100 BACTERIA AND SOIL FERTILITY 



ciation for the Advance of Science in 1898, to predict dire calam- 

 ity to the human race if science were not able to utilize atmos- 

 pheric 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 corn crop every year for 500,000 years, whereas the sup- 

 ply- of carbon is sufficient for such a crop for only two years. 

 Nevertheless, carbon has no commercial value as plant- food, while 

 nitrogen in available form is worth from 15 to 20 cents a pound 

 in the market. 



The same atom of nitrogen at different times plays many dif- 

 ferent 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 nitrogen 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 the simpler compounds — 

 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 ammonification. 



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 

 story, the quantity of combined nitrogen in the world would 



