By this study, we evaluate, by incubation in the laboratory, the hydrolysis and the kinetics of mineralization of the urea used in Arenoferralsol and Ferralsol, as well as to appreciate, at the same time, the activity of microorganisms by the quantity of CO2 released over 15 days. In Arenoferralsol, the sharp rise in pH observed during the first 7 days of incubation denotes a strong hydrolysis of urea inhibiting the activity of microorganisms with, as a consequence, a low mineralization of nitrogen. The sharp rise in pH correlates with the massive volatilization of NH3 recorded during this time span. Beyond the 7th day, the decrease in volatile NH3 began, the resumption of microbial activity and the gradual increase in nitrification. On the other hand, in Ferralsol, it was recorded, in the same space of time, a weak rise in pH indicating a weak release of NH3, but, at the same time also a weak microbial respiration and a weak transformation of urea nitrogen. The pH, although raised to an average of 5.5, nevertheless remained strongly acidic. The drop in pH observed from the 11th day of incubation did not, however, stimulate the formation of NH4+-N and CO2, which still remained low. Thus, the NO3--N content experienced some increase. Hence, the conclusion that nitrification in this type of soil is very slow due to its high acidity. Taking into account the results observed in the two soils, nitrification as well as volatilization seem to be more active in Arenoferralsol compared to Ferralsol. Because, the acid state of Ferralsol as well as the quality of the organic matter combine to inhibit nitrification by destroying microbial life, especially those responsible for nitrification while progressive nitrification is noticeable in Arenoferralsol.