Attributes of natural and synthetic materials pertaining to slow-release urea coating industry

Urea is one of the spirited input materials for plant growth. However, more than half of conventional urea applied to the soil may not reach the plants and be washed off by rain and irrigation water. The high lost proportion results not only in economic losses but also in environmental pollution. Alleviation in cost and nitrogen pollution is possible by mitigating the nitrogen release rate and synchronizing it with the plant's need. A wide range of synthetic and natural materials are being tested for production of slow-release fertilizers. Each one of these materials has its own merits, with accompanying disadvantages. This study critically reviews the most featured natural and synthetic materials pertaining to the slow-release urea industry. The focused review was aimed at elaborating the benefits and limitations of polymeric and nonpolymeric coating materials, release mechanism of conventional and coated urea, methods to characterize the coated urea, slow-release characteristics imparted by a coating to the coated urea, and release patterns of conventional and coated urea. This discussion was further extended to commercially available coatings and slow-release potential of superabsorbent, urethane, and starch-based coatings.