164 CONTRIBUTIONS TO OUR KNOWLEDGJE OF SOIL-FERTILITY, XV., 



While using the Lipman-Brown medium for enumerating soil- 

 bacteria, certain colonies were found growing on the under- 

 surface of the agar, and preventing the growth of bacterial 

 colonies upon the upper surface. These toxic bacteria, termed 

 provisionally T.P.2, grew scantily upon the surface of the 

 Lipman-Brown agar, but well upon ordinar}- nutrient agar. 

 This bacillus and the three Actinomycetes were tested upon 

 sterilised soil to determine their possible toxic action upon the 

 soil-flora. 



A quantity of garden-soil was heated for an hour at 100'', and 

 divided into portions which, after having had the moisture- 

 content raised to 10% with suspensions of the micro-organisms, 

 were put into wide-mouthed, sterile bottles which were plugged 

 with cotton-wool, weighed, and set aside in the laboratory. The 

 temperature varied from 22' to 26° except towards the end of 

 the experiment, when it fell gradually to 16°. The moisture- 

 content was maintained by the occasional addition of sterile 

 water, and portions were abstracted from time to time, and the 

 bacteria counted in the usual manner. The medium employed 

 for the enumeration was Lipman-Brown agar with the addition 

 of 0*05% of Lemco meat-extract, which gave a more uniform 

 count in the triplicate plates. 



Bacteria in millions per gram of dry soil. 



" By an accident, the water-content was raised to 14%. 



In counting the colonies, it is not always easy to distinguish 

 a bacterial colony from a mould; especially is this the case with 

 the Actinomyces-colonies when they are deep in the agar, but, 

 so far as was possible, the moulds were excluded in the counts. 

 The colonies of Actino. No. 10 were easy to recognise on account 

 of their yellowish colour. 



