234 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



to assimilate the nitrogen of the air, the nitrogen cycle is 



completed. The plants, or their seeds, furnish food for 



animals and men, which nitrogenous food, as we have seen, is 



broken down, first by the digestive processes of the body, and 



afterwards by micro-organisms, producing first ammonia, and 



finally nitrates, to serve once more as food for plants. It 



must be remembered, besides, that apart from the leguminous 



plants used in this way for food, a large proportion of the total 



growth of the plants of this order must suffer decay, and their 



nitrogen be returned directly to the soil. Indeed this method 



of returning nitrogen to the soil constitutes one of the ordinary 



processes of agriculture, and is part of what is known as the 



rotation of crops. After a crop has been grown, such as 



wheat, which tends to exhaust the soil of its nitrogen, it is 



customary to grow a crop of clover, which is afterwards 



ploughed into the soil. The clover in its growth absorbs 



large quantities of nitrogen from the air ; when it is ploughed 



into the soil it rots, and once more, through the changes 



which have been described, this nitrogen is converted into 



nitrate, which will again serve as food for wheat. 



The series of changes which has been discussed in the 

 foregoing chapters may be usefully siunmarised in the 

 following diagram, which is self-explanatory. The application 

 of the knowledge, summarised in this diagram, to the practical 

 problems of agriculture and sewage disposal will be more 

 fully discussed in Chapters XVI and XVII. 



