14 PRINCIPLES OF SOIL MICROBIOLOGY 



genie bacteria in the soil and their importance as carriers of infection. 

 They quite properly, from their point of view, used the methods of 

 medical bacteriology. But even the excellent and stimulative researches 

 of Koch and those following him 21 could not lay a proper foundation of 

 soil microbiology, due primarily to the lack of proper methods. 



The meat-extract-peptone agar or gelatin, found so valuable in patho- 

 genic bacteriology, is entirely inappropriate for soil work, for various 

 reasons, chief among which is the fact that the medium is not standard 

 in composition and that it allows a rapid development of a few organ- 

 isms which readily overgrow the plate and thus may prevent entirely 

 the development of others. The distinct inferiority of bouillon agar or 

 bouillon gelatin can be seen from the results of Engberding, 22 who found 

 that a soil giving 99 colonies with Heyden agar, gave 39 with bouillon 

 agar and only two with bouillon gelatin. But even the Heyden agar is 

 not definite in composition, although it is often used for counting soil 

 bacteria. 



The media used for the determination of numbers of microorganisms 

 in the soil (those that develop on the plate) should allow the development 

 of the greatest possible number of organisms and should be standard in 

 composition, so that every batch made up in the same laboratory or 

 at any other laboratory will be like every other batch. This means that 

 inorganic salts should be used. If organic substances are necessary, 

 they should be pure, stable, and standard if possible, as in the case of 

 the carbon and nitrogen sources. Various sugars or organic acids used 

 as sources of carbon can be obtained in a standard form. Nitrogen 

 substances should also be as standard as possible and used in as small 

 amounts as possible. Agar in itself should not serve as a nutrient and 

 should be, therefore, as pure as possible. The objection to gelatin 

 is that it serves also as a nitrogen source for many microorganisms, 

 thus making the medium not standard. It should, therefore, be used 

 only in qualitative work or in special instances, as in the study of the 

 number of gelatin-liquefying organisms in the soil. To hold in check 

 the development of certain rapidly growing organisms, which prevent 

 the growth of the numerous but slow growing bacteria in the soil, the 

 organic matter content of the media had to be reduced to a minimum. 



21 Houston, A. C. Chemical and bacteriological examination of soils. Local 

 Gov't. Board, Rept. 27: 251-296. 1898. 



"Engberding, D. Vergleichende Untersuchungen liber die Bakterienzahl im 

 Ackerboden in ihrer Abhangigkeit von aussern Einfiiissen. Centrbl. Bakt. II, 

 23: 569 642. 1909. 



