196 Agricultural Bacteriology. 



tion, the soil tends to become poorer and poorer in nitro- 

 gen, because the supply of this element is being used by 

 succeeding crops of plant life, and unless some means 

 are taken to restore the supply, the soil soon becomes a 

 barren one. If every thing that the soil produced was 

 returned to it, its fertility could be maintained indefi- 

 nitely. The soil in certain parts of Japan and China has 

 been cultivated for thousands of years and is today as 

 fertile as ever because every bit of the waste matter of 

 human and animal life has been returned to it, and has 

 not been sent down to the sea in the form of sewage or 

 exported to other lands in the form of raw and manufac- 

 tured products, as is being done in our own country. It 

 seems impractical for us to be as careful as are the peo- 

 ples mentioned in returning to the soil all waste matter. 

 Yet it is absolutely necessary for us to maintain the ni- 

 trogen supply of the soil in every possible way, for this 

 is the element that is most likely to be lost, and the one 

 which is the most costly to replace in the form of com- 

 mercial fertilizers. 



The air, composed as it is of four-fifths of nitrogen and 

 one fifth of oxygen, furnishes the source from which it 

 becomes possible to draw unlimited supplies of nitrogen. 

 The nitrogen of the air is not directly available for most 

 green plants. It is, however, much more economical for 

 the farmer to learn how to utilize this supply than to 

 rely for his store of nitrogen on the commercial ferti- 

 lizers that may be purchased. 



Fixation of nitrogen. Within the last few years 

 methods for the fixation of the nitrogen of the air by 

 electrical means have been perfected. "Where cheap 

 power can be procured, as near water-falls, nitrogenous 

 fertilizers can be made cheap enough to compete with the 



