Sulfur coated urea (SCU) is a controlled-release nitrogen fertilizer developed to overcome several drawbacks associated with conventional urea fertilizers. Unlike conventional urea, SCU regulates the release of nitrogen over an extended period, matching plant needs. This helps reduce nitrogen losses and improves fertilizer use efficiency.

Chemical structure and properties
Sulfur Coating Urea  particles consist of a compact shell or coat made of elementary sulfur surrounding prilled urea. The sulfur shell acts as a barrier, controlling the diffusion of nitrogen from the urea core. Various manufacturing techniques are used to coat urea particles with different thicknesses and properties of sulfur shells. Thicker shells result in slower nitrogen release, extending the fertilization period from a few weeks for conventional urea to 3-6 months for SCU.

Advantages over conventional urea
The slow and extended release of nitrogen from SCU offers several agronomic and environmental advantages compared to applying the same amount of nitrogen through multiple applications of conventional urea fertilizer.

- Higher nitrogen use efficiency: SCU minimizes nitrogen losses through leaching, volatilization and runoff. Studies show 15-30% higher nitrogen recovery from SCU than conventional urea.

- Fewer fertilizer applications: Due to its controlled-release properties, SCU requires fewer field applications compared to soluble nitrogen sources. This saves on labor and reduces machinery passes over the field.

- Improved synchronization with crop demand: Nitrogen from SCU is released to match the uptake pattern of most field crops, preventing deficiencies or excess nitrogen at any growth stage.

- Reduced environmental impacts: Less nitrogen losses from SCU means lower contamination of ground and surface water resources from leaching and runoff. Nitrous oxide emissions, a potent greenhouse gas, are also reduced.

Manufacturing processes
The manufacturing process of SCU involves coating solid urea prills with molten elemental sulfur using various techniques.

- Pan or tumbling coating: Urea prills are tumbled or agitated inside a pan or rotating drum as molten sulfur is sprayed over them to form a uniform coating.

- Fluidized bed coating: Compressed air fluidizes urea particles inside a coating vessel and molten sulfur is sprayed to coat particles as they tumble in the air stream.

- Prilling and coating: In a dual process, urea solution is simultaneously prilled and coated with sulfur as it solidifies.

- Layering: Alternate layers of urea and molten sulfur are rapidly packed into various shapes like briquettes or cubes which harden.

Quality control during manufacturing ensures uniform sulfur coatings of defined thickness are achieved. Advanced processes can tailor products for different soil types and crop stages. Coated samples are tested for dissolution rates under various temperatures and conditions before bulk production.

Applications in various crop production systems
Over the past few decades, SCU has emerged as a popular controlled-release nitrogen fertilizer globally formany agricultural uses:

- Cereal crops: Wheat, rice, maize, barley, adapted well to SCU application due to its ability to satisfy crop nitrogen demand throughout the growing season. Studies report yield increases of 5-15%.

- Horticultural crops: Tree crops like citrus, tropical fruits, vegetables, berries, turfgrass benefit from less number of SCU applications and fewer incidences of nitrogen deficiency or excess.

- Pastures and forage crops: Extended nitrogen supply ensures continuous regrowth of grasses for grazing livestock. SCU maintains higher quality and quantity of forage over conventional urea.

- Precision farming: GPS-controlled variable-rate application matches SCU release to spatially varying crop needs, minimizing wasted nitrogen inputs.

- Organic farming: Allowed as a "regulated synthetic" input in some countries for transitioning conventional farms to organic.

Market growth and future scope
The global SCU market has grown at an average annual rate of over 5% in the past decade and is projected to increase further due to rising demand from large crop producing regions. Major manufacturers are increasing production capacities and introducing new product variants tailored for tropical and subtropical conditions. Wider availability and promotion of SCU use in Integrated Nutrient Management and 4R nutrient stewardship programs will support further market expansion. Overall, SCU will continue playing a vital role in sustainable and efficient crop fertilization practices worldwide.

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