CHAPTER XXIII. 

 AZOFICATION. 



THE maintenance of the nitrogen supply of the soil is the phase 

 of soil fertility which has received greatest consideration both from 

 the scientist and from the practical agriculturist. Nitrogen is one 

 of the more expensive commercial fertilizers and is, in the majority 

 of soils, the limiting factor of crop production. The supply of com- 

 bined nitrogen on the earth is comparatively small and it is possible 

 to calculate approximately the time necessary for its exhaustion. 

 Basing his conclusion on such a calculation, at least one scientist 

 has predicted dire calamity to the human race were science not able 

 soon to solve this problem. Science has measured up to its require- 

 ments in this regard, for the synthetic production of combined 

 nitrogen has been accomplished, and this in a manner so highly 

 satisfactory that it is able to compete successfully with the product 

 of natural deposits. Advancements have also been made in our 

 knowledge of the underlying principles influencing the natural proc- 

 esses which govern the fixation of nitrogen in the soil. Although 

 there is much yet to be learned in this field it is upon the control 

 of these natural processes that ultimate success will be based. 



Historical. It has been known for generations that uncropped soils 

 increase in fertility. Less ancient, however, is the knowledge that 

 this increase may be due to a gain of nitrogen in the abandoned 

 soils. Even more recent than this is the knowledge that it may be 

 due to bacteriological action. 



In the middle of the nineteenth century Boussingault wrote: 

 "Vegetable earth contains living organisms germs the vitality 

 of which is suspended by drying and reestablished under favorable 

 conditions as to moisture and temperature." He also hinted at the 

 fact that these microorganisms take part in the process of nitrogen 

 fixation. He spread out thinly 120 gm. of soil in a shallow glass dish 

 and for three months moistened it daily with water free from 

 nitrogen compounds. At the end of this time analysis showed that 

 it had lost carbon, but had gained nitrogen. It was not until thirty 

 years later that Hellriegel and Wilfarth made their discovery of 

 nitrogen-fixation by symbiotic organisms. At that time the labora- 

 tory technic of modern bacteriology was still undeveloped. Since 

 then, however, we have learned much concerning the relationship 

 of plants to free and combined nitrogen of the air and of the soil. 



