654 PLANT GROWTH AND PLANT COMMUNITIES 



tioned. It is usual to recognize the role of soil organisms in organic- 

 matter transformations but to ignore their presence when considering 

 the physico-chemical events taking place on root surfaces. 



Soil ecology 



The world of the soil is a biological community of which the 

 higher plant, supported by the soil, is a most important member, con- 

 tributing as it does to the energy supply for the vast mass of the soil 

 population. The microorganisms within the soil are, in truth, largely 

 dependent on the higher plants, though at times one senses a desire on 

 the part of microbiologists to prove that the higher plants are depend- 

 ent on the soil microorganisms. It is only incidental that the activities 

 of some organisms contribute to the nutritional support of higher 

 plants. 



There is a growing field of soil ecology in which consideration is 

 given to the whole community of the soil. The microbiologist has iso- 

 lated from soils an enormous array of organisms— bacteria, actino- 

 mycetes, fungi, algae, protozoa, etc.— with physiological capabilities of 

 bewildering diversity, which in the great majority of cases cannot be 

 related directly to the welfare of the higher plant. More is heard about 

 disadvantageous organisms— the pathogens, parasites, cutworms, and 

 nematodes that cause root injury to seedlings or established plants— 

 than about the great company of unclassified soil inhabitants. Much of 

 the time there is hardly a whisper about the soil fauna, which may ac- 

 count for a considerable part of the total living mass in the soil, 



Garrett ( 1956 ) pointed out astutely that soil ecology is analogous 

 to the better-known field of plant ecology, but in reverse. In the se- 

 quence of events leading to the climax situation, the successful organ- 

 isms prepare the way for their replacement through the autogenic 

 changes they produce in the habitat. "But whereas an autogenic suc- 

 cession of higher plants tends to improve the capacity of the habitat to 

 support growth of the more specialized types ... a succession of 

 heterotrophic micro-organisms progressively depletes its substrate be- 

 cause this is finite and exhaustible, and so the end-point of the micro- 

 bial succession is not a climax association but zero." 



It is immaterial that plants can be grown satisfactorily in an en- 

 vironment that excludes all other organisms— as can animals. This is 

 merely an experimental trick, in view of the unchallengeable fact that 

 the normal environment is one in which organisms are numerous. 

 Plant roots not covered by the rhizosphere mantle of microorganisms 

 would be abnormal, but there is no clear evidence that this is either 

 good or bad; it is only inevitable. 



