50 ELEMENTARY AGRICULTURE 



Both are colorless, odorless and invisible. About one- 

 fifth of the air is oxygen and the other four-fifths 

 nitrogen. Oxygen is a very active element, combining 

 readily with other substances. It is the oxygen that 

 causes iron to rust, coal to burn, or wood to decay. If 

 the air were pure oxygen, any fire once started could 

 never be put out, and even our bodies would take fire 

 and burn. 



Nature of Nitrogen. On the other hand, nitrogen 

 is a very inactive element and does not combine readily 

 with other substances. Its presence in the air dilutes 

 the oxygen and makes it less active. It is well known 

 that tea can be made so strong that no person can 

 drink it. It may be readily diluted and its strength 

 greatly lessened, however, by the addition of water. It 

 is much the same way with oxygen. It is so active that 

 it must be mixed with nitrogen before it can be used by 

 man and animals. It is mixed in the air, there being, 

 as has been said, about four times as much nitrogen as 

 oxygen in it. Farm crops cannot use this "free" nitro- 

 gen in the air. 



The Use of Bacteria. There are, however, little 

 plant-like germs, called bacteria, which live in the soil, 

 that can and do feed upon this free nitrogen in the air. 

 These germs are a kind of parasite and are usually 

 found associated with the legumes, i. e., with peas, 

 beans, clover and the like. They fasten themselves to 

 the roots of these plants and build their homes there. 

 Their little "nests" look like tiny potatoes and are 

 called tubercles. They are about as large as pinheads 



