Long-term fate of fertilizer sulfate- and elemental S in co-granulated fertilizers. Nutrient Cycling in Agroecosystems 120: 31–48
Degryse, F., Baird, R., Andelkovic, I., McLaughlin, M.J. 2021.
Summary by Dobermann, A.
Sulfur (S) deﬁciency has become more common since the 1980s due to reduced atmospheric deposition, increasing crop yields and less use of S-containing fertilizers. Sulfur fertilizers are either sulfate-S (SO4-S) or elemental S (ES) based. Elemental sulfur has the advantage that it does not leach and is 100% S. However, it needs to be oxidized to sulfate in order to be available for plant uptake.
Commercial products usually consist of ES cogranulated with other macronutrient fertilizers. In this paper, a team from the Fertilizer Technology Research Centre at the University of Adelaide, Australia assessed the fate of S-fortiﬁed monoammonium phosphate (MAP) fertilizers. Across four field sites in Argentina, Brazil, Canada and the USA over two years, the sulfur recovery by the crop ranged from 16 to 28% for MAP-ES and from 9 to 86% for MAP-SO4-S.
The data allowed constructing a model which takes into account organic S cycling, SO4-S leaching and ES oxidation to explain the observed crop recoveries, and make longer-term predictions. The model demonstrated that total recoveries of ES will eventually reach those of SO4-S or exceed them if there is SO4-S leaching. Hence, long-term trials are needed to evaluate the true effectiveness of a slow-release fertilizer source such as ES.
The authors also point out that the dynamics of soil organic S, which is a major contributor to S uptake, are still poorly understood.
Classic Plant Nutrition Paper
Nitrogen release from granules of sulfur coated urea. PhD Thesis, Oregon State University
Jarrell, W. 1976.
Summary by Dobermann, A.
Sulfur coated urea (SCU) was the first slow-release fertilizers developed in 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 the 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 on 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, holes and cracks are opened in the coatings, and 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 the data from his experiments to develop a series of mathematical equations (there are 55 in his thesis) and a computer program for predicting 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, arriving at results with practical value. Those who wish may also explore the nicely annotated FORTRAN program!