Sulfur coated urea (SCU) was the first slow-release fertilizer developed in the 1960s by the Tennessee Valley Authority (TVA) in Muscle Shoals, Alabama. It was designed to reduce the rate of dissolution of urea granules when applied to soil, but a wax sealant had to be applied on top of the sulfur coating in order to fill in imperfections. This raised the question of how this would impact nitrogen release.
Wesley Jarrell’s PhD thesis, supervised by Larry Boersma at Oregon State University, provides an elegant answer to that. It had been known that after SCU is applied to soil, microbes degrade the coatings and open holes and cracks in the coatings so that urea diffuses out of the granules through these openings.
The rate at which SCU breaks down and releases its urea depends on soil temperature, water content, and the characteristics of the coatings. Jarrell established quantitative relationships between release patterns and soil temperature, soil water content, and coating characteristics of a given material. He used his experimental data to develop a series of mathematical equations (there are 55 in his thesis) and a computer program to predict the availability of nitrogen from SCU as a function of time, allowing manufacturers to produce materials for different conditions.
This is an excellent example of innovative PhD research. It addresses a concrete problem through a strong theoretical foundation, systematic experimentation and mathematical modeling, leading to results with practical value. Those who wish may also explore the nicely annotated FORTRAN program!
