Oct. 9, 1916 
Nitrifying Powers of Humid mid Arid Soils 
79 
If the hypothesis briefly explained is correct, the difference in the 
behavior of the humid and arid soils toward the nitrification of dried- 
blood nitrogen is at once explicable, since the amounts of organic and 
inorganic colloids are usually much larger in the humid than in the arid 
soils and would act toward the ammonia produced from dried blood as 
above explained. Even if it should prove desirable to use smaller 
quantities of dried blood in cultures to determine its availability in 
arid soils, the mejthod used heretofore may serve as an excellent means 
for the comparison of groups of soils and as an index to the soil's internal 
surface and its status with reference to colloid content. 
In connection with this discussion it is cogent to refer to the results 
obtained by Sackett (10), showing that Colorado soils are superior in 
nitrifying power to 22 soils from localities outside of that State. These 
results would seem to imply that for some reason Colorado soils are 
in general superior in nitrifying efficiency to other soils. A comparison 
of Sackett's data with the writers', however, does not bear out such an 
implication when the nitrification of the soil's own nitrogen and not 
that of fertilizer nitrogen is considered. Thus, for example, in the 
Pasadena area out of 33 soils about 45 per cent produced 7 mgm. or more 
of nitrate in 100 gm. of soil in a month's incubation period. Of the 23 
Colorado soils tested by Sackett only 21 per cent of such soils were found; 
yet the average total nitrogen content of the Pasadena soils is prob¬ 
ably below that of the Colorado soils and Sackett's incubation period 
was six weeks and the writers' only four weeks. When a relative instead 
of an absolute basis of comparison is used, similar results are obtained 
in other California areas. But the humid soils outstrip the Colorado 
soils even farther than the Pasadena soils and show clearly a very supe¬ 
rior nitrifying power as a class for the soil's own nitrogen to that 
possessed by the Colorado soils. 
So far as the nitrification of sulphate-of-ammonia nitrogen is con¬ 
cerned, the Colorado soils do seem to be superior to other soils if the soils 
chosen by Sackett are fairly representative of Colorado soils. They 
are, however, to be considered as a class only slightly superior to the 
Riverside and Pasadena soils when it is considered that we employed a 
much shorter period of incubation, much less sulphate of ammonia, and 
did not inoculate our soils with fresh infusions. In fact, it appears now 
that an equal comparison of the Riverside or Pasadena soils with the 
representative Colorado soils would probably show them to be very 
similar in respect to nitrifying powers for sulphate of ammonia. While 
it would be difficult to establish any fixed criterion, it would seem, how¬ 
ever, that the soil's own nitrogen would for all ordinary purposes best 
subserve the purposes of soil fertility. If such a criterion is adopted, 
then the Colorado soils as well as the arid soils of California can not only 
be said not to be superior in nitrifying power to the humid soils but it 
is barely possible that they are appreciably inferior in that respect. 
