CHAP, vii.] TYPES OF BACTERIAL ACTION 193 



in the soil materials have, however, been associated 

 with particular organisms or groups of organisms, and 

 many of these changes are of fundamental importance 

 in the ordinary nutrition of plants. The organisms in 

 the soil which so far have received the chief attention 

 are those concerned with the supply of nitrogen to the 

 plant. Certain organic compounds of nitrogen, chiefly 

 of a protein nature, become gradually broken down by 

 the action of soil bacteria into simpler compounds, e.g., 

 into amino-acids, and then into ammonia, which latter 

 substance is seized upon by other organisms and oxidised 

 successively to nitrous and nitric acid. As nitric acid 

 is generally the form in which the higher plants 

 obtain the nitrogen they require, the fertility of the 

 soil is wholly bound up in the maintenance of this 

 cycle of change. Under certain conditions the work 

 of other organisms intervenes, and the nitrogen com- 

 pounds, instead of becoming nitric acid, are converted 

 into free nitrogen gas, and are lost to the soil. Per 

 contra, another group of organisms possesses the power 

 of " fixing " free nitrogen, i.e., of taking the gaseous 

 element nitrogen and combining it with carbon, 

 hydrogen, oxygen, etc., into forms available for the 

 higher plants. Such organisms sometimes act when 

 living in " symbiosis " with plants possessing green 

 carbon-assimilating tissue : the two form a kind of 

 association for mutual support, the bacteria deriving 

 the carbohydrate which they must consume from the 

 higher plant supplied by them with combined nitrogen. 

 Other symbiotic processes have been traced in the 

 soil, and may yet be made to play an important part 

 in the nutrition of field crops. Indeed, a number of 

 tentative trials have already been made with the view 

 of increasing the productiveness of the soil by introduc- 

 ing either useful organisms that were wanting, or 



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