252 HOW CROPS FEED. 
In the soil, nitric acid is always combined with an 
alkali or alkali-earth, and never exists in the free state in 
appreciable quantity. We speak of nitric acid instead of 
nitrates, because the former is the active ingredient com- 
mon to all the latter. Before considering its formation 
and nutritive relations to vegetation, we shall describe 
those of’ its compounds which may exist in the soil, viz., 
the nitrates of potash, soda, lime, magnesia, and tron. 
Nitrate of Potash (K NO,) is the substance com- 
mercially known as niter or saltpeter. When pure (refin- 
ed saltpeter), it occurs in colorless prismatic crystals. It 
is freely solub'e in water, and has a peculiar sharp, cooling 
taste. Crude saltpeter contains common salt and other 
impurities. Nitrate of potash is largely procured for in- 
dustrial uses from certain districts of India (Bengal) and 
from various caves in tropical and temperate climates, by 
simply leaching the earth with water and evaporating the 
solution thus obtained. It is also made in artificial niter- 
beds or plantations in many European countries. It is 
likewise prepared artificially from nitrate of soda and 
caustic potash, or chloride of potassium. The chief use 
of the commercial salt is in the manufacture of gunpowder 
and fireworks. 
Sulphur, charcoal, (which are ingredients of gunpow- 
der), and other combustible matters, when heated in con- 
tact with a nitrate, burn with great intensity at the ex- 
pense of the oxygen which the nitrate contains in large 
proportion and readily parts with. 
Nitrate of Soda (Na NO,) occurs in immense quantities 
in the southern extremity of Peru, province of Tarapaca, 
ag an incrustation or a compact stratum several feet thick, 
on the pampa of Tamarngel, an arid plain situated in a 
region where rain never falls. The salt is dissolved in hot 
water, the solution poured off from sand and evaporated to 
the crystallizing point. The crude salt has in general a 
