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Abstract Urea diffuses through holes in the coatings of granules of sulfur‐coated urea (SCU) into the surrounding soil. The ability to predict the rate at which urea is released from SCU granules aids in matching the slow release characteristics of the fertilizer to specific climatic and soil conditions. When openings are small and the concentration of urea immediately outside each opening is very low, a two‐stage release pattern is proposed. During the initial stage, solid urea is dissolving and urea diffuses out of the granule at a constant rate. When all of the urea inside the granule has dissolved, the rate of release decreases logarithmically with time. The equations describing rates of release during the two stages are urn:x-wiley:03615995:saj2sssaj198003615995004400020042x:equation:saj2sssaj198003615995004400020042x-math-0001 for the constant rate period, and urn:x-wiley:03615995:saj2sssaj198003615995004400020042x:equation:saj2sssaj198003615995004400020042x-math-0002 for the falling rate period, where m r = m/M o with m being the amount of urea released and M o the amount of urea initially present in the granule, D is the diffusion coefficient, A p is the cross sectional area of the pore through which diffusion occurs, Δ x p is the length of the pore, ρ is the density of urea, and c sat is the solubility of urea. The diffusion coefficient D and the solubility of urea c sat are temperature‐dependent, according to urn:x-wiley:03615995:saj2sssaj198003615995004400020042x:equation:saj2sssaj198003615995004400020042x-math-0003 where T is absolute temperature and T c is temperature in degrees centigrade. The temperature dependence of the release rate during the constant‐rate period as determined by experiment agreed well with that predicted from theory. These equations may be used to predict the rate of release provided the physical properties of the given SCU are known.
Jarrell et al. (Sat,) studied this question.