CHAPTER VII. 



THE MICRO-ORGANIC POPULATION OF THE SOIL AND ITS 

 RELATION TO THE GROWTH OF PLANTS. 



The soil is inhabited by a great variety of micro-organisms, 

 but their precise relationship to the growing plant is difficult 

 to determine because we know so little about them. The 

 micro-organic population is certainly highly complex : it is 

 known to contain many kinds of bacteria, moulds, protozoa 

 and other organisms, and new members are discovered almost 

 every month. 



Usually they are picked out by some culture method, and 

 their physiological effect is studied in an arbitrary culture 

 solution : sometimes the results are applied straightway to 

 the soil without further ado. The method is defective for 

 two reasons. Firstly, micro-organisms are considerably in- 

 fluenced by the medium in which they happen to find them- 

 selves, and may effect one change under one set of circumstances 

 but quite another change under other circumstances. Secondly, 

 most micro-organisms exist in two states : an active or trophic 

 state, and a resting state, and it is reasonable to suppose that 

 the resting forms are comparatively unimportant. Probably 

 in many cases no sharp line exists between the two, the active 

 forms changing to the resting stage or back again as the soil 

 conditions alter ; but it is never safe to assume without proof 

 that any organism discovered by culture methods is active in 

 the soil. The main difficulty in applying the results is that 

 soil cannot be sterilised ^ because of its chemical instability, 

 nor can it be made up artificially ; in consequence one cannot 

 begin with a sterile soil and inoculate into it a particular set 



1 Intermittent sterilisation at 80° causes less decomposition, but it does pro- 

 duce change (D. A. Coleman, H. C. Lint, and N. Kopeloff, Soil Sci., igi6, 1, 259). 



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