NUMBERS OF MICROORGANISMS 



37 



Although by the plate method the number of Azotobacter cells 

 present in the soil (when they are present at all) is rather small, amount- 

 ing to only a few hundreds (800 to 1000) 85 or a few thousands 86 per 

 gram, the direct microscopic examination reveals a great abundance of 

 these organisms in soils properly limed and buffered. Clostridium 

 pastorianum (Bac. amylobacter A. M. et Bred.) may be found in num- 

 bers ranging from 1 to 5 millions per gram of soil. In poorly buffered 

 and in acid soils, Azotobacter is absent altogether and its place is taken 

 by the Clostridium, as shown later. 



For the determination of the numbers of bacteria belonging to the 

 genus Rhizobium, a medium containing 2 grams levulose, 0.06 gram 

 asparagine, 0.1 gram sodium citrate, 0.1 gram potassium citrate and 2 

 grams of agar in 100 parts of tap water may be used. 87 When the plates 

 are poured, 3 to 5 drops of normal solution of sodium carbonate are 



TABLE 4 

 Numbers of physiological groups of bacteria in 1 gram of soil 



I. Peptone decomposing 

 II. Urea decomposing. . . 



III. Nitrifying 



IV. Denitrifying 



V. Nitrogen-fixing 



added to 10 cc. of medium. Most bacteria and molds do not develop 

 on this medium; 50 to 8G per cent of the colonies are Rhizobia which 

 are small or punctiform, white, somewhat stiff and gummy. To con- 

 firm the diagnosis, the colonies are transferred to plates of nutrient 

 agar or plant extract (beans) agar. The numbers of Rhizobia in the 

 soil were found to run parallel with its fertility, and as many as 3 to 4 



85 Truffaut, G., and Bezssonoff, H. Augmentation du nombre des Clostridium 

 Pastorianum (Winogradsky) dans les terres partiellement sterilisees par le sulfure 

 decalcium. Compt. Rend. Acad. Sci. 172: 1319-1324. 1921;173:868-870. 1921. 

 La Science du Sol. 1: 3-61. 1922. 



86 Razoomov, A. S. Method of counting soil bacteria according to their physi- 

 ological groups (Russian). Trans. Institute of Fertilizers, No. 28, Moscow. 

 1925. 



87 Greig-Smith, R. The determination of rhizobia in the soil. Centrbl. Bakt. 

 II, 34: 227-229. 1912. 



