420 
Journal of Agricultural'Research 
Vol. VII, No. io 
crops, the effects of alkali salts, the relative nitrifiability of different fer¬ 
tilizing substances, seasonable variation in nitrification, the movement 
of nitrates, and other phases of this question are being studied. The 
results obtained in the early stages of this work strongly emphasized the 
need for further study of the methods to be used. 
In the previous studies on nitrification in semiarid soil the Remy- 
solution method, with certain modifications, has been used to a limited 
extent; but usually the direct-soil method has been used, in which actual 
soil is employed as the culture medium. 1 Different investigators, how¬ 
ever, have modified the details to suit their own ideas. These modi¬ 
fications have to do mainly with variations in temperature, moisture 
content, and periods of incubation on the one hand, and differences in 
the ratio of soil to nitrogenous materials on the other. Regarding the 
latter it is noteworthy that the percentage of nitrogenous, organic mate¬ 
rial employed has been varied from about 0.7 to 2 per cent. In the 
case of ammonium sulphate the variations have ranged from 0.1 to 1 
per cent. 
Likewise, widely variant percentages of actual nitrogen from different 
sources have been added by one and the same investigator. For example, 
Lipman and Burgess (24) employed equal weights (1 per cent) of calcium 
cyanamid, dried blood, bone meal, high-grade tankage, cottonseed meal, 
manure, etc.; but since the nitrogen content of these materials ranged 
from 2.46 per cent to 16.55 P er cent, the actual quantities of nitrogen 
added* must have varied accordingly. Ammonium sulphate was added 
at the rate of 0.2 per cent. On the basis of the data thus obtained, 
deductions were made concerning the relative nitrifiability of these mate¬ 
rials. Likewise, McBeth and Smith (27) added dried blood and ammo¬ 
nium sulphate at the rates of 1 and 0.08 per cent, respectively; and 
while the absolute amounts of nitrate formed from the latter were con¬ 
siderably larger than from the former, no particular notice was given to 
it, an average of the results from the two forms being recorded in many 
cases. 
Before presenting the full data bearing upon the specific subjects 
named above, the present paper will be devoted to a discussion of the 
methods commonly employed in laboratory studies on nitrification with 
special reference to the concentration of nitrogenous materials and period 
of incubation used. In most cases the phenol-disulphonic-acid method 
was used for the determination of nitrate. At frequent intervals through¬ 
out this investigation the aluminum reduction method as outlined by 
Burgess (5) was also used for the purpose of checking the results obtained 
by the colorimetric method. The results by the two methods were found 
to agree closely in all cases, except where high concentrations of nitroge- 
1 It is not deemed necessary to discuss in detail the advantages and disadvantages of these two methods. 
The reader is referred to a paper by Tohnis and Green (25) in which a critical review of the subject is given. 
Some of the points emphasized below were also strongly emphasized by them two years previously. 
