10 PRINCIPLES OF SOIL MICROBIOLOGY 



soil particles, especially in case of clay soils, and of separating living from 

 dead bacteria make accurate counts impossible. The method can, 

 therefore, not be used as yet for quantitative work, but is quite applica- 

 ble for qualitative purposes, to show the types of microorganisms which 

 exist in the soil in an active form. The microscopic method may be 

 used for counting bacteria in culture media, especially in a liquid form, 

 but even here the total volume of microbial cells may prove 9 a better 

 index of the activities of the organisms than their numbers. 



Organisms found in the soil by the direct microscopic method. Conn 

 demonstrated that the actual number of bacteria found in the soil, by 

 the use of the microscope, is probably five to twenty times as great 

 as that indicated by the culture plate method. This discrepancy is 

 due to that fact that a large number of soil bacteria do not grow on the 

 plates. By far the greatest number of microorganisms found in the 

 soil, by the use of the microscope, consists of the minute non-spore- 

 forming rods and cocci. The large spore-forming bacteria (as Bac. 

 megatherium and Bac. cereus) have been found in normal soil only in the 

 form of spores, which make up a very small proportion of the total 

 bacterial flora of the soil. Filaments of actinomyces have also been 

 found, but to a lesser extent than the spores of these organisms. Fungus 

 mycelium was not found in any soil, except when an unusual amount of 

 organic matter is present. The spore-forming bacteria become 10 active 

 in the soil only when a great excess of easily decomposable organic 

 matter has been added or when the moisture content of the soil is high. 

 The minute non-spore-forming rods and cocci are considered 11 to form 

 the autochtonous microflora of the soil. 



Other investigators found that the microscopic examination of soil 

 bacteria allows the differentiation of three distinct groups, 12 namely (a) 

 cocci and short rods, (b) typical large cells of Azotobacter and (c) 

 bacillary forms. The first two groups are largely connected with the 



9 Skar, O. Mikroskopische Zahlung und Bestimmung des Gesamtkubikin- 

 haltes der Mikroorganismen in festen und fliissigen Substanzen. Centrbl. Bakt., 

 II, 57 : 327-344. 1922. Fries, K. A. Eine einfache Methode zur genauen Bestim- 

 mung der Bakterienmengen in Bakteriensuspensionen. Centrlbl. Bakt. I (Orig.), 

 86: 90-96. 1921. 



10 Winogradsky, S. Sur la microflore autochtone de la terre arable. Compt. 

 Rend. Acad. Sci. 178: 1236-39. 1924. 



11 Joffe, J. S., and Conn, H. J. Factors influencing the activity of spore form- 

 ing bacteria in soil. N. Y. Agr. Exp. Sta. Bui. 97. 1923. 



12 Richter, A. A. and B. A. To the question of microscopic soil investigation. 

 (Russian). Utchonie Zapiski, Saratov Univ. 4: No. 1. 1925. 



