Sulfur-coated urea (SCU) is a matrix-based composite controlled-release fertilizer. Its core feature is a thin film of sulfur coated on the surface of urea granules. Some products also have an additional wax or resin sealing layer. By controlling the decomposition rate of the coating, nitrogen release is slowed down, while simultaneously supplementing crops with essential sulfur. It possesses the dual functions of “nitrogen control and sulfur supply,” making it a commonly used low-cost slow-release fertilizer for field crops and cash crops.
I. Core Preparation Process
The core of sulfur-coated urea production is precise coating spraying technology. Strict control of temperature and spraying rate is required to ensure uniform coating thickness. The specific process is as follows:
Urea Pretreatment:Urea granules with uniform particle size are preheated to a specific temperature to enhance the adhesion between the granules and molten sulfur.
Molten Spraying:Sulfur is heated to a molten state and evenly sprayed onto the surface of the preheated urea granules in a rotating drum. Depending on the product’s release cycle requirements, an additional layer of wax or resin can be sprayed as a sealing layer to improve coating stability.
Cooling and Shaping: After spraying, the granules cool and solidify, forming a two- or three-layer structure of “urea core + sulfur coating (optional wax/resin sealing layer)”.
Thickness Control: The coating thickness is controlled by adjusting the sulfur melting temperature, spraying rate, and roller speed—a thin coating corresponds to rapid release (suitable for short-term growing crops), while a thick coating corresponds to long-term release (suitable for perennial crops or field basal fertilizer).
II. Nutrient Release Mechanism: The release of nitrogen and sulfur from sulfur-coated urea relies on the natural decomposition process in the soil environment. The release process is highly compatible with the crop’s nutrient requirements and consists of two simultaneous stages:
Coating Decomposition Stage: Water vapor in the soil permeates the coating, while soil microorganisms (such as sulfur-oxidizing bacteria) gradually decompose the sulfur coating. If a wax/resin sealing layer is present, it will first break down under microbial or physical action, initiating the decomposition of the sulfur layer.
Nutrient Release Stages
Nitrogen Release: As the coating develops pores, soil moisture enters the granules, dissolving the urea. Nitrogen slowly diffuses into the soil through these pores, with a release cycle controllable within 8-12 weeks, avoiding the problem of “one-time release and massive loss” common with ordinary urea.
Sulfur Release: Decomposed sulfur is converted into sulfate, which is absorbed and utilized by crops. Sulfur is a key medium-element for protein synthesis and enzyme activity activation, promoting crop metabolism and improving the quality of agricultural products.
III. Core Advantages
Extended Nitrogen Supply Cycle, Improved Utilization Rate
Ordinary urea releases a large amount of nitrogen within 1-2 weeks after application, with a utilization rate of only 30%-40%. Sulfur-coated urea releases nitrogen slowly, increasing the utilization rate to over 50%, reducing nitrogen loss due to leaching and volatilization, and continuously supplying the crop’s needs throughout its entire growth period.
Dual-purpose fertilizer, supplementing sulfur
Unlike polymer-coated controlled-release fertilizers, sulfur-coated urea’s coating itself is a sulfur source, effectively addressing sulfur deficiency-related issues such as leaf yellowing and weakened growth in crops. It is particularly suitable for sulfur-deficient soils (such as sandy soils and fields where sulfur-required crops are grown long-term).
Reduced fertilization costs, saving labor and time
The long-lasting nitrogen release allows for single-application as base fertilizer, eliminating the need for multiple topdressings. This significantly reduces labor and fuel inputs, meeting the needs of large-scale mechanized planting of field crops such as wheat, corn, and cotton.
Environmentally friendly, meeting regulatory requirements
The slow-release nitrogen minimizes farmland runoff pollution and ammonia volatilization, reducing the impact on eutrophication of water bodies and the atmospheric environment, aligning with agricultural policies aimed at reducing fertilizer use and increasing efficiency.
IV. Production and Quality Control Key Points
Key control parameters: During production, precise control of the sulfur melting temperature (ensuring complete melting without decomposition), spray uniformity (avoiding excessively thick or thin coatings in certain areas), and cooling rate (preventing coating cracking) is crucial.
Customized Release Curves: By adjusting the coating thickness and sealing layer type, different nutrient release curves can be customized. For example, thin-coated products are suitable for topdressing crops, while thick-coated products are suitable for base fertilizer of perennial crops such as fruit trees and seedlings.
V. Precautions for Use
Impact of Soil Acidity: Long-term, large-scale application of sulfur-coated urea will lower the soil pH due to the decomposition of sulfates, leading to soil acidification. It is recommended to use it in acidic soils in conjunction with alkaline materials such as quicklime or wood ash, or to alternate it with alkaline fertilizers to maintain soil acid-base balance.
Application Method: Trench or hole application is required; avoid surface spreading—high temperature and drought on the surface will accelerate coating damage and impair the controlled-release effect. Also, do not crush it before use, otherwise the coating structure will be destroyed, and the controlled-release function will be lost.
Suitable Crops: Prioritized for field crops such as wheat, corn, and rice, as well as crops with high nitrogen requirements and sulfur needs such as fruit trees and lawns. Not suitable for aquatic crops (such as lotus root), as rapid decomposition of the coating will lead to nitrogen loss.
Conclusion: Integrating SCU into Modern Fertilizer Production Systems
In conclusion, sulfur-coated urea represents a significant advancement in controlled-release fertilizer technology, offering an economical solution for sustainable nutrient management. Its successful npk fertilizer manufacturing process can be integrated into broader npk fertilizer production lines to create specialized formulations. While SCU production primarily employs drum coating, other key technologies like the double roller press granulator are essential in modern npk fertilizer production technology. For NPK compound fertilizers, dry granulation processes using a fertilizer roller press machine offer an alternative for heat-sensitive materials, with the npk bulk blending machine enabling precise nutrient ratio customization.
The scalability of SCU production complements the high NPK compound fertilizer production capacity demanded by global agriculture. When evaluating the NPK fertilizer production price, the long-term agronomic benefits and reduced environmental impact of controlled-release products like SCU justify the investment. Ultimately, integrating SCU technology with npk fertilizer granulator machine equipment and npk granulation machine technology creates versatile production systems capable of manufacturing a spectrum of products—from specialized controlled-release fertilizers to blended NPK formulations—supporting efficient, precise, and sustainable agriculture worldwide.
