4 8 
Journal of Agricultural Research 
Vol. VII, No. 2 
involved, which indicate that an increase in the nitrate content of soils, 
such as those described by Headden (i, 2) in Colorado, is always accom¬ 
panied by an increase in the quantity of the other alkali salts. Not only 
on the basis of his own data obtained as above but also on those of 
Headden himself, Stewart was able to make out a strong case in sup¬ 
port of his explanation anent the origin of the heretofore-mysterious 
“niter-spots.” 
It will be noted that both Hilgard and Stewart employed actually or 
by implication an indirect method for arriving at their conclusions. 
While Stewart's method was less indirect in that seasonal variations in 
nitrate accumulations were studied, thus allowing something like a meas¬ 
urement of the soil's nitrifying power in the field, it can at best be pro¬ 
ductive only of results which call for further investigation. In other 
words, the determination of nitrates in field soils, no matter how fre¬ 
quently made, as a basis for determining the power of the soil to nitrify 
any nitrifiable material, is subject to more objections because of a lack 
of control of conditions than is a direct, if arbitrary, method of deter¬ 
mining the nitrifying power of a soil in the laboratory. In an attempt, 
therefore, to obtain data by a more direct method, which might serve to 
reveal the truth about the nitrifying powers of humid and arid soils, the 
writers have carried out a series of studies the results of which form the 
principal topic of this paper. 
PLAN, MATERIAL, AND METHODS OF EXPERIMENTS 
In the experiments the plan was to compare under controlled and 
uniform conditions in the laboratory the nitrifying powers of a large 
number of representative soils from both humid and arid regions. In 
seeking for a scheme to use as the basis for the selection of such soils, it 
occurred to the senior author that it could be arranged by employing 
one soil type at least from every State in the Union to compare with a 
large number of California soils collected in connection with the soil- 
survey work of the State. Fortunately, there had just been completed 
a collection of soils from all the States and Territories in the Union 
(at least one soil and one subsoil from each State or Territory) and the 
writers were therefore supplied with what might be regarded as repre¬ 
sentative soil material. For the illustration of arid soils the samples 
collected to represent the different types of four soil-survey areas in 
California were employed. Approximately 45 humid were compared 
with 150 arid soils. 
The nitrifying power of each soil was determined, using the nitrogen in 
the soil, in sulphate of ammonia, in dried blood, and in cottonseed meal 
for every soil. The last-named was employed in 100-gm. portions in 
tumblers, as in the methods now common among soil bacteriologists. The 
soils containing as nearly as possible optimum amounts of water were 
