CHAPTER VI 



THE RELATIONSHIP BETWEEN THE MICRO-ORGANIC POPULATION OF 

 THE SOIL AND 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 animals, 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 : some- 

 times the results are applied straightway to the soil without further 

 ado. The method is defective for two reasons. Firstly, micro- 

 organisms are considerably influenced by the medium in which they 

 happen to find themselves, 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 of organisms so as to observe their behaviour under natural 

 conditions. These difficulties have not proved insuperable, and a 

 considerable number of organisms have been discovered which have 

 been divided into three great groups. The first include those that 

 affect the growth of plants either by direct action on the plant, or by 

 bringing about some change of fundamental importance to the plant. 

 The second produce no effect themselves on the plant, but act on 

 those that do. The third have no action direct or indirect on the 



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