|Abstract or Summary
- 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
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