132 



Partial Sterilisation of Soil 



Repetition of the experiment with another soil and with gelatine 

 plates gave similar results. The plates showed sharp differences in 

 flora ; fluorescent bacteria predominated on the plates poured from the 

 untreated soil cultures but were absent from the others: B. mycoides 

 and zopfii were most numerous on the plates poured from the toluened 

 soil. It is shown later on that B. mycoides decomposes peptone much 

 more rapidly than B. fluorescens. 



This rate of change is at first slow, then it rapidly increases and 

 finally it slackens. The rapid period sets in some time earlier in the 

 toluened soil than in the untreated, but is delayed considerably in the 

 heated soil probably because of the small number of organisms which 

 survive a temperature of 98° C. 



It was found that a little toluene reduced the decomposition rate 

 almost to zero, thus affording further proof, if more were needed, of the 

 bacterial origin of the change. 



§ 25. It is by no means necessary that soil should be used as the 

 inoculating material in these experiments. The filtered liquid from the 

 peptone cultures readily decomposes peptone, and in this case also the 

 culture obtained from the toluened soil is more potent than that from 

 the untreated. 



Again, the extracts obtained by shaking some of the soil with water 

 and filtering through cotton-wool show the same kind of difference in 



