SOIL PROPERTIES AND THE ABILITY OF THE UREASE INHIBITOR N-(N-BUTYL) THIOPHOSPHORIC TRIAMIDE (NBTPT) TO REDUCE AMMONIA VOLATILIZATION FROM SURFACE-APPLIED UREA

A laboratory study was made to evaluate the ability of the urease inhi bitor N-(n-butyl) thiophosphoric triamide (nBTPT) to reduce NH3 volati lization from surface-applied urea. Urea was amended with 0.0, 0.01, 0 .058 or 0.28% nBTPT (w/w) and was applied, at a rate equivalent to 100 kgN ha-1, to the surface of 16 grassland soils, selected to show a wi de range of different chemical and physical characteristics. Daily NH3 loss was measured for 9 days, after fertilizer application, using ven tilated enclosures at 13-degrees-C. The influence of soil properties o n the effectiveness of nBTPT as a urea amendment was investigated usin g a modelling approach followed by stepwise multiple regression analys is. NH3 volatilization from unamended urea ranged from 5.8 to 38.9% of the N applied and was greatest from a soil with a high pH and low tit ratable acidity. The % inhibition of NH3 loss by nBTPT was highly depe ndent on soil type, being effective on some soils even at the 0.01% nB TPT level. The % inhibition at the 0.28% level ranged from 99.4 to 54. 4%. Modelling the relationship between total NH3 loss and inhibitor co ncentration showed that the % nBTPT required to achieve a given % decr ease in NH3 volatilization was constant for all soils. For example, 0. 092% nBTPT was predicted to lower total NH3 loss by 90% from any given soil. The response to increasing inhibitor concentration in lowering NH3 volatilization was greatest in a soil with low organic matter cont ent and high pH. Stepwise multiple regression analysis showed that 4 s oil properties (titratable acidity, pH-H2O, urease activity and cation -exchange capacity) contributed significantly to explaining 90.6% of t he variation in total NH3 loss. In all soils the % urea remaining at t he end of the incubation was 2.6, 4.6, 10.9 and 25.5% for 0.00, 0.01, 0.058 and 0.28% nBTPT, respectively. The persistence of urea treated w ith nBTPT was particularly marked in soils with a high pH and could ha ve physiological implications for plants. It is particularly interesti ng that nBTPT was most effective in soils which showed high NH3 volati lization from unamended urea.