Waksman — 140 — Actinomycetes 



currence and were detected only seldom in one or two soils (Table 31). 

 With an increase in depth of soil, there was a remarkable increase in the 

 proportion of actinomycetes, as compared with the total soil microbiologi- 

 cal population. Based on an average of three soils, the relative number 

 of actinomycetes increased from 9.2 per cent in the surface layer of 2 cm 

 to 65.6 per cent at a depth of 75 cm, although the total number of 

 organisms decreased from 743,000 to 240,000 per gram between these 

 two depths. 



Jensen (187) made a detailed study of the abundance of actinomy- 

 cetes in various Danish soils. The total numbers were found to range 



Table 32 : Distribution of microorganisms in different soils from 



Bikini and Kongelap Islands (194): — 



Total numbers per gram of soil 



between a few thousand to 13 million per gram. The acid soils, of a 

 pH less than 5.0, had only very few actinomycetes. The numbers 

 increased with a decrease in acidity, largest numbers occurring in soils 

 of pH 6.8-8.0. The relative abundance of actinomycetes as compared 

 to the total population varied from 0, for acid peat soils, to 37 per cent. 

 Some of the species were very common and were found in many soils, 

 whereas others were observed in only very few instances. In a studv of 

 Bikini soils, which are very alkaline in reaction, Johnstone (194) 

 found that the microbiological population is made up largely of actino- 

 mycetes (Table 32). 



Influence of soil treatment upon abundance of actinoviycetes.— 

 Treatments of soil, especially the use of fertilizers which result in 

 changes in soil reaction and the use of the organic manures and cover 

 crops, greatly influence the abundance of actinomycetes in the soil 

 (Table 33). 



