UNFAVORABLE FOOD SUPPLY 147 



substances, as soda, potash, lime, magnesia and iron. 

 Ammonia, which is a gaseous compound of nitrogen and 

 hydrogen, is also used to some extent by plants. Free 

 nitrogen, the most abundant constituent of the air, plays 

 no direct part in plant nutrition (259). 



255. The sources of nitrates in the soil are : 



(a) Nitrification, by which the nitrogen contained by 

 organic matter and ammonium sulfate in the soil is changed 

 to nitric acid through the agency of microscopic plants 

 (bacteria). The nitric acid thus formed combines with 

 certain substances (bases) in the soil, as potash and lime, 

 forming nitrates (254) ; (b) symbiosis on the roots of 

 leguminous crops, through which atmospheric nitrogen is 

 changed to nitric acid (259, 112) ; (c) deposits from the 

 atmosphere in rain or snow (260) ; (d) ammonium salts or 

 nitrates applied to the soil (261). 



256. The conditions affecting nitrification are similar 

 to those affecting plant life in general, since nitrification 

 results from plant life. As it takes place below the 

 surface of the soil, it is favored by the same conditions 

 that favor the root growth of land plants, viz., aeration, 

 warmth and moisture. In general, it is active during 

 the growing season, but at a standstill during the dor- 

 mant period. It does not proceed rapidly in spring until 

 the soil has become sufficiently warm to promote active 

 root growth. 



Nitrification also releases the other food materials 

 contained by organic matter (92). 



257. Soil aeration promotes fertility by favoring 

 nitrification. Thus cultivation and drainage (of heavy 

 soils) not only directly promote the growth of plants 

 by assisting aeration (93), but they actually increase 

 plant-food. Early plowing in spring promotes nitrifi- 



