92 THE PRINCIPLES OP AGRICULTURE 



however, which have the power of drawing 

 upon this supply for their nitrogen. They are 

 the leguminous plants, and include the clovers, 

 peas and beans (110). These plants have knobs 

 or nodules growing upon their roots. These 

 nodules are the homes of germs; and these 

 germs seize upon the nitrogen of the air and 

 turn it over to the plant. This process is 

 known as the fixation of nitrogen. Then if 

 these crops are plowed under they not only 

 add humus from their vegetable substance, 

 but nitrogen which has been gathered from 

 the air. 



139. The nitrogen added in green -crops or 

 humus must go through the process of nitri- 

 fication before it is available to the plant. 

 Sometimes this process does not furnish nitric 

 acid fast enough to supply rapidly growing 

 plants, and then a form of available nitrogen 

 may be added direct. This can be done by 

 using nitrate of soda or sulfate of ammonia. 

 The former is mined in Chile ; the latter is a 

 substance obtained from gas works. The am- 

 monia formed from the nitrogen that was_ in the 

 coal or wood is caught in sulfuric acid (generally 

 known as oil of vitriol). These two substances, 

 together with dried blood from the slaughter 

 houses, constitute the best commercial sources 

 of nitrogen. 



