ATMOSPHERIC AIR AS THE FOOD OF PLAXTS. B83 
every flame and fire, every decaying plant and animal, 
the organic matters that exhale from the skin and lungs 
of living animals, or from the foliage and flowers of plants, 
especially, perhaps, the volatile oils of cone-bearing trees, 
are, indirectly, means of converting a portion 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 Nitrogen Compounds in the Atmosphere. 
—c. From free nitrogen by ozone accompanying the oxy- 
gen exhaled from green foliage in sunlight. 
The evidence upon the question of the emission of ozone 
‘by plants, ov 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 facts 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 fertilizers, while oth- 
ers are comparatively indifferent to them. Thus the cereal 
grains and grasses ate 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 turnips flour- 
ish best, as a rule, when treated with phosphates and alka- 
line substances, and are not manured with animal fertiliz- 
ers so cconomically 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 produce a good 
wheat crop, in which, at 30 bu. of grain and 2,600 lbs. of 
straw, there is contained 45 lbs. of nitrogen; a crop of. 
elover nay be produced -without- nitrogenous manure, in 
