NUMBERS OP MICROORGANISMS 13 



media, while the autotrophic and nitrogen-fixing organisms were found 

 not to be able to develop readily in high dilutions. Each of these two 

 methods (plate and dilution) for determining the number of microor- 

 ganisms in the soil has certain advantages and disadvantages. 



The plate method consists in diluting the soil with sterile tap water, 

 making a series of dilutions, so that 1 cc. of the final dilution, when 

 plated out with nutrient agar or gelatin, will allow 40 to 200 colonies 

 to develop on the plate. The dilution method consists of diluting the 

 soil first with sterile water, as with the plate method, but transferring 

 1 cc. of several of the final dilutions into special sterile nutrient media 

 adapted for the growth of particular groups of microorganisms. The 

 number of microorganisms will be found to lie between the two highest 

 dilutions, one of which gives positive and the other negative growth. 

 This allows us to determine approximately the number of organisms 

 belonging to each group and present in the particular soil. 19 The latter 

 method is rather cumbersome, since it involves the preparation of a 

 large number of media and the use of a number of containers for the 

 development of various physiological groups of organisms for making 

 the various dilutions, also, it involves great variability in the results. 20 

 The plate method is convenient, but its chief limitation is the fact that 

 it allows the development of only the heterotrophic, non-nitrogen fixing, 

 aerobic bacteria and of yeasts, molds and actinomyces. The dilution 

 method can be used for the study of practically all known soil forms. 

 The two methods may then be used each for its particular purpose, 

 particularly in view of the fact that, for those microorganisms that 

 develop on the common culture plate, the dilution method was not 

 found 18 to give higher results than the plate method. The latter 

 method should, therefore, be utilized for a general study of the numbers 

 of microorganisms in the soil, keeping in mind its limitations, while the 

 dilution method should be used for the determination of the abundance 

 of special groups of microorganisms which do not develop on the plate. 



Culture media. With the introduction by Koch in 1881 of the gelatin 

 plate for counting bacteria in general, an impetus was also given to the 

 study of soil bacteria. But, unfortunately, Koch himself and prac- 

 tically all the bacteriologists following him for the next fifteen years were 

 medical men interested particularly in the possible presence of patho- 



19 Lohnis, F. Zur Methodik der bakteriologischen Bodenuntersuchung. 

 Centrbl. Bakt. II, 14: 1-9. 1905. 



20 Fischer, H. Zur Methodik der Bakterienzahlung. Centrbl. Bakt. II, 25: 

 457-459. 1910. 



