EFFECTS OF THIOSULFATE AND TETRATHIONATE ON UREASE ACTIVITY AND SEEDLING GROWTH

Interest in use of ammonium thiosulfate (ATS) in conjunction with urea as a fertilizer has been stimulated by reports that ATS retards hydrolysis of urea by soil urease. We recently found, however, that ATS significantly retarded urea hydrolysis in soil only when applied at very high rates (greater-than-or-equal-to 2,500-mu-g/g soil) that adversely affected seedling development. Because ATS is rapidly oxidized in soil, we compared the effects of thiosulfate and its oxidation products (tetrathionate, sulfite, and sulfate) on urea hydrolysis and seedling development in soil and hydrolysis of urea by jackbean urease. We found that the inhibitory effect of thiosulfate on urea hydrolysis in soil is due to tetrathionate formed by oxidation of thiosulfate and that both thiosulfate and tetrathionate have an adverse effect on seedling growth of wheat and corn in soil. Tetrathionate at concentrations of 2,500 and 5,000-mu-g/mL inhibited hydrolysis of urea by jackbean urease, whereas thiosulfate had no inhibitory effect at these concentrations. We could not confirm a hypothesis that the inhibitory effect of ATS on soil urease is due to Fe2+ and Mn2+ formed during oxidation of thiosulfate in soil.