Dec. 4, 1916 
Nitrification in Semiarid Soils 
429 
The above data show that nitrification set in within the first four days 
and continued in the untreated portions throughout the 94 days of the 
experiment. When allowance is made for the nitrate originally present, 
it will be seen that almost as active nitrification took place in the un¬ 
treated soil from the control plot as in that from the manured plot. 
But with the addition of iomgm. of nitrogen in the form of dried blood, 
the more active nitrification took place during the first nine days in the 
soil from the manured plot. After this time nitrate formation took place 
the more vigorously in the soil from the control plot. 
Ammonium sulphate was most actively nitrified in the manured soil 
during the first 31 days. After this time the rate in the control plot 
exceeded that in the manured plot. At every time interval, with the 
exception of the 15-day period, ammonium sulphate was found to undergo 
more active nitrification than dried blood. 
If the rates of nitrification in the two soils be compared on the basis of 
the data obtained upon the ninth day, 1 it would seem reasonable to con¬ 
clude that the manured plot is capable of supporting more active nitrifi¬ 
cation of either dried blood or ammonium sulphate than the check plot. 
If a later period be chosen the inference seems equally reasonable that 
the two soils are about equal in ability to nitrify dried blood. The data 
obtained from the untreated portions, however, would seem to indicate 
that the floras of the two soils are quite similar, so far as their ability to 
produce nitrate is concerned. In a subsequent paper this point will be 
more fully discussed. 
The preceding data strongly emphasize the importance of studying 
the formation of nitrates in laboratory studies during different intervals 
of time and in the presence of varying concentrations of different nitroge¬ 
nous materials. Just as different concentrations of nitrogenous materials, 
as already stated, may lead to widely different conclusions, the above 
results show that almost any conclusion may be drawn regarding the 
relative rates of nitrification of dried blood and ammonium sulphate in 
a given soil or of dried blood in different soils, provided the incubation 
periods be carefully chosen. 
ACCUMULATION OF NITRITES IN LABORATORY EXPERIMENTS ON 
NITRIFICATION 
From the classical experiments of Winogradsky it is generally con¬ 
sidered that nitrification proper begins with ammonia and takes place in 
two stages, each stage being brought about by a different set of bacteria. 
The nitrite bacteria oxidize the ammonia to nitrous acid, and the nitrate 
bacteria complete the oxidation to nitric acid. In field soils, however, 
the activity of the latter is usually sufficiently great to complete the oxida¬ 
tion of nitrite almost, if not quite, as fast as it is fothied. Hence, nitrite 
rarely accumulates in notable amounts in arable soils. 
1 Such comparisons have previously been made (8, 27) upon the basis of a 10-day incubation period. 
