THE MICRO-ORGANIC POPULATION OF THE SOIL 251 



of organisms so as to observe their behaviour under natural 

 conditions. These difficulties have not proved insuperable, 

 and a number of organisms have been discovered which nor- 

 mally lead a trophic life in the soil. These organisms may be 

 regarded as the micro-organic population of soil. 



The essential conditions for the life of a micro-organic 

 population are adequate supplies of food and energy materials, 

 sufficient air and water, a proper range of temperature, and 

 absence of harmful factors. With the exception of the 

 energy supply these requirements are the same as for higher 

 plants ; and as many of the nutrients are common to the plant 

 and to micro-organisms it is not surprising that soils well 

 suited to the growth of plants often carry a large population 

 of micro-organisms also. 



Investigators have long realised that soils contain numbers 

 of bacteria, and in Chap. V. some of the changes they bring 

 about are described. Evidence has been accumulated that 

 other groups of micro-organisms are normally present also 

 — fungi, algae, protozoa, nematodes, etc. 



In previous editions of this book the organisms were 

 classified according as they were or were not useful to plants. 

 This scheme is useful as a first approximation, but it does 

 not admit of fuller development : it is therefore discarded in 

 the present edition. The members of the soil population 

 must be regarded as leading their own lives and, with few 

 exceptions, possessing powers of adaptation which enable 

 them to draw sometimes on one compound and sometimes 

 on another for food and for energy, and therefore not necessarily 

 always producing the same substances. The mycoides, for ex- 

 ample, where they are active, can produce ammonia from 

 protein and would then be helpful to plant growth ; under 

 other circumstances, however, they assimilate ammonia and 

 therefore compete with growing plants. 



Certain broad relationships have been established between 

 the various members of the soil population, and between the 

 population as a whole and the growing plant : there is evidence 

 for the following propositions : — 



