New York Agricultural Experiment Station. 199 



A still greater weakness results from the fact that the culture 

 media in general use are such that there is no means of being sure 

 in any case to what extent these conditions have been met. The 

 fault of the ordinary culture media is that they contain materials of 

 indefinite composition, different lots of which undoubtedly vary 

 sufficiently in composition so that conditions are at times more 

 favorable for bacterial growth than at others. The result is that 

 the counts obtained by different workers, or by the same worker 

 when using different batches of media, may vary greatly, even though 

 there be no variation in the actual number of bacteria present. 



The fact that the plate method gives incomplete counts when 

 applied to soil has been illustrated by comparing it with the only 

 other method that has been proposed for counting soil bacteria. 

 Hiltner and Stormer 2 suggested that liquid instead of solid media 

 be used for making quantitative determinations. They recom- 

 mended the use of four different liquid media in making each test, 

 each medium adapted to the growth of some particular group of 

 soil bacteria. Their method was to inoculate each medium with 

 small portions of soil infusion of many different dilutions, some of 

 them so dilute as to cause no reaction to take place in the medium 

 into which they were introduced. Having determined how great 

 a dilution was necessary in inoculating each medium before tubes 

 could be obtained in which bacteria adapted to that medium were 

 lacking, a simple calculation sufficed to show the approximate 

 number of each of these groups present in the soil investigated. 

 This dilution method, according to Lohnis, 3 gives higher counts 

 than the plate method, a fact which suggests that many bacteria 

 are overlooked when the latter method is used. 



In spite of this well-known weakness of the plate method, it is 

 in common use today, while Hiltner and Stormer's method is 

 rarely employed. This is partially to be explained by the relative 

 convenience of the two methods. Hiltner and Stormer's method is 

 cumbersome, while poured plates furnish a simple and convenient 

 means of testing several samples in a comparatively short time. 

 A second advantage of the plate method is that it is possible to 

 isolate pure cultures of the various bacteria from the colonies that 

 develop on the plates. The study of these pure cultures gives 

 a more thorough qualitative knowledge of the soil flora than can 

 be obtained by Hiltner and Stormer's method. 



This second advantage of the plate method must be made even 

 greater before the procedure becomes of the greatest possible value 



2 Hiltner, L., and Stormer, K. Studien uber die Bakterienflora des Ackerbodens, mit 

 besonderer Beriicksichtigung ihres Verhaltens nach einer Behandlung mit Schwefel- 

 kohlenstoff und nach Brache. Kaiscrliches Gesundheitsamt. Biol. Abt. Land. u. 

 Forstw., 3:445-545, 1903. 



3 Lohnis, F. Zur. Methodik der bakteriologischen Bodenuntersuchung II. Centbl. 

 Bakl. Abt. II, 14:1-9, 1905. 



