A model of oxidation of an elemental sulfur fertilizer in soils

Elemental sulfur (S) fertilizer (S-0) has to be oxidized by microorganisms to produce plant available SO4-2-S. This biological reaction is affected by many factors that influence the soil microbial population and activity and is difficult to describe mathematically. In simulating a S-0 fertilizer application in the field, we discovered that by adding an exponent (n) to the incubation time (t) of the traditionally accepted first-order equation, the SO4-2-S production could be predicted. The equation that describes this relationship is S-t = S-0 . [1-exp(-kt (n))], where S-t is cumulative SO4-2-S produced from applied S-0 at time t, S-0 is S-0 applied when t = 0, k is the oxidation rate constant, and n is a time function modification factor. The equation was tested in two cultivated chernozemic soils receiving fertilizer-size granule S-0 (2.00-3.36 mm. diameter) and fine-particle S-0 (<44 <mu>m diameter) incubated at 5, 10, and 20 degreesC at a moisture content of 60% of field water holding capacity. Between 88 and 99% of the variation between predicted and measured SO4-2-S (P < 0.001) was explained, and the predicted values were at a range of 99 to 102% of the measured values with limited variation (SE <less than or equal to> 4% of the mean). The modified equation offers a promising tool to predict the available S provided over time based on the amount of S-0 fertilizer applied and may have the potential for use in industry and in environmental studies.