KINETICS OF FIELD OXIDATION OF ELEMENTAL SULFUR IN NEW-ZEALAND PASTORAL SOILS AND THE EFFECTS OF SOIL-TEMPERATURE AND MOISTURE

The effectiveness of elemental sulfur (S-degrees) as a fertilizer is g overned by its rate of microbial oxidation in soil to the sulfate form for absorption by plants. Some 80 field oxidation rate experiments we re conducted under grazing throughout New Zealand for one year by meas uring the residual S-degrees in soil at approximately two-month interv als. The S-degrees was applied as particles 75 to 150 mum in diameter at the rate of 30 kg ha-1. The simple cubic oxidation rate model (no a llowance for temperature or moisture changes) accounted for more than 80% of the variance at nearly 70% of sites. The mean annual soil tempe rature (10-cm depth) was the most important factor found affecting the mean annual oxidation rate constants, accounting for 38 % of the vari ance 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 varian ce among the sites is the oxidizing efficiency of the associated micro organisms. 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 inc ubated at 25-degrees-C and field capacity moisture for 10 weeks or mor e. Field soils, therefore, had an average moisture for the oxidation r ate equivalent to that at field capacity. This would explain the lack of contribution of soil moisture to the variance, and would support th e use of the mean annual oxidation rate constant (from the mean annual soil temperature) for calculating the optimum particle size range of S-degrees fertilizer. In confirmation, S-degrees particle size recomme ndations from field rate constants for pastoral fertilizer were consis tent with those from earlier agronomic experiments.