THE NITROGEN CYCLE 183 



the carbon cycle and rendering the carbon-dioxid content of the 

 atmosphere nearly constant. 



The Nitrogen Cycle. Since nitrogen occurs as an essential part of 

 the structure of every plant and animal, it is found in all crops and 

 crop residues. It occurs in the top soil in proteins, protein decom- 

 position products, ammonia, nitrites, and nitrates. It is not found 

 in the mineral matter of the earth except in shales and other deposits 

 containing the residues of plant and animal bodies. Hence, the 

 quantity in the combined form is not great when compared with 

 other essential elements. Yet it is required by all living organisms 

 in large quantities. Many of these are returning it to its inert 

 atmospheric form. This fact led Sir William Crooks, in his famous 

 address before the British Association for the Advancement of 

 Science in 1898, to predict dire calamity to the human race if science 

 were not able to utilize atmospheric nitrogen. 



In the free form, nitrogen occurs in enormous quantities; four- 

 fifths of the atmosphere is composed of it. Dr. Hopkins has pointed 

 out that the total supply of nitrogen over each acre of the earth's 

 surface, if available, would meet the needs of a hundred-bushel crop 

 of corn every year for 500,000 years, whereas the supply of carbon 

 is sufficient for such crops for only tw r o years. Nevertheless, 

 carbon has no commercial value as plant-food, while nitrogen in 

 available form is worth from 15 to 20 cents a pound on the market. 



The same atom of nitrogen at different times plays many different 

 roles. One of the triumphs of agricultural bacteriology is the 

 advancement which it has made in following nitrogen through its 

 cycle. 



Nitrogen occurs in the plant and animal mainly in the form of 

 protein. The plant protein may be eaten by the animal and produce 

 animal protein. Either may reach the soil and decay. The nitro- 

 gen eaten by animals may be deposited as tissues of the animal or 

 excreted as urea, hippuric or uric acid. These products are acted 

 upon by bacteria with the formation of ammonia. 



Either the plant or animal proteins may reach the soil where 

 decay sets in with the formation of albumoses, proteoses, peptones, 

 peptids, and amino-acids. The amino-acids are then deaminized 

 with the formation of an acid and ammonia. The process is spoken 

 of as amrnonification. 



The ammonia does not accumulate in the soil, but is acted upon 

 by other bacteria, the nitrosomonas, with the formation of nitrous 

 acid. This is quickly taken up by the nitrobacter and oxidized 

 to nitric acid which reacts with bases in the soil with the formation 

 of nitrates. The nitrates are the main source of nitrogen for the 

 plants which build from them and carbon dioxid, amino-acids, 

 peptids, peptones, proteoses, albumoses, and finally plant proteins 

 and the nitrogen has completed its cycle. If this were the whole 



