254 HOW CROPS FEED. 
Nitrates of alumina are known to the chemist, but have 
not been proved to exist in soils. Nitrate of ammonia 
has already been noticed, p. 71. 
Nitric Acid not usually fixed by the Soil.—In its deport- 
ment towards the soil, nitric acid (either free or in its salts) 
differs in most cases from ammonia in one important par- 
ticular. The nitrates are usually not fixed by the soil, but 
remain freely soluble in water, so that washing readily and 
completely removes them. The nitrates of ammonia and 
potash are decomposed in the soil, the alkali being retain- 
ed, while the nitric acid may be removed by washing with 
water, mostly in the form of nitrate of lime. Nitrate of 
soda is partially decomposed in the same manner. Free 
nitric acid unites with lime, or at lcast is found in the 
washings of the soil in combination with that base. 
As just remarked, Knop has observed that certain soils 
‘containing much organic matters and sesquioxide of iron, 
appeared to retain or decompose a small portion of nitric 
acid (put in contact with them in the form of nitrate of 
potash). Knop leaves it uncertain whether this result is 
simply the fault of the method of estimation, caused by 
the formation of basic nitrate of iron, which is insoluble in 
water, or, as is perhaps more probable, due to the de- 
composing (reducing) action of organic matters. 
Nitrification is the formation of nitrates. When vege- 
table and animal matters containing nitrogen decay in the 
soil, nitrates of these bases presently appear. In Bengal, 
during the dry season, when for several months rain sel- 
dom or never falls, an incrustation of saline matters, 
chiefly nitrate of potash, accumulates on the surface of 
those soils, which are most fertile, and which, though culti- 
vated in the wet season only, yield two and sometimes 
three crops of grain, ete. yearly. The formation of ni- 
trates, which probably takes place during the entire year, 
appears to go on most rapidly in the hottest weather, 
