K. J. Russell and H. B. Hutchinson 119 



§ 10. We are now in a position to account for all the changes 

 brought about by partial sterilisation. 



The micro-organic flora of the ordinary arable soil with which 

 we started is very mixed, and includes a wide variety of organisms per- 

 forming very different functions. For our purpose 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 actual 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 bring about liberation of free nitrogen during the 

 decomposition of organic matter, and are to this extent injurious, 

 but such action is either much restricted, or is counterbalanced by the 

 fixation process, and does not affect our general statement. The 

 phagocytes, and similar organisms, on the other hand, must be detri- 

 mental to fertility because they limit the number of the organisms 

 and therefore the rate of ammonia production. 



Between these two classes of organisms there is an equilibrium 

 under natural conditions ; the bacteria cannot multiply indefinitely, 

 but are kept in check by the phagocytes ; the phagocytes, on the other 

 hand, are kept in check by the limited amount of food, and no doubt 

 also by other adverse conditions, such as lack of water^ In these cir- 

 cumstances bacteria effect only a limited amount of decomposition, 

 much less, in fact, than might be expected from the total amount of 

 organic matter present. 



When toluene is added, or when the soil is heated to 98°, the 

 phagocytes are killed, but the bacterial spores are not. On removing 

 the toluene and adding moisture, 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 large organisms ; 

 they even appear to decompose the dead organisms. There is evidence 

 to show 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 largely increased amount of ammonia is 

 produced. Some of the groups of organisms suffer, such as the 



1 On this view it is easy to explain Rahn's results, which have hitherto remained very 

 obscure. He found that drying the soil at ordinary temperature increased its productiveness 

 but did not cause what he considered sufficient alteration in the bacterial flora or the food 

 supply (i.e. the immediate food supply), CeiUr.fiir Bakteriologie, 1908, ii. Bd. 20, S. 38. 



