SOIL-PLANT RELATIONSHIPS AND PLANT NUTRITION 37I 



vicinity of plant residues, or fragments of organic material, dense colonial 

 growth may occur. Moreover, microbial cells, as they die, themselves form 

 food for other organisms. 



It has long been recognized that in the immediate vicinity of plant roots 

 micro-organisms are particularly abundant. Indeed it would be correct to say 

 that plant roots are virtually encompassed by a microbial mantle. The zone 

 immediately adjacent to plant roots, which has been called the rhizosphere, 

 is therefore a zone of intense microbial activity. The organisms present seem 

 often to be qualitatively as well as quantitatively different from those present 

 at a distance from the roots. Furthermore, there is reason to believe that 

 different plant species growing in the same soil do not have identical rhizo- 

 sphere populations. These organisms presumably develop because soluble 

 organic substances are liberated from plant roots and different species may 

 not liberate the same compounds. In a sense, plants may be said to determine 

 their microbiological associates. In sterile systems a number of amino acids 

 have been identified as coming from plant roots, but the physiology of this 

 phenomenon has not been fully studied. Only from the ubiquity of rhizosphere 

 populations can it be deduced that organic excretions, emanations, or exuda- 

 tions from plant roots are general. 



There is no evidence that the rhizosphere population is in any way essential 

 to plant growth. Growth in sterile soil or sterile nutrient solutions is at least 

 as good as in non-sterile environments. However, this does not mean that the 

 presence and activity of the rhizosphere population can be ignored when 

 considering the nutrition of the plant. The microbiology, and presumably 

 therefore the biochemistry, of a cropped soil is quite different from that of a 

 soil free of vegetation. 



Some special relationships between soil micro-organisms and plants ought 

 to be mentioned here. The rhizosphere population at times contains pathogenic 

 organisms that can directly injure root tissues. Most organisms of this type 

 are relatively unspecific and cause injury to diverse plant species. The damp- 

 ing-off of seedlings is frequently due to the presence of pathogenic fungi. Root 

 rots may be caused by bacterial invasion. 



There are, however, beneficial relationships between soil organisms and 

 plant roots that involve the transfer of nutrients. The best-known and in a 

 sense the least understood of these is the case of the nitrogen-fixing bacteria 

 (rhizobia) which invade and cause nodules to develop on the root system of 

 some leguminous plants and which, by synthesizing an organic nitrogen com- 

 pound utilizable by the plants, release them from complete dependence on 

 the supply of nitrate in the soil. The mycorrhizal relationship between certain 

 fungi and many forest trees also is believed to affect the supply of one or 

 more nutrients to the plant, the roots of which are invaded or enveloped 

 by mycelial strands. The mycorrhizal association and the symbiotic system 

 in leguminous plants, though beneficial, are not essential. Ecologically they can 



