyurethane coating can also introduce degradable segments, allowing the membrane material to gradually degrade over 2 to 5 years. The raw material cost is slightly higher than that of polyolefins, at US$1200 to US$2500 per ton.
II. Sulfur Coating: A Mature and Economical Controlled-Release Solution for Nitrogen Fertilizers
Sulfur coating is the oldest and most widely used inorganic coating material, particularly suitable for urea coating. Preheated urea granules are sprayed with molten sulfur (melting point 115-120°C), which solidifies on the granule surface to form a yellow coating. A sealant (usually paraffin or polymer emulsion) is sprayed onto the sulfur layer to seal the micropores. The coating rate is typically 10%-15%, and the cost of sulfur is approximately $200-400 per ton, significantly lower than polymer solutions. The release period is 40-60 days, with a slightly faster initial release (approximately 15%-25%), making it suitable for crops such as rice and forage grasses that have high early-stage nitrogen requirements.
Three precautions for sulfur coating: Sulfur converts to sulfate in acidic soils, exhibiting sulfur fertilizer effects, but its effectiveness decreases in alkaline soils; the storage temperature should not exceed 40°C, otherwise the sulfur layer will soften, causing granules to stick together; sulfur-coated urea is not suitable for paddy fields where it is flooded for extended periods after application, as sulfur reduction under anaerobic conditions will produce hydrogen sulfide.
III. Natural Polymer Coatings: An Emerging Eco-Friendly Option
To address the microplastic problem, coating materials made from natural polymers are rapidly developing. Chitosan (a deacetylated product of chitin) has excellent film-forming properties and is completely biodegradable, with a release period of 30 to 60 days. The amino groups on its molecular chain can form weak bonds with fertilizer nutrients, further delaying release. The raw material cost for chitosan coatings is relatively high, ranging from $2,500 to $5,000 per ton, and it is currently mainly used in high-value-added applications such as horticulture and organic agriculture.
Starch-based coatings enhance water resistance through esterification or cross-linking modification, achieving a coating rate of 5% to 12% and a release period of 20 to 50 days. The cost of modified starch is approximately $800 to $1,500 per ton, falling between that of sulfur and polymers. However, its water resistance is still weaker than that of synthetic polymers, and it is prone to moisture absorption and clumping during storage and transportation in high-humidity environments. Blending starch with polylactic acid or polycaprolactone can balance degradability and water resistance, and is currently a hot research area.
IV. Mineral and Wax Coating: A Simple Physical Barrier Solution Mineral materials such as bentonite and talc, mixed with binders, are sprayed onto the surface of granules to form a physical barrier layer, delaying moisture penetration. The coating rate is typically 8% to 15%, and the cost is low (US$100 to US$300 per ton), but release control precision is poor, with a release cycle of 15 to 35 days. Mineral coating is suitable for low-value-added slow-release fertilizers for field crops, or as a bottom sealing layer for polymer coatings.
Paraffin wax and polyethylene wax coating is the simplest physical barrier solution—molten paraffin is sprayed and then solidifies into a hydrophobic layer. The release cycle is 5 to 15 days, achieving only “slow release” rather than “controlled release.” Paraffin itself is non-degradable, but the amount used is small (coating rate 3% to 6%), resulting in limited environmental impact. It is mostly used to prevent clumping rather than precise controlled release, or as a sealing layer for sulfur coatings.
Three Decision-Making Criteria for Coating Raw Material Selection:
Based on Target Market and Crop Selection: For high-end cash crops (tea, flowers, fruit trees), polyurethane or chitosan coating is recommended; for field crops such as rice and corn, sulfur coating or polyolefin coating is recommended; organic agriculture must choose chitosan or modified starch coating.
Based on Release Cycle Requirements: Sulfur coating is recommended for 30 to 45 days; polyolefin or polyurethane coating is recommended for 60 to 90 days; for release cycles exceeding 90 days, a customized polyurethane solution with a high coating rate is required.
Based on Equipment Compatibility: Sulfur coating requires a sulfur melting tank, spray rollers, and solvent recovery system; polymer coating requires hot air preheating and solvent evaporation devices; natural polymers are mostly water-based solvent systems, requiring stricter drying temperature control. It is recommended that new production lines prioritize the polyurethane route, as it offers stable raw material sources, mature technology, and minimal environmental impact.
Coating as the Value-Adding Final Stage
Slow-release coating transforms commodity fertilizer granules into precision agronomic tools, extending nutrient availability from weeks to months while reducing environmental losses. The integration of an advanced npk fertilizer coating system—whether deploying sulfur, polymer, or natural polymer films—requires seamless upstream compatibility with the fertilizer coating machine and the granulation platform that feeds it. A robust npk fertilizer granulator machine or npk fertilizer granule machine must first deliver uniform, mechanically stable pellets within the 2–4.75 mm target range, as coating thickness and release precision depend critically on substrate consistency. For producers seeking maximum flexibility, coating can also be applied to granules produced via an npk bulk blending machine, where pre-coated nitrogen sources are physically combined with phosphate and potash to create customized slow-release blends without chemical reaction. By selecting coating materials and application parameters that align with target crops, soil conditions, and regulatory environments, manufacturers convert a standard npk fertilizer granulator machine output into premium, differentiated products that command 30%–100% price premiums over uncoated equivalents.