236 THE LIVING ORGANISMS OF THE SOIL [chap. 



low temperatures, and by the various antiseptics 

 mentioned above, so that they are entirely absent from 

 soils heated to ioo, and present only in relatively 

 small numbers of a few species only in the less 

 drastically treated soils. Their addition to culture 

 media containing bacteria only has been found to limit 

 the multiplication of the bacteria and the production 

 of ammonia. 



Much further work is yet required to clear up the 

 whole story, but the main outlines are clear enough. 

 Ordinary soil, in addition to its flora of bacteria pre- 

 paring ammonia and nitrates from the organic reserves 

 as food for the crop, contains also a fauna of protozoa 

 which prey upon the bacteria and limit their multiplica- 

 tion. Whenever the soil is exposed to conditions 

 favourable to the development of the bacteria, the same 

 conditions also favour the development of the protozoa, 

 which continue to limit the multiplication of the bacteria 

 and therefore the preparation of available plant food, 

 i.e., the fertility of the soil. The partial sterilisation 

 processes heat, treatment with antiseptics, etc., cause 

 a selective killing - off of the protozoa, and therefore 

 remove the limitation to the succeeding development 

 of the bacteria. In consequence, they develop to 

 numbers unattainable in the presence of the protozoa, 

 cause a corresponding increase in the production of 

 ammonia, etc., and therefore in the fertility of the soil. 



The extraordinary enhancement of the fertility of 

 the soil, an enrichment that requires no additions to 

 the stock of plant food, since it is brought about by 

 the more rapid and complete utilisation of what is 

 there, that can be produced in laboratory experiments 

 by the process of partial sterilisation cannot but give 

 rise to visions of a vastly greater productiveness to be 

 attained from the land under cultivation. But many 



