Identification of Ammonium Thiosulfate as a Nitrification and Urease Inhibitor

Incubation studies at 25°C with a Williams loam (Typic Argiboroll) illustrated that the thiosulfate ion is an inhibitor of nitrification. Inhibition of the nitrification of 10 mmol N kg −1 (as ammonium sulfate) ranged from 55–80% with an addition of 1 mmol S kg −1 (as sodium thiosulfate) to 82–91% with a 10 mmol S kg −1 addition. Percent inhibition was greater after 28 than 14 d of incubation. Residual soil ammonium measurements confirmed the influence of thiosulfate on nitrification. A separate study illustrated that a 5% (v/v) inclusion of commercial‐grade liquid ammonium thiosulfate (ATS) to urea‐ammonium nitrate solution (UAN) inhibited nitrification by 51% in a 28 d incubation. Another experiment also illustrates that small additions of ATS to UAN dramatically reduced the rate of urea hydrolysis. Incubations were performed for 2 and 4 d at 25°C with a Williams loam and a Bearden silty clay loam (Aeric Calciaquoll). Levels of inhibition ranged from 15–35% with a 1% (v/v) addition of ATS to UAN to 40–52% with 10% (v/v) inclusion. ATS did not inhibit urea hydrolsis by purified jackbean urease. Three commercial sources of ATS were compared to reagent sodium thiosulfate, at equal rates of sulfur, and all sources inhibited soil urease similarly. Although the nitrification and urease inhibition properties of ATS are less than have been reported for some other materials, ATS deserves further laboratory and field evaluation as a possible nitrification and urease inhibitor. As commercial batches of UAN sometimes contain ATS, each lot should be tested for ATS contamination before use in research. A procedure for such a test is described.