Effect of combining urea fertilizer with P and K fertilizers on the efficacy of urease inhibitors under different storage conditions.

Purpose
Urease inhibitors provide a simple solution to mitigate ammonia loss from fertilized soil. Consumption of bulk blend fertilizers and compound fertilizers were increased in recent year and its enhanced efficiency and stabilized techniques were urgently required. However, it is essentially unknown if and how the efficacy of urease inhibitors is influenced by the inclusion of phosphorus (P) and potassium (K) fertilizers with urea.

Materials and methods
A laboratory study was therefore conducted to assess the impact of P and K (bulk blend scenario: combing urea with di-ammonium phosphate (DAP); compound fertilizer scenario: nitrogen (N)-P-K proportion as 16-16-16 (CN16) or 32-0-6 (CN32)) and additionally the impact of fertilizer storage duration and temperature on the efficacy of two different urease inhibitors NBPT and Limus® in reducing ammonia volatilization following application to soil.

Results and discussion
Both urease inhibitors significantly reduced ammonia loss from urea regardless of storage temperature and time. However, mixed storage of urea and DAP with urease inhibitors significantly decreased the efficacy of NBPT and Limus® in reducing ammonia loss. Ammonia loss increased exponentially with DAP addition rate and with storage time of the mixture. Storage at a higher temperature (30 °C compared with 20 °C) also reduced the efficacy of the inhibitors. Adding magnesium sulfate (MgSO4) to urea plus Limus® significantly mitigated the negative effect of DAP mixed storage on the efficacy of Limus® regardless of storage temperature and time. The urease inhibitors did not significantly reduce ammonia loss from CN16, but were effective for reducing ammonia loss from CN32.

Conclusions
The efficacy of urease inhibitors was compromised by P fertilizer. Urease inhibitor inclusion in the production of CN32, urea, and its blends (DAP + MgSO4) are recommended as an effective means of reducing the environmental cost causing by intensive agricultural production.