Abstract
Knowledge of elemental sulfur (S 0 ) oxidation kinetics for commercial S 0 sources is required before they can be recommended for use as an S fertilizer source or soil acidulent. A wide number of S 0 products are manufactured and differ in physical traits that influence oxidation rate and their relative effectiveness to supply S or acidify the soil. The objectives of this research were to determine the oxidation rate of four commercial S 0 sources and to measure soil pH and electrical conductivity (EC) changes due to S 0 oxidation in an alkaline Hillemann (fine-silty, mixed, thermic Albic Glossic Natraqualf) silt loam. Water-extractable SO 4 2- -S, soil pH, and EC were measured at five sample dates after S 0 application in three laboratory incubation studies. Products evaluated included wettable S (WS90), Tiger 90 (T90), Disper-Sul90 (DS90), and the experimental product S92. The proportion of SO 4 2- -S recovered was regressed over the 90- or 94-d incubation time using a straight-line model to determine oxidation rate constants (k). An individual k explained oxidation of each product, except WS90, which required two k values to model WS90 oxidation. The first k (0.0589 mg SO 4 2- -S mg S -1 d -1 ) for WS90 represented a rapid oxidation phase between application and the first sample date at 10 d. The second k (0.01359 mg SO 4 2- -S mg S -1 d -1 ) described the slower oxidation rate observed for the remainder of the study. Of the four products tested, WS90 had the highest k. Oxidation of S92 (0.00063 mg SO 4 2- -S mg S -1 ) tended to be more rapid than DS90 (0.00021 mg SO 4 2- -S mg S -1 ) or T90 (0.00032 mg SO 4 2- -S mg S -1 ), which were not different. Only WS90 resulted in agronomically significant reductions in soil pH. Compared with the control, soil pH was reduced from 8.1 to 6.7 by application of 1000 kg S ha -1 '. Oxidation of S° followed zero-order kinetics. For each individual product, k was similar among application rates. These results suggest that commercial S 0 products have different rates of oxidation. Knowledge of the oxidation kinetics of the different commercial S° sources will aid in developing use recommendations to growers for acidification of alkaline soils.
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