364 SCIENCE PROGRESS 



factor can be suppressed in ordinary field soils by any 

 economical and practicable method. 



It is possible in the light of these results to form a clearer 

 picture of the microscopic life of the soil. The micro-organic 

 flora of an ordinary soil is very mixed, including as it does a 

 wide variety of organisms performing very different functions. 

 They may be divided roughly into two classes : saprophytes 

 which live on and effect the decomposition of organic matter, 

 and a class comprising (a) phagocytes which consume living 

 bacteria, (b) large organisms inimical in other ways to bacteria. 

 The action of the saprophytes tends to increase the fertility of 

 the soil, e.g. they produce ammonia, fix nitrogen and so on. 

 It is true that some of them cause a liberation of nitrogen 

 during the decomposition of organic matter and are to this 

 extent injurious. Such action, however, is either much re- 

 stricted or is counterbalanced by fixation processes. On the 

 other hand the phagocytes and similar organisms are detri- 

 mental to fertility, because they limit the number of bacteria 

 and therefore the rate at which ammonia is produced. 



Between these two classes of organisms there is an equi- 

 librium under natural conditions ; the bacteria cannot multiply 

 indefinitely but are kept in check by the phagocytes, which, 

 in turn, are kept down by the limited amount of food, water, 

 etc. In these circumstances bacteria effect only a limited 

 amount of decomposition — much less, in fact, than might be 

 expected from the quantity of organic matter present. 



When toluene is added or when the soil is heated to 98 C. 

 the phagocytes are killed but not the bacterial spores. On 

 removing the toluene and moistening the soil, the spores 

 germinate and the resulting organisms multiply with great 

 rapidity, since they are now freed from the attacks of their 

 enemies and the competition of other larger organisms ; they 

 even appear to decompose the dead organisms. There is 

 evidence that the individual species may be less virulent than 

 the old races, but they more than make up for any deficiency 

 in this direction by their enormously increased numbers. The 

 rate of decomposition is considerably hastened and a larger 

 amount of ammonia is produced. Some of the groups of organ- 

 isms suffer, such as the nitrogen-fixers, whilst the nitrifying 

 organisms are exterminated. 



It might be thought that the removal of nitrifying organisms 



