CHAPTER X 



THE NITROGEN CYCLE 



Gaseous nitrogen is fixed by certain bacteria with the 

 formation of organic nitrogenous matter. This organic 

 material is decomposed by other organisms with the production 

 of ammonia. Ammonia is oxidised to nitrate by the nitrifying 

 autotrophes, and certain Chromobacteria have the property 

 of liberating gaseous nitrogen from nitrate. So nitrogen 

 completes a cycle: the steps in this cycle are indicated below 

 and will now be considered in greater detail. 



(a) Nitrogen-fixation 



In Chap. V it was shown that certain organisms are able 

 to utilise atmospheric nitrogen as a source of nitrogen for 

 growth purposes. This fact has been used since very early 

 days of agriculture as a means of fertiHsing soil. The majority 

 of plants lead an autotrophic type of existence and draw upon 

 the inorganic nitrogen of the soil for their nitrogen require- 

 ment. Consequently the growth of a heavy crop of grain 

 results in the depletion of the soil-nitrogen, and cropping of 

 the same soil year after year results in a steadily decreasing 

 yield of grain until eventually such cultivation is no longer 

 economical. Since the times of Virgil it has been known 

 that this depletion can be countered in one of two ways: 



176 



