Oct. 9, 1916 
Nitrifying Powers of Humid and Arid Soils 
49 
incubated for one month at 28° to 30° C. The usual devices were em¬ 
ployed for preventing an undue evaporation of moisture, the mainte¬ 
nance of a uniform water supply, and for mixing soil with fertilizer 
materials. The soil nitrogen was, of course, employed as naturally occur¬ 
ring in the 100-gm. soil portions, sulphate of ammonia (in solution) was 
employed at the rate of 0.2 per cent, dried blood at the rate of 1 per 
cent, and cottonseed meal at the rate of 1 per cent, all based on the air- 
dry weight of the soil. The phenoldisulphonic-acid method for deter¬ 
mining nitrates, as described by Lipman and Sharp (8), was employed 
throughout the experiments, except as otherwise stated. The nitrate 
content of the original soil was subtracted in all cases, and calculations 
were made of the absolute amounts of nitrates produced, of the total 
nitrogen present (whether soil nitrogen alone or soil nitrogen plus fer¬ 
tilizer nitrogen), and of the percentage of the latter which was trans¬ 
formed in a month’s incubation period into nitrates. For the purposes 
of the last-named determination only the complete whole numbers for 
the percentage concerned were computed, a plus sign being used after 
every one to indicate that the exact percentage was less than 1 per cent 
in excess of the number given. 
Throughout these experiments the writers have been cognizant of 
the weaknesses which obtain in any method yet devised to obtain results 
in the laboratory with soil-bacterial activities which are directly trans¬ 
latable into terms of field conditions and magnitudes. For example, 
the fact that uniform moisture and such exceptional air conditions as 
are present in a constant-temperature incubator are not to be found in 
the field has not been overlooked. Nor yet have the writers assumed 
that the large amounts of fertilizer employed by them in the experiments 
exercise the same effect as the much smaller quantities employed in farm 
practice. There has been employed as a basis what seems to be the 
reasonable hypothesis that soils and fertilizers, particularly the former, 
bearing a certain relationship to one another as regards nitrifying powet 
in the laboratory, should bear approximately the same relationship to one 
another in the field. This should be particularly so as regards results 
obtained with the soil’s own nitrogen. 
Further, the writers also realize two other serious difficulties which 
beset the investigator engaged on problems such as the one in hand. 
Seasonal variations in the nitrifying powers of soils are of great magni¬ 
tude. This has been demonstrated by numerous investigators. Not the 
least disquieting data on that subject are in the hands of the writers, 
representing the most extensive study yet carried out on seasonal varia¬ 
tions in the ammonifying and nitrifying powers of soils. The other 
difficulty is that the soils which are compared with arid soils were 
collected in most of the States on Experiment Station lands, which 
are not truly representative of average conditions obtaining among 
