NUMBERS OF MICROORGANISMS 



11 



colloidal soil particles, in the form of zooglea, surrounded by slimy 

 capsules; the third group is found mostly in the soil solution, but the 

 representatives of this group occur frequently also in the form of 

 clumps, especially on decomposing organic matter. A comparative 

 study of the occurrence of these three groups at different depths of soil 

 has given the following results : 



TYPE OF SOIL 



Forest soil. 



Brown loam soil. 



Sandy soil. 



Surface 

 10 cm. 

 20 cm. 



Surface 

 10 cm. 



Surface 

 10 cm. 

 20 cm. 



PIECES OF 



FUNGUS 

 MYCELIUM 



millions 

 per gram 



47 

 34 



7 



3 



19 

 3 



The non-spore forming bacteria are thus found to be most abundant, 

 the large rods or bacillary forms coming next, especially in soils rich in 

 decomposing organic matter. Fungus mycelium is also abundant in 

 such a soil. 



Cultural methods for demonstrating the kinds of organisms active in the 

 soil. The cultural methods for the study of soil microorganisms are 

 divided into methods for (a) quantitative study of soil microorganisms, 

 (6) qualitative studies, and (c) for the study of microbiological activities, 

 both in pure culture and in the soil. 



Winogradsky 13 suggested to use the silica gel plate, to which a specific 

 substance is added, for demonstrating the existence of specific organisms 

 in the soil. 



Pure, colorless potassium silicate is dissolved in water to a specific gravity of 

 1.06 (6 to 8 Beaume). A dilution of HC1 equivalent to a specific gravity of 1.10 

 (13 Beaume) is also prepared. An equal volume of the silicate is poured into the 



13 Winogradsky, S. Sur une methode pour apprccier le pouvoir fixateur de 

 l'azote dans les terres. Compt. Rend. Acad. Sci. 180: 711-716. 1925. 



