66 THE SOIL POPULATION AND ITS DISTRIBUTION 



soil, the decrease is more moderate than that of the numbers of 

 bacteria. Consequently, at the deep layers of many soils the plate 

 count reveals a proportionally large abundance of actinomyces. 

 This may be due to their greater tolerance to low concentrations 

 of oxygen as well as to their abihty to thrive upon the organic sub- 

 stances of a complex nature which may be washed into these low 

 levels. The earthy odor of plowed sod land is due to certain 

 aromatic substances elaborated by the actinomyces. 



Since actinomyces are essentially aerobic organisms they are 

 absent in peat bogs, except near the surface, since the bog condi- 

 tions favor the development of anaerobic organisms. Most 

 actinomyces are sensitive to acidity, and are, therefore, practically 

 absent from raw-humus forest soils and unlimed highmoor peats. 

 Certain few species of actinomyces, however, are known to thrive 

 at reactions even more acid than pH 4.0. Since they are quite 

 resistant to dry conditions they are found abundantly in arid and 

 semi-arid soUs as well as in sandy soUs. 



In general, the numbers of actinomyces range from a few 

 thousands to many millions per gram of soil. However, in view 

 of the fact that they occur in the soil both in the form of spores and 

 vegetative mycehum, a large part of which may not develop readily 

 upon artificial media, the numbers determined by the plate method 

 are only relative, and serve merely to indicate the approximate 

 abundance of these organisms in a given soil under a given set of 

 conditions. 



From observations of the actinomyces in differently treated 

 plots of soil which were originally the same (Table 11), it is apparent 

 that, when the soil had become acid following repeated additions of 

 ammonium sulfate, the numbers of actinomyces diminished even 

 to such a low level as 370,000 per gram. In the same soil kept at 

 pH 6.7 by liming (19B), the organisms were as numerous as 

 2,820,000 per gram. However, with the actinomyces as with 

 the bacteria, the organic matter content of the soil is an important 

 factor in determining their abundance. In Plot 5A, with a pH 

 of even 5.4, as many as 2,920,000 actinomyces were found per gram 

 of soil. 



Development of Fungi in Soil. — Fungi occur in soil in the 

 form of both vegetative mycelium and spores; this is true both of 

 soils acid in reaction and rich in organic matter, or neutral and 

 even alkaline in reaction and containing little organic residues. As 



