THE CIRCULATION OF NITROGEN 233 



254. Nitrification. The decay of proteid matter involves, 

 first, the change of the insoluble proteids into soluble substances 

 called peptones, a similar process to that of digestion in the 

 stomach. This liquefaction is due to the secretion of special 

 ferments by certain bacteria. Then follow further complicated 

 changes until the nitrogenous substances are broken down, and 

 ammonia (NH 3 ), a relatively simple compound, is formed, together 

 with various organic acids and other compounds. Two forms of 

 bacteria which are abundant in almost all soils cooperate to trans- 

 form the ammonia first into nitrous acid, and then into nitric 

 acid, the latter forming at once nitrates, or salts of nitric acid. 

 The nitrates are the chief' source of the nitrogen supply of green 

 plants. The process by which the ammonia of decay becomes 

 available through the nitrates for plant use instead of passing 

 into the air is called nitrification. 



255. The circulation of nitrogen. There is a circulation of 

 nitrogen in nature, which is indicated in the diagram (Fig. 207). 

 This circulation starts with the nitrates, which are taken up 

 in solution by the cells of green plants, in the higher plants, 

 of course, through the root system. The nitrogen in the nitrates 

 is combined with carbon compounds obtained from the carbo- 

 hydrate food manufactured by the processes of photosynthesis. 

 Hydrogen, oxygen, sulphur, and often phosphorus also enter 

 into the resultant substances, which are proteids. The energy 

 which makes possible this building up of the complex proteids 

 comes from the sunlight, as is indicated in the diagram. Animals 

 are able to carry the building-up processes somewhat higher, 

 obtaining their energy from food which comes directly or indi- 

 rectly from plants. Then the breaking-down process begins 

 through the decay of nitrogenous waste products and of dead 

 matter, and this is accomplished as described in the previous 

 sections through the activities of fungi and chiefly the bacteria. 

 Finally, simple ammonia is produced, and this, by the process of 

 nitrification, enters into the formation of nitrates, and the nitro- 

 gen is then available again for green plants. 



