NUMBERS OF MICROORGANISMS 39 



million cells were found per gram of soil, comprising 0.4 to 6.75 per cent 

 of the total flora developing on the plate. Since Rhizobium was found 

 to fix 3 to 5.6 mgm. of nitrogen for 100 cc. of solution, it was believed 

 to be identical with Bad. radicicola. 



The use of starch agar for determining the numbers of organisms in 

 the soil capable of utilizing starch has also been suggested. 89 



A detailed report of the abundance of various physiological groups of 

 bacteria in different soils is given in table 5, based upon three deter- 

 minations made by Diigelli 88 in 1920, using two samples for each de- 

 termination. 



Numbers of actinomyces in the soil. The synthetic media (p. 16) 

 best adapted for the growth of bacteria are also well adapted for the 

 growth of actinomyces, and their numbers can be determined on the 

 same plate used for bacterial numbers. If the reaction of the media 

 favorable for the study of numbers of the three groups of soil organisms 

 are compared, the optimum for actinomyces is found to be pH 7.0 to 

 7.5, for bacteria pH 6.5 to 7.0, for fungi pH 4.0. Of course, these are 

 not the optima for the growth of the particular organisms in pure 

 culture. The incubation period for counting actinomyces should be at 

 least 7 days and, if possible, 14 days, at 25° to 30°C. 



The actinomyces are, next to the bacteria, the most abundant group 

 of organisms in the soil, as far as forms developing on the plate are 

 concerned. Since a colony arises from a conidium, a chain of conidia 

 or a piece of vegetative mycelium, larger numbers may not necessarily 

 indicate more abundant vegetative growth, but a larger abundance of 

 conidia. 



Hiltner and Stormer observed an increase in the numbers of actino- 

 myces in the autumn, relative to the other groups of microorganisms, 

 due to the increase of the content in undecomposed organic matter in 

 the soil. They were found to form in the spring 20 per cent, in the fall 

 30 per cent, dropping in the winter to 13 per cent, of the total soil 

 microbial flora developing on the plate. The addition of stable manure, 

 due to its straw content, results in an increase in the numbers of 

 actinomyces. 



88 Diiggeli, M. Die Bakterien des Waldbodens. Schweiz. Ztschr. f. Forstwes. 

 1923; Forschungen auf dem Gebiete der Bodenbakteriologie. Landw. Vortrage. 

 Verlag Huber, Frausenfeld. 1921. H. 3. 



89 Hoffmann, C. A contribution to the subject of soil bacteriological analytical 

 methods. Centrbl. Bakt. II, 34: 386-388. 1912. 



