FIELD MICROPLOT AND LABORATORY EVALUATION OF UREA HYDROLYSIS INHIBITION BY AMMONIUM THIOSULFATE

The degree of urea hydrolysis inhibition by ammonium thiosulfate [ATS; 12-0-0-26S; (NH4)2S2O3] soil is affected by fertilizer application method. Thiosulfate is not itself a urease inhibitor, so the effect of fertilizer application method on S2O32- oxidation may be important. Our objectives were to 1) evaluate the effects of fertilizer application method, S2O3 rate, and environmental conditions on urea hydrolysis inhibition by ATS and 2) to determine the levels of tetrathionate (S4O62-) produced by S2O32- application and the inhibitory activity of applied S4O62-. Urea ammonium nitrate (UAN, 32-0-0) +/- S2O3 or S4O6 was applied to a Kahola silt loam soil (finesilty, mixed, mesic Cumulic Hapludolls) in field microplot and laboratory experiments. Application methods were band (0.25 ml droplets), broadcast (0.005 ml droplets), or uniform (1 or 2 ml diluted UAN added to 5 g of soil). Urea hydrolysis inhibition was greater with band-applied UAN + ATS (13-35%) than with broadcast application (0-20%). A 10:1 ratio of fertilizer urea-N:ATS-S produced maximum inhibition of urea hydrolysis with band or uniform application methods. At a soil moisture of 137 g kg-1, S4O62- was the major S anion present after S2O32- application; thiosulfate oxidation to S4O62- was limited at a soil moisture of 400 g kg-1. Tetrathionate applied with UAN inhibited urea hydrolysis to the same extent as S2O32-. For future field experiments, we recommend that ATS be added to UAN at 5% by volume and applied as a surface band. In the laboratory, aerobic incubation methods are recommended to ensure S2O32- oxidation to S4O62-.