^TMOSPHEUIC AIU AS THE FOOD OF PLANTS. 83 



every flame and fire, every dcicaying plant and animal, 

 the organic matters that exhale from the; skin and Inngs 

 of living animals, or from the foliage and flowers of plants, 

 especially, i)erhaps, the volatile oils of cone-bearing trees, 

 are, indirectly, means of converting a jiortion of free ni- 

 trogen into nitrous and nitric acids, or ammonia. 



These topics will be recurred to in our discussion of 

 Nitrification in the Soil, p. 254. 



Formation of Nitroi^en ('orapouii<ls in the Atmospliere. 



— c. From free nitrogen by ozone accompanying the oxy- 

 gen exhaled from green foliage in sunlight. 



The evidence xipon the question of the emission of ozone 

 by plants, or of its formation in the vicinity of foliage, has 

 been briefly presented on page 68. The present state of 

 investigation does not permit us to pronounce definitely 

 upon this point. There are, however, some ficts of agri- 

 culture which, perhaps, find their best explanation by as- 

 suming this evolution of ozone. 



It has long been known that certain crops are especially 

 aided in their growth by nitrogenous fLTtilizers, while oth- 

 ers are comparatively indifierent to them. Thus the cereal 

 grains and grasses are most frequently benefited by appli- 

 cations of nitrate of soda, Peruvian guano, dung of ani- 

 mals, fish, flesh and blood manures, or other matters rich 

 in nitrogen. On the other hand, clover and turnij^s flour- 

 ish best, as a rule, when treated with phos])hates and alka- 

 line sxxbstances, and are not manured with animal fertiliz- 

 ers so economically as the cereals. It has, in fact, become 

 a rule of practice in some of the best farming districts of 

 England, where systematic rotation of crops is followed, 

 to apply nitrogenous manures to the cereals and phos- 

 phates to turnips. Again, it is a fact, that whereas nitro- 

 genous manures are often necessary to ]ii-oduce a good 

 wheat crop, in which, at 30 bu. of grain and 2,000 lbs. of 

 straw, there is contained 45 lbs. of nitrogen ; a crop of 

 clover may be produccl without nitrogenous manure, in 



