Plants as Affected by Insufficient Food. 143 
The nitric acid thus formed combines with certain sub- 
stances (bases) in the soil, as potash and lime, forming 
nitrates (255), 
b — Symbiosis (sym-bi-o’-sis) on the roots of leguminous 
crops, through which atmospheric nitrogen is changed to 
nitric acid. 
c— Deposits from the atmosphere in rain or snow. 
d— Ammonium salts or nitrates applied directly to the soil. 
257. The Conditions Affecting Nitrification are 
similar to those affecting plant life in general, since nitrifica- 
tion 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 dormant 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 con- 
tained by organic matter. 
258. Soil Aeration Promotes Fertility by favoring 
nitrification. Tbus cultivation, and drainage (of heavy soils) 
not only directly promote the growth of plants by assisting 
aeration (94), but they actually increase plant food. Early 
plowing in spring promotes nitrification by favoring warm- 
ing of the soil. Cultivation in dry weather further pro- 
motes plant nutrition by preventing the accumulation of 
soluble plant food in the dry surface soil, where it is de- 
posited by evaporation above the reach of roots. 
259. Partially Decomposed Organic Manures Act 
More Promptly than fresh ones, because nitrification has 
already commenced in these materials, 
