The nitrification inhibitor "AM" was tested in four different soils of São Paulo State, in order to determine the effective dose for nitrification inhibition for each soil. The doses of "AM" used (0, 12, 24, 36, 48 and 60 ppm) were not sufficient to act significantly in the Latossol Roxo and Podzolizado of Lins and Marília, var. Marilia soils; however, positive results were obtained in the Podzolizado of Piracicaba and Regossol Intergrade soils. For the first soil, doses of 48 to 60 ppm were sufficient to act for a period of 60 days and for the second, the "AM" was effective with 24 ppm. for a same period of time.