MICROSITE REACTIONS OF UREA-NBTPT FERTILIZER ON THE SOIL SURFACE

Nitrogen transformations that occur at the microsite of urea granule placement in soil as affected by the use of the urease inhibitor n-(N-butyl) thiophosphoric triamide (nBTPT) were studied. Urea granules containing 0, 0.5 or 0.05% w/w nBTPT were placed on the soil surface in plastic cups for up to 6 days. Soils were then frozen in liquid N2 and a 0.9 cm thick vertical slice was cut through the fertilizer placement site. A 3.6 cm wide x 2.0 cm deep section of this slice was cut into 45 squares (0.4 x 0.4 cm) and analyzed for soil pH and extractable ammonium, nitrate and urea concentrations at the microsite where the fertilizer had been placed. In a sandy soil (pH 5.2), it was found that urease inhibitors lowered soil pH and soil NH4+ concentrations at the placement site compared to urea alone and allowed more diffusion of urea away from the fertilizer microsite. In a clay soil (pH 8.2), the effect of nBTPT was not as pronounced, and high concentrations of NH4+ (450 mug N g-1) developed in a zone of high pH. The experiments show that a significant factor in the effectiveness of urea inhibitors is their capacity to improve diffusion of NH4+-N away from the zone of high soil pH associated with urea hydrolysis. The effectiveness of these inhibitors depends on the capacity of the soils to permit diffusion.