16 THE FERTILITY OF THE SOIL 



question now remaining was what had given this tre- 

 mendous stimulus to the multiplication of the ammonia- 

 making bacteria ; by various steps which need not here 

 be enumerated, the two investigators arrived at the 

 conclusion that the cause was not to be sought in any 

 stimulus supplied by the heating process, but that the 

 normal soil contained some negative factor which limited 

 the multiplication of the bacteria. Examination along 

 these lines then showed that all soils contain un- 

 suspected groups of large organisms of the protozoa 

 class, which feed upon living bacteria. These are 

 killed off by heating or treatment by antiseptics, and 

 on their removal the bacteria which partially escape the 

 treatment, and are now relieved from their attacks, 

 increase to the enormous degree that we have specified. 

 According to this theory the fertility of a soil containing 

 a given store of nitrogen compounds is limited by the 

 rate at which these nitrogen compounds can be con- 

 verted into ammonia, which in its turn depends upon 

 the number of bacteria present effecting the change, and 

 these numbers are kept down by the larger organisms 

 preying upon the bacteria. The larger organisms can 

 be removed by suitable treatment, whereupon a new 

 level of ammonia production, and therefore of fertility, 

 is rapidly attained. Curiously enough one of the most 

 striking of the larger organisms is an amoeba akin to 

 the white corpuscles of the blood the phagocytes which, 

 according to METCHNIKOFF'S theory, preserve us from 

 fever and inflammation, by devouring such intrusive 

 bacteria as find entrance into the blood. The two cases 

 are, however, reversed; in the blood the bacteria are 

 deadly, and the amoeba therefore beneficial, whereas in 

 the soil the bacteria are indispensable, and the amoeba 



