Vertical distribution of microorganisms in soils was studied. The largest populations of bacteria (10⁸/g of dried soil), actinomycetes (10⁷/g) and fungi (10⁵/g) were observed in soils from the top layer (0∼30cm in depth) of black color loam. Reduced numbers of bacteria and actinomycetes, but few fungi, were detected in soils of the second upper layer (50cm∼5m) of black loam. Bacteria (10⁶/g) were also found even in silty sand soils of 40m depth. Vertical distribution of actinomycetes was examined in detail in soils of 0∼100cm depth taken at a 10cm interval. The actinomycete population was the largest in soils of the surface layer (0∼10cm), and decreased gradually as the depth increased. A comparative survey of all the isolates showed that ca. 80% of different kinds of actinomycete cultures were isolated from surface soils. It was demonstrated that actinomycetes peculiar to individual soil layers existed.

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