Abstract
We reported previously a mechanism of nitrogen release from resin-coated urea based on the differences in vapor pressure between inside and outside of the coated granules. In the case of coated compound fertilizers, however, the proposed mechanism did not explain fully the differences in release rates of individual nutrients, nitrogen, phosphate and potassium. The release rates of nutrients from 4 resin-coated compound fertilizers were determined periodically in water at 30℃. Ammonium, nitrate and chloride were released earlier in all samples. Phosphate and potassium release were different with the samples. The release of sulfate was the latest in all samples. The constituent compounds in the samples were determined quantitatively by using the X-ray powder diffraction method. Based on the quantities of the salts and double salts, and their estimated solubilities, we concluded that release rates were proportional to the solubilities. The most soluble compound in the granules governed the initial permeation of water vapor into the granules and released the nutrients earlier. Accordingly, release from the less soluble compounds was delayed.
