Calorimetry was used to study the heat evolution associated with microbial growth in tea field soils (growth thermograms) during the microbial degradation of added organic matter. From the actual heat evolution curves (f(t) curves) derived from the growth thermograms under various conditions of pH and water content of the tea field soils, the apparent growth rate constant (mu') of soil microbes was determined as an index of their growth activity. In addition, mu' was compared between tea field soils treated with organic versus compound fertilizer. From the results, the following conclusions were made: (i) In acidic tea field soil (pH less than about 6.0), the growth activity of soil microbes declined with decreasing pH, and mu' was twofold higher at pH 6.0 than at pH 4.3. (ii) In tea field soils (Andosol) with a water content less than about 50%maximum water holding capacity (MWHC), the growth activity declined with decreasing water content, and mu' at 52.5%MWHC was double that at 35.2%MWHC. (iii) The growth activity of microbes in tea field soil treated with organic fertilizer was approximately 1.2-fold higher than that treated with compound fertilizer.