142 Partial Sterilisatioii of Soil 



The results are plotted on Curve 4. 



Extract of Tolueiied soil 



Extract of Toluened soil + 



Extract of iiiitrcatecJ soil 



(some large organisms jiresent) 



Extract of 'rolucned soil + 



culture of large organisms from 



untreated soil 



40 60 80 



Time in hours 

 Curve 4. Effect of large organisms from untreated soil on the rate of decomposition 

 of peptone by soil bacteria (Table 14). 



I 40. Not only does partial sterilisation kill these destructive and 

 competing organisms and thus make the conditions more favourable 

 for the new bacterial flora, but it probably also increases the food 

 supply. We have been able to observe under the microscope a dis- 

 solution of the killed protozoa by the bacteria. It is not possible as 

 yet to form any estimate of the amount of nitrogen thus supplied 

 as food, but it cannot be anything like the amount of ammonia 

 ultimately produced in the soil. 



§ 41. As already remarked, toluene does not kill all the large 

 organisms but leaves at least one which in course of time developes. 

 It is probable that this organism is concerned in the falling off in 

 activity of the bacterial soil after a long period as indicated by the 

 second crop (Table 1), the drop in the rate of oxidation (§ 20) and 

 the fall in bacterial numbers. 



§ 42. While the evidence must be regarded as fairly complete 

 that the removal of large unfavourable organisms is one cause of 

 the improvement effected b)' partial sterilisation, we by no means 

 wish to imply that it is the only one. It is quite possible that there 

 are other factors involved. We found, for instance, a nitrogenous 

 substance in the soil which was very soluble in toluene, the distribution 

 of which would no doubt be affected by toluening. Some of the 

 catalytic changes brought about by soil, e.g. the decomposition of 



