Oct, 9, 1916 
Nitrifying Powers of Humid and Arid Soils 
81 
SUMMARY 
A study was made of the nitrifying powers, under incubator conditions, 
of about 40 humid and about 150 arid soils. The soil was used as a 
medium and the forms of nitrogen employed were soil nitrogen, sulphate 
of ammonia plus soil nitrogen, dried blood plus soil nitrogen, and cot¬ 
tonseed meal plus soil nitrogen. The humid soils were obtained from 
the different State and Territorial Experiment Stations, one type of 
surface soil from each one being used in these experiments. The arid 
soils employed represented the soil types of four typical soil-survey 
areas in California. The results obtained appear to justify the follow- 
lowing statements: 
(1) The conclusion appears ineluctable that the nitrifying powers of 
soils of the arid region are no more intense than those of the humid 
region. This denies Hilgard's (3) teaching to the contrary and confirms 
the statement of Stewart (11), which was based on more indirect and 
less extensive evidence. 
(2) While indications are not by any means positive now, it is possible 
that the data of the writers justify the further conclusion that the 
nitrifying powers of humid soils are greater than those of arid soils. 
If such a conclusion could be drawn, it would have to be based merely 
on the nitrification of soil nitrogen and dried-blood nitrogen. The 
former being the natural source of supply of most of the available 
nitrogen obtained by crops, it should really be regarded as the most 
valuable basis for forming a judgment. 
(3) Arid soils, on the other hand, nitrify the nitrogen of sulphate of 
ammonia and cottonseed meal with much greater vigor than do the 
humid soils. A reversal of efficiency is manifest between the two groups 
of soils as regards sulphate of ammonia and cottonseed meal, on the one 
hand, and dried blood and soil nitrogen, on the other. 
(4) These results not only throw new light on the question of nitrifica¬ 
tion in soils of different climatic regions but also tend to confirm the 
earlier findings of the writers and others on the important relation of 
climate to soil properties (9). 
(5) The foregoing considerations apply to the two groups in general 
and not to parts of such groups in particular. For example, the Bay 
and Ukiah soils do not nitrify soil nitrogen as efficiently as the more arid 
Pasadena and Riverside soils. This may, however, be accounted for by 
the seasons at which the collection of the soils was made and by the 
physical condition in which the Bay and Ukiah soils were received. 
