75 
oct. 9 ,1916 Nitrifying Powers of Humid and Arid Soils 
By the different arrangement of the data in Tables VIII to X a reversal 
of indications in minor ways has perhaps been brought about. But, 
in general, certain differences of a marked character in the nitrifying 
powers of humid and arid soils are obvious. Thus, by whatever method 
compared, the soil nitrogen of humid soils seems to become nitrified more 
readily than that of arid soils. Likewise, the opposite is true of sulphate- 
of-ammonia and cottonseed-meal nitrogen and their effects on the nitri¬ 
fication of soil nitrogen, and, as pointed out, the difference may amount 
to veritable inverse relationships. On the other hand, the opposite of 
the effects noted for the forms of nitrogen just discussed is true in gen¬ 
eral for dried-blood nitrogen. The most marked differences are, of course, 
evident between the foreign soils on the one hand and the Pasadena and 
Riverside soils on the other, because the latter are more distinctly arid 
in character than the Ukiah and Bay soils. 
REVIEW OE RESULTS 
Since the results obtained by the writers are very striking and offer the 
first direct evidence, so far as they are aware, of the differences between 
the nitrifying powers of humid and arid soils, it is essential that their find-. 
ings be viewed critically. The first question which arises is that of the 
representative nature of the samples of soil employed with respect to the 
different climatic regions. The four soil areas chosen to represent arid 
soils in California may be taken as representative because they exemplify 
as nearly as possible interior valley and coast conditions in both southern 
and northern California. Since more than twice as much rainfall is 
normally received by the Bay and Ukiah areas as that received by the 
Riverside and Pasadena areas, these soils, all of which are none the less 
arid, should be considered as representative of average conditions in the 
State. If, however, it is desired to apply the term “arid” only to regions 
receiving less than 20 inches of rain a year, it is necessary to consider only 
the Riverside and Pasadena soils. So far as soil nitrogen and nitrifica¬ 
tion are concerned, the Riverside area will fairly represent a large, if not 
the largest, part of the San Joaquin Valley, a small part of the Sacra¬ 
mento Valley, the Coachella and Imperial Valleys, and nearly all of the 
Mojave and the Colorado Desert regions besides. The Pasadena and 
the Bay areas may be taken as nearly representative of the coast valley 
conditions from San Francisco to San Diego, the first being only in 
behavior more typical of the southern and the second of the northern 
valleys. The Ukiah area will partly represent a large portion of the 
northern half of Sacramento Valley and much of the coast region above 
San Francisco. In general, therefore, the four soil areas are fairly 
representative of California conditions, and in particular may serve, as 
above indicated, to represent the typically arid conditions of the State 
if only a certain area or areas be chosen. 
