Sulfur-Coated Urea: A Dual-Action Slow-Release Fertilizer

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.

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