PRESIDENTIAL ADDRESS. 55 



bacteriologically the proof must be made with pure cultures. 

 There are so very many organisms of all kinds in raw soil 

 and any one of them might produce the limiting factor. It 

 would be a long and tedious process to test all the micro- 

 organisms of the soil, and it is therefore probable that the 

 organisms that were tested, did not include the active 

 organism. Indeed this was recognised by Russell and 

 Hutchinson in their first paper, for there the protozoa were 

 only provisionally identified with the detrimental agent, 

 and even the term "protozoa" was not rigidly interpreted. 

 In other words the limiting factor was considered to be 

 only probably protozoal. 



Although Russell and Hutchinson carefully safeguarded 

 themselves, the head of the Rothamsted Experimental 

 Station, in which the work was carried out, said in a presi- 

 dential address before the British Association, that the 

 phagocytic action of the protozoa in limiting the growth of 

 the soil bacteria, was the greatest discovery in soil bacteri- 

 ology since Hellreigel and Willfarth's historical paper upon 

 the nodule bacteria. 



The addition of raw soil to toluened soil brings about a 

 curious result with regard to the flora. A toluened soil, 

 capable of supporting some 43 millions per gram of dry soil 

 and no more, when treated with raw soil, became capable of 

 supporting another 30 millions of bacteria. This does not 

 augur very well for the activity of the introduced protozoa. 

 It rather indicates some condition regarding the balance 

 of the bacterial groups, that, according to the kinds of 

 bacteria, there is a certain normal number supportable by 

 the soil. 



The Bacteriotoxins in the Soil. 

 Upon reading Russell and Hutchinson's paper, I was 

 struck by the many divergences from a common law, by 

 the strenuous pushing forward of the protozoal hypothesis, 



