70 DRY FARMING 



pound containing oxygen. Another species of bacteria 

 (the nitric acid bacteria) then converts this nitrite into 

 nitrate, through the further addition of oxygen to the 

 nitrogen. As nitrate,, the nitrogen may then be used 

 again by plants. 



Certain losses of nitrogen through bacterial action are 

 likely to occur, especially in the manure pile. "Firing" 

 or "fire-fanging", due to the rapid decomposition of the 

 manure, results in the escape of nitrogen to the air in 

 gaseous form. The process is called denitrification and 

 is due to bacterial action. A similar process occurs in 

 soils, especially when water-logged, but the losses from 

 this source in well-tilled soils are not appreciable. 



On the other hand, additions to the soil's supply of 

 nitrogen may occur in several ways. Small amounts are 

 brought down from the atmosphere by the rain and 

 snow, chiefly as ammonia and nitric acid. In the soil 

 certain bacteria (Azotobacter) living upon the carbon- 

 aceous material in the soil, have the ability to extract 

 free nitrogen gas from the air and build it up into their 

 bodies. These sources, however, are not of great 

 magnitude. A third source, the fixation of nitrogen by 

 leguminous plants, will receive due consideration. 



49. The Mineral Plant Food Elements. In the decay 

 of organic matter, carbonic acid and many other organic 

 acids are formed. In the nitrification process nitrous 

 and nitric acid are formed, not as free acids but united 

 with the basic substances, among them the essential 

 plant food elements as calcium, magnesium and potas- 

 sium. These substances, occurring in the soil in in- 

 soluble compounds, are rendered soluble by this process. 

 Phosphorus occurring chiefly in the soil as the insoluble 

 tricalcium phosphate becomes soluble and available to 



