Graduate Thesis Or Dissertation
 

Nitrogen release from granules of sulfur-coated urea

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  • Sulfur-coated urea (SCU) is a slow release nitrogen fertilizer manufactured by the Tennessee Valley Authority. Urea granules are sprayed with molten sulfur, then holes in the sulfur coating are sealed with a layer of wax sealant. The sulfur coatings are mixtures of amorphous and crystalline allotropes of sulfur. Coating thickness varies over the surface of each granule. The distribution of coating thickness among granules in a given lot of SCU depends on the conditions under which it is manufactured. A model was constructed to predict the rate at which urea is released from SCU as a function of time. It was assumed that microorganisms break down coating materials until the urea is exposed to water. The urea dissolves and diffuses out of the granule into the soil. Soil temperature, soil water content, and coating characteristics affect the rate of release. Diffusion of urea out of open SCU granules was examined theoretically and experimentally. Theoretical equations were developed which predict that diffusion of urea through small holes in the coating occurs in two stages. The initial release rate was shown to be constant. After all the urea has dissolved, the rate of release decays exponentially. To test the theory, granules of SCU-31 were immersed in water and the rate of release of urea from these granules was measured for 70 days. It was found that the cumulative amount of urea released was an exponential function of time. Urea was released 1.25 times faster at 35 C than at 5 C, while the theory predicted that release should be 2.33 times faster at 35 C than at 5 C. The large variability in hole sizes among open granules may account for the difference between predicted release rates and those measured experimentally. Granules of SCU-31 which were alternately wetted and dried at 35 C released more urea than granules which were constantly immersed. The breakdown of coatings may have been caused by removal of sealant from holes in the sulfur shell. Alternatively, a portion of the amorphous sulfur in the coating may have crystallized as the surface dried, causing cracks to open in the sulfur shell. Granules in water, the temperature of which was varied between 5 and 35 C each day, released urea at the same rate as samples maintained at constant temperature. Equations were developed to predict the rate at which microbial populations increase and the rate at which each organism breaks down the coating materials. The rate of coating breakdown is the product of organism numbers and organism activity. To establish a relationship between the amount of coating removed and the fraction of granules which have been opened, granules were dipped in benzene for 1 to 10 minutes to remove portions of their coatings. After dipping in benzene, the amount of coating removed was measured and the percentage of open granules was determined. Based on the resulrs of this experiment, it was hypothesized that all SCU granules fall into one of three classes. Class I: granules are initially open and release urea immediately upon exposure to water. Class 2: sealant must be removed from openings in the sulfur shell before urea is released. Class 3: sulfur must be removed from the coatings before urea is released. The relationship between the amount of coating removed and the number of granules opened was linear for both Class 2 and Class 3 granules. A computer program. was written which predicted the rate at which urea is released from SCU as a function of time. To test the accuracy of these predictions, release of urea from SCU-4, SCU-20, SCU-30, and SCU-23 was measured at the soil temperatures of 25 and 35 C in soil at: -0..3 bar soil water potential for 100 days. Release from SCU-4 was also measured at 5 and 15 C and in soil at -15 bars soil water potential. The agreement between predicted and measured release indicated that for the temperature and soil water conditions studied, the rate of release was accurately predicted using the computer program.
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