THE NITRIC ACID OF THE SOIL. 251 
‘ question—“ How great is the ammonia-content of good 
manured soil lying fallow ?”—made repeated determina- 
tions of ammonia (17 in all) in the same soil (well-ma- 
nured, sandy, calcareous loam exposed to all rains and 
dews but not washed) during five months. The moist 
soil varied in its proportion of ammonia with the greatest 
irregularity between the extremes of 0.0008 and 0.0003° |,. 
Similar observations were made the same summer on the 
loamy soil of a field, at first bare of vegetation, then cov- 
ered with a vigorous potato crop. In this case the fluctu- 
ations ranged from 0.0009 to 0.0003°|, as irregularly as in 
the other instance. 
Knop and Wolff examined the soil last mentioned at 
various depths. At 3 ft. the proportion of ammonia was 
scarcely less than at the surface, At 6 ft. this loam, and 
at_a somewhat greater depth an underlying bed of sand,. 
contained no trace of ammonia. This observation ac- 
cords with the established fact that deep well and drain- 
waters are destitute of ammonia. 
Boussingault has discovered (Agronomie, 3, 195) that 
the addition of caustic lime to the soil largely increases its 
content of ammonia—an effect due to the decomposing ac- 
tion of lime on the amide-like substances already noticed. 
§ 5. 
NITRIC ACID (NITRATES, NITROUS ACID, AND NITRITES) OF 
THE SOIL. 
Nitric acid is formed in the atmosphere by the action 
of ozone, and is brought down to the soil occasionally in 
the free state, but almost invariably in combination with 
ammonia, by rain and dew, as has been already described 
(p. 86). It is also produced in the soil itself by processes 
whose nature—considerably obscure and little understood 
will be discussed presently. 
