90 HOW CROPS FEED. 
until in some way chemically decomposed, belongs to the 
soil or to the rivers and seas. 
Nitrous Acid in the Atmospheric Wuters.—In most of the researches up- 
on the quantity of nitric acid in the atmosphere and meteoric waters, 
nitrous acid has not been specially regarded. The tests which serve to 
detect nitric acid nearly all apply equally well to nitrous acid, and no 
discrimination has been made until recently. According to Schénbein 
and Bohlig, nitrates are sometimes absent from rain-water, but nitrites 
never. They occur, however, in but minute proportion. Pineus and 
Réllig observed but traces of nitrous acid in the waters gathered at 
Insterburg. Reichardt found no weighable quantity of nitrous acid in 
a sample of hail, the water from which contained in 10 million parts, 32 
parts ammonia and 5g parts of nitric acid. It is evident, then, that 
nitrous acid, if produced to any extent in the atmosphere, does not re- 
min as such, but is chiefly oxidized to nitric acid. 
In any case our data are probably not incorrect in respect to the 
quantity of nitrogen existing in both the forms of nitrous and nitric 
acids, although the former compound has not been separately estimated. 
The methods employed for the estimation of nitric acid would, in gen- 
eral, include the nitrous acid, with the single error of bringing the latter 
into the reckoning as a part of the former. 
Nitric Acid as Food of Plants.—A multitude of obser- 
vations, both in the field and laboratory, demonstrate that 
nitrates greatly promote vegetable growth. The extensive 
use of nitrate of soda as a fertilizer, and the extraordinary 
fertility of the tropical regions of India, whose soil until 
lately furnished a large share of the nitrate of potash of 
commerce, attest the fact. Furthermore, in many cases, 
nitrates have been found abundantly in fertile soils of tem- 
perate climates. 
Experiments in artificial soil and in water-culture show 
not only that nitrates supply nitrogen to plants, but dem- 
onstrate beyond doubt that they alone are a sufficient 
source of this element, and that no other compound is so 
well adapted as nitric acid to furnish crops with nitrogen. 
Like ammonia-salts, the nitrates intensify the color, and 
increase, both absolutely and relatively, the quantity of 
nitrogen of the plant to which they are supplied. Their 
effect, when in excess, is also to favor the development of 
foliage at the expense of fruit. 
