EFFECT OF AMMONIUM-THIOSULFATE AND AMMONIUM-POLYPHOSPHATE ON UREASE ACTIVITY AND AMMONIA LOSS FROM UREA-FERTILIZED SOILS

Ammonium thiosulfate (ATS), a common liquid N and sulfur fertilizer, has been identified as a urease inhibitor. Urease inhibitors as fertilizer additives can reduce ammonia (NH{dollar}sb{lcub}3{rcub}{dollar}) volatilization when urea or urea ammonium nitrate (UAN) are applied to soil surfaces. A series of microplot experiments were conducted to ascertain the agronomic utility of ATS as a urease and ammonia volatilization inhibitor. The efficacy of ATS was studied as a function of ratios of UAN:ATS, different liquid fertilizer droplet sizes, and the addition of a pH buffer (ammonium polyphosphate). Experiments were conducted under field and laboratory conditions from the summer of 1984 until January of 1987.; The results indicate that the addition of ATS to UAN reduced ammonia volatilization in most experiments. The best mixing ratio was 2 to 5% (vol/vol) ATS to UAN solution. Volatilization of ammonia was also significantly reduced by adding 20% (vol/vol) addition of ammonium polyphosphate (APP) to UAN. Ammonia losses increased as fertilizer droplet size was reduced suggesting that increasing droplet size helps prevent ammonia volatilization. A series of laboratory experiments were performed to assess the effect of ATS upon urea hydrolysis under varying soil moisture contents. The overall rates of urea hydrolysis by the two soils tested were greatly affected by the soil moisture, being reduced dramatically at wilting point as compared to field capacity. In all cases, ATS successfully slowed urea hydrolysis. It is not known how ATS interferes with urea hydrolysis. It is probable that ATS acts as a general microbial inhibitor instead of a specific urease inhibitor.