Crop Rotation 285 
of corn and oats, or one under a two-year rotation in which clover and 
cowpeas were used as a green manure. 
3. There was an indication that the influence of the crop present 
on the soil was greater than the influence of the previous cropping on 
the bacteria in the soil, and this was probably due to the difference not 
only in the crop but in the treatment of the soil and preparation for 
the crop.” 
TECHNIC. 
The object of this investigation was to determine the effect of 
different crops and crop rotations on the nitrate content and nitrifying 
power of the soil. 
To accomplish this end samples were taken monthly from each plot 
over a period from March, 1920, to October, 1920, inclusive, and these 
samples were analyzed for moisture, nitrates and nitrifying power. 
Samples were taken at as near the same date each month as cir- 
cumstances would permit. In order to eliminate as nearly as possible 
the factor of denitrification, no samples were taken when the soil was 
wet enough to “ball up”. 
Samples were evenly distributed over the plot as shown by dots on 
the accompanying map, and were taken from about the same places each 
month. In order to eliminate the factor of fertilizer, the samples 
were taken from the check plots of each series. (The check plots on 
the south side of Series V West were used because those on the north 
side were considerably lower than those of the other series.) 
Moisture determinations were made by drying in an oven at 100° C 
for several hours, duplicate samples of 10 gms. soil each in 30 c.c. 
tarred crucibles. 
The nitrate content was determined by the phenol-di-sulphonic 
method, as modified by Noyes’ for soil investigation work. 
To determine the nitrifying power of the soil, duplicate samples of 
100 gms. each were placed in glass tumblers covered with petri plates, 
and incubated at room temperature for two weeks. In any instance 
where the moisture content was noticeably lower than normal, which 
ran about 20 per cent, distilled water was added to bring it back to 
normal. At the end of two weeks nitrate determinations were made as 
in the case of the fresh samples. 
In tabulating the data the averages of the duplicates were used, 
and the nitrate content calculated on the basis of moisture-free soil and 
in terms of parts per million parts of water-free soil. 
DISCUSSION OF RESULTS. 
Before entering into a discussion of the tables it would be well to 
note that the factors affecting bacterial activities and hence nitrate 
formation in the soils as determined by previous investigators are, 
moisture, temperature, aeration, crop growth, organic matter and plant 
food content of the soil, presence of toxic substances, cultivation, pre- 
vious cropping and treatment, and type of soil. Of these many factors, 
those which may be taken as different in the different plots are aera- 
tion, kind of crop grown, cultivation, previous crop and treatment, and 
