Oct. 9, 1916 
Nitrifying Powers of Humid and Arid Soils 
77 
and in nitrifying power from the sample of the same name described 
in Table XI. This question of variation in any one type is now being 
investigated. 
In general, therefore, it appears possible that some States may con¬ 
tain very few good nitrifying soils, others very few poor nitrifying soils, 
and still others contain a fair proportion of both. If this possibility is 
allowed, then the samples described in this paper as the foreign soils 
must approach closely the average conditions of humid soils, despite 
the criticisms which are above suggested. 
The next question is that of the influence of the seasonal variation 
in the nitrifying activity of soils on the validity of nitrification data, and 
particularly of those above presented. As pointed out in the intro¬ 
ductory part of this paper, there are now in the writers' possession many 
data on the monthly variation in the nitrifying powers of several dif¬ 
ferent soil types. These data, which cover a period of 1^ years of 
monthly tests, indicate even more strikingly than former results in this 
regard the great variability to which such determinations are subject. 
Hence, the low nitrifying powers of the Bay and Ukiah area soils which 
were gathered from November to December, 1914, may be regarded as 
due to the depressing effect of the conditions of the late fall, particularly 
as regards a lack of moisture. It is also realized that the much greater 
relative activity of the soils of the Pasadena and Riverside areas may 
in part be accounted for by the collection of the samples in June and 
July, when conditions were more favorable to nitrification. The humid 
soils were collected in various parts of the year, but the bulk of them 
arrived at Berkeley between September 15, 1914, and January 1, 1915, 
and soon thereafter were tested. If anything, therefore, the seasonal 
effects should have caused the samples from the humid soils to be some¬ 
what depressed in nitrifying efficiency, but they show themselves su¬ 
perior as a class to the arid soils in the nitrification of soil nitrogen. 
In view of this fact and of the opportunity offered for the comparison 
of a variety of seasonal effects in both arid and humid soils, great signifi¬ 
cance can not be attached to the influence of the seasonal variations, 
which are characteristic of nitrification determinations, on the validity 
of the results. 
The quantity of dried blood employed in the cultures in which that 
material was tested will next be considered. It has recently been 
pointed out (5) for dried blood that the nitrification of small quantities 
may proceed normally in certain soils, as has also previously been shown 
to be true for calcium cyanamid and for goat manure (7), whereas large 
quantities would not only permit no nitrification but would actually 
induce losses of nitrate from the soil. In comparative studies like those 
described previously the absolute values obtained are not significant 
except as side issues, provided all classes of soils tested are treated alike. 
55856°—16 - 3 
