92 The Colorado Experiment Station. 
optimum amount is probably below rather than materially above 
this percentage. The determination of the optimum amount or 
moisture under specific and varied conditions would be interesting 
but is a detail not included in the general features which I am 
endeavoring to present. We have the maximum nitrification in 
the sample containing 20 percent of water at the end of the experi¬ 
ment. My judgment is that this amount had scarcely varied at all 
during the experiment. On the other hand we have the maximum 
fixation in the two samples containing 13.5 and 17.5 percent re¬ 
spectively. These results may be accidental but they agree well 
with my observations in the field. The amount of fixation, how¬ 
ever, is quite uniform and the addition of water, at least up to 
17.5 percent has clearly promoted it. 
The chemical composition of this soil is fairly represented by 
our Laboratory Nos. 724 and 725, representing the soil and sub¬ 
soil from a portion of this same tract. These analyses were given 
in Bulletin 155, p. 36. 
ANALYSES 
XCII 
XCIII 
Soil 
Subsoil 
laboratory 
laboratory 
No. 724 
No. 725 
Percent 
Percent 
Insoluble . 
. 54.653 
57.068 
Silicic acid . 
. 19.805 
12.754 
Sulfuric acid . 
. 0.047 
0.049 
Chlorin . 
. 0.032 
0.059 
Phosphoric acid . 
. 0.120 
0.127 
Carbonic acid . 
. 3.048 
6.312 
Lime .. 
. 6.100 
8.465 
Magnesia . 
. 1.355 
1.448 
Sodic oxid . 
. 0.290 
0.432 
Potassic oxid . 
. 0.872 
0.742 
Ferric oxid . 
. 5.601 
3.499 
Aluminic oxid . 
. 3.738 
5.397 
Manganic oxid . 
. 0.118 
0.026 
Ignition . 
. 5.072 
3.887 
Sum . 
100.265 
Oxygen equiv. to chlorin. 
. 0.007 
0.013 
Total . 
100.252 
Total nitrogen . 
. 0.147 
0.069 
Humus . 
While these analyses are not made on the soil actually used 
they serve to represent its composition as well as a sample taken 
in any fair-sized field may represent its soil. 
The amount of nitrogen fixed is very large and seems incred¬ 
ible. Taking 10.54 milligrams as the quantity fixed in 100 grams 
of soil in twenty-seven days and calculating the amount that would 
be fixed in the top two inches of the soil per acre, we have 937 
pounds, or for an acre-foot 5,622 pounds, equivalent to about 17.5 
tons of proteids (N x 6.25) per acre-foot, a quantity which seems 
extremely large, but this is the result actually obtained. 
