The type of organic matter (OM) plays an essential role in nutrient cycling in agricultural soil systems. Microbial activity in tropical soils was calorimetrically followed as a useful tool in this investigation. Tropical soil samples with different textures: Rhodic eutrudox (R), Typic eutrudox (V) and a Quartzipsamment (Q) from Brazil were amended with 25% cattle manure (E), municipal refuse compost (L), earthworm casts (H), the agrochemical trifluralin (T); (23 mug, equivalent dose of 1.25 kg ha(-1)) were explored. The microbial activity was determined by calorimetry and simultaneously by fumigation-extraction (microbial biomass carbon, C) to compare both methods. The results for R, Q, and V soils were: (212.04(A), 195.99(B) 204.47(A)) for microbial biomass C and (0.692(B), 0.714(B), 0.784(A)) for thermal effect with P < 0.05, respectively, over a period of incubation of 91 days. The microbial activity of the modified soils decreases in the order: E, H, L and T. Both methods showed a coefficient of correlation r = 0.7443 and the statistical probability of occurrence of the event, P < 0.0001. From this correlation the utility of both methods for measuring the microbial activity in soils could be deduced.