Kinetics of field oxidation of elemental sulfur in New Zealand pastoral soils and the effects of soil temperature and moisture

Abstract

The effectiveness of elemental sulfur (S°) as a fertilizer is governed by its rate of microbial oxidation in soil to the sulfate form for absorption by plants. Some 80 field oxidation rate experiments were conducted under grazing throughout New Zealand for one year by measuring the residual S° in soil at approximately two-month intervals. The S° was applied as particles 75 to 150µm in diameter at the rate of 30 kg ha⁻¹. The simple cubic oxidation rate model (no allowance for temperature or moisture changes) accounted for more than 80% of the variance at nearly 70% of sites.

The mean annual soil temperature (10-cm depth) was the most important factor found affecting the mean annual oxidation rate constants, accounting for 38% of the variance through the Arrhenius equation. Other factors, including rainfall, soil moisture, pH and soil groups accounted for only a further 2% of variance. The factor most likely to account for the balance of variance among the sites is the oxidizing efficiency of the associated microorganisms.

The rate constant versus temperature relationship for field oxidation in the five New Zealand climate regions was consistent with mean rate constants of soil groups from a similar set of 47 soils incubated at 25°C and field capacity moisture for 10 weeks or more. Field soils, therefore, had an average moisture for the oxidation rate equivalent to that at field capacity. This would explain the lack of contribution of soil moisture to the variance, and would support the use of the mean annual oxidation rate constant (from the mean annual soil temperature) for calculating the optimum particle size range of S° fertilizer. In confirmation, S° particle size recommendations from field rate constants for pastoral fertilizer were consistent with those from earlier agronomic experiments.