Some soils collected in the Amazonian State of Brazil were studied in an attempt to interpret thermodynamically a growth reaction representing balanced exponential microbial growth. The aim is to calculate and to interpret the metabolic enthalpy change per mole of glucose catabolically consumed by soil microorganisms, Delta H-met, to explain the changes in the microbial soil activity due to the deforestation and burning suffered by the Amazonian soils. Locations with primary forest, pastures and agricultural plantations were chosen for sampling. Power/time curves were recorded for 1 g of soil samples supplemented with 0.2 ml of a nutrient solution containing 1.5 mg of glucose and 1.5 mg of ammonium sulphate. From the areas limited by power/time curves the total heat change of the microbial growth reaction in soil, Q(T), was calculated. The metabolic enthalpy change, Delta H-met, was quantified by the equation: Delta H-met=Q(T)/S-0 where S-0 is the initial quantity of glucose added. Results showed differences in the values of Delta H-met calculated for the different soil samples. Soils from primary forests poor in organic matter content have higher values of Delta H-met than soils collected in primary forests rich in organic matter. The introduction of agriculture and pasture also caused changes in values of Delta H-met. It seems that the above mentioned manipulations strongly modify the microbial soil activity and the microbial population in Amazonian soils. From the values of Delta H-met it was possible to quantify the percentage of energy invested in growing biomass. The results showed that some plantations could be threaten by immobilization of nutrients while others could accelerate the process of desertification in that soil.