66 AGRICULTURAL BACTERIOLOGY. 



trogen. So important is this factor that it is not an exaggera- 

 tion to say that the whole problem of life centers around the 

 question of the nitrogen compounds. Without a proper form 

 of nitrogen compound in the soil, plants are ordinarily unable 

 to live. They may obtain their carbon, oxygen and hydrogen 

 from the air, but their nitrogen is furnished by the soil. It is 

 the nitrogen element of the food supply whose exhaustion is 

 threatened and whose replacement offers the greatest difficulty. 

 We can therefore best understand the humus and the relation 

 of bacteria to the soil by centering our discussion around the 

 transformation of nitrogen. We would not imply by this that 

 the other factors are not of great importance, but the problem 

 of nitrogen has been so thoroughly studied, is so clearly the 

 basis of plant life and the fertility of the soil, and withal so 

 closely associated with the life of bacteria, that it may well 

 serve as the center of our discussion. 



The condition of life on the ordinary farm is such as to 

 cause a constant loss of nitrogen. The sources of this loss 

 are numerous but the most evident one is the gradual removal 

 from the farm of the nitrogenous material in the crops which 

 are sold. The farmer cannot retain all his crops upon his soil. 

 If he could sell the cream from his milk, alone, he would sell 

 the carbon compounds and retain most of the nitrogen, which 

 is left largely in the skim milk. When, however, he sells 

 whole milk he is sending off a considerable portion of the ni- 

 trogen which his plants extracted from the soil and furnished 

 to his cattle. Everything sold removes some of it and the 

 methods are numerous by which the farmer is getting rid of 

 nitrogen. This produces an inevitable drain upon the soil 

 and an inevitable tendency toward the condition of nitrogen 

 starvation, a condition in which the soil is unable to support 

 plant life unless supplied with high-priced nitrogen fertilizers. 



But the exhaustion of tin- soil is certainly unnecessary. 

 Althoguh there may be a constant drain of nitrogen it is clear 



