Chapter VIII —139— Distribution 



In general, a close correlation has been obtained between the abun- 

 dance of actinomycetes in the soil and the amount and extent of de- 

 composition of available organic matter. Heinze (167) spoke of 

 actinomycetes as playing an essential role in the soil "fermentation" proc- 

 esses and in humus formation. Mac6 (279) demonstrated that ac- 

 tinomycetes are capable of decomposing proteins to a very marked 

 extent, thus pointing to their importance in the decomposition of plant 

 residues in the soil. 



FousEK (125) made a comprehensive study of the ability of actino- 

 mycetes to break down plant and animal residues, and came to the con- 

 clusion that these organisms play an important role in decomposition 

 processes. He obtained considerable variations among the different soils: 

 loam soils contained much larger numbers than sandy soils, uncultivated 

 soils more than cultivated soils, and the numbers were higher in fall 

 than in spring. This was also explained by the increase, in the fall, of 

 fresh undecomposed plant residues which serve as food for the actinomy- 

 cetes. These organisms were also abundant in forest soils (24 to 27 per 

 cent), pointing to their important role as agents of decomposition and 

 humus formation. 



According to Conn (7'^'), soils contain much larger total numbers 

 of actinomycetes, namely, 12 to 14 millions per gram, than those reported 

 by HiLTNER and Stormer, namely, 2.5 millions. Actinomycetes were 

 found to be particularly abundant in old sod soils, where they made up 

 nearly 40 per cent of the total soil microflora, as compared to about 20 

 per cent of the corresponding microflora in cultivated soil. Conn 

 found that the addition of grass roots to soil stimulated the development 

 of actinomycetes. 



It may be reasonably concluded that the exact number of actinomy- 

 cetes in any one soil depends, not only upon the nature of the soil and 

 upon its treatment, but also upon the medium that is used for making 

 the plates and upon the conditions of incubation. This explains why 

 HiLTNER and Stormer and Conn reported the numbers of actinomy- 

 cetes in the soil to be greatly in excess of one million per gram of soil, 

 whereas Krainsky (230), who used for plating purposes a very special 

 medium, not favorable to the development of large numbers of acti- 

 nomycetes, found only 20,800 organisms per gram. 



By the use of simple synthetic media, such as nitrate-sucrose agar or 

 egg-albumen agar it was found (Table 30) that the numbers of actino- 

 mycetes varied in 25 soils from 7,500 per gram, for an acid cranberry 

 soil, to 2,400,000 for an Oregon adobe soil; these numbers represented 

 3.5 and 15.4 per cent of the total microbial population developing on 

 the plate respectively. Soils rich in organic matter contained a micro- 

 bial population which was made up of 39 to 46 per cent actinomycetes. 



There was a marked difference in the nature of actinomyces t)^es 

 found in the different soils. Some of the species were very abundant 

 and were found in several soils, whereas others were only of limited oc- 



