32 
Colorado Experiment Station. 
One step toward the proof of this lay in the demonstration of the 
relation of the moisture content of the soil to the fixation of nitrogen. 
To this end, six deep culture dishes were prepared with varying 
amounts of sterile distilled water each containing sufficient to give ioo 
grams of air dried soil with a moisture content of 2.1 per cent the 
following degrees of moisture: 2.1, 10, 20, 25, 30, and 48 per cent 
respectively. 100 grams of air dried soil which was known to possess 
nitrogen fixing powers in situ were added to each dish. It wili be 
noted that the first dish contained the air dried soil only, whie the 
last, with 48 per cent water, was saturated. The weight of each 
dish and its contents was determined, and every day the loss of water 
by evaporation was restored with sterile distilled water. The soils 
were all kept in the incubator at a temperature of 28°C. to 30°C. for 
thirty days at the end of which time the total nitrogen was determined 
for each. The results of the experiment are given in Table No. 6. 
TABLE No. 6. Relation of Soil Moisture to Nitrogen Fixation in Soil. 
Per cent moisture 
Milligrams nitrogen per 100 g. 
soil 
Milligrams nitro¬ 
gen fixed per 100 
g. soil in 30 days 
At beginning 
After 30 days 
2.1 
73.89 
78.84 
4.95 
10.0 
73.89 
82.11 
8.22 
20.0 
73.89 
80.90 
7.01 
25.0 
73.89 
79.13 
5.24 
30.0 
73.89 
78.49 
4.60 
48.0 
73.89 
73.85 
• • • • 
The experiment indicates that the optimum moisture content for 
maximum fixation lies between 10 per cent and 20 per cent; that the 
amount of fixation gradually decreases as the saturation point of the 
soil is approached at which it is zero. These results are in perfect 
harmony with our field observations which have pointed clearly to the 
detrimental effect of excessive moisture both on the production of 
brown color and the formation of high nitrates. 
THE RELATION OF NITRATES AND AZOTOBACTER CHRO- 
OCOCCUM TO THE BROWN COLOR. 
The continual occurrence of the brown color on high nitre soils, 
which have been shown to possess nitrogen fixing power, is too con¬ 
stant an association to be regarded as a mere accident or coincidence. 
This relation needs no further exposition since it has been referred 
to repeatedly in the preceding pages, but before entering into any dis¬ 
cussion of the subject, it should be understood clearly and emphatic¬ 
ally that we have no intention of appealing to the nitrates or the nitro¬ 
gen fixing flora of the soil to explain every brown spot or similar dis¬ 
coloration that may be found. There are at least two other recognized 
agents that may be responsible for a similar condition. I refer to the 
