It was compared the grade of inhibition of two inhibitors products of the nitrification (Sulfatiazol, N - Serve) and the tendencies on the conversion of ammonium into nitrate in the ground and the concentration of this ion on Pennisetum purpureum Schum. Depending on the source (urea, Nitron 26) and the quantity of nitrogen (100, 200 kq/ha). The concentrations of nitrates on the grounds and the vegetal tissue stretched to intensity at the same time as were increase the levels of nitrogenous. The effectiveness of the inhibitors is determined for the employed levels and in some cases for the present environment conditions.