Using calorimetric method, heat evolutions associated with the growth of soil microbes (growth thermograms) during microbial decomposition of added organic matters (yeast extracts) in various kinds of soils were studied. Soils tested were soils from vegetable garden, from mountain area, from a playground, from tea garden and sea sands from beach. From the actual heat evolution curves (f(t) curves) derived from the growth thermograms, the "apparent growth rate constant (m')" of soil microbes was determined. As the m' value of soil microbes can be regarded as a decomposition rate constant (kd) of yeast extracts1) added, the abilities of decomposing the yeast extracts were compared for each of the soils. From the results, the followings were obtained: (1) The calorimetry of solid microbes is a usable technique to quantitatively estimate the ability of organic matter decomposition in soils. (2) The ability of organic matter decomposition is considerably different depending on the kind of soils. (3) The decomposition ability of sea sand solids from beach is much higher than those of the other soils. (4) The decomposition ability of tea garden soil is considerably lower than that of the others. On the basis of the above results, we conclude that the method employed in this study provides more quantitative information about the ecological potential than any other existing methods and that the present calorimetric method contributes to the land improvement in terms of the material cycling as well as the agricultural production.