THE NITRIFYING EFFICIENCY OF CERTAIN COLORADO SOILS 
9 
was not collected according to the usual methods employed by soil 
bacteriologists, I may say that no one recognizes the value of using 
standard methods, wherever possible, more than we do, and we have 
the greatest respect for the investigator who holds firmly to this position. 
However, for our purpose, I believe that our soils were just as nearlv 
representative of their respective localities as if they had been taken 
by an experienced person under aseptic conditions. 
In Colorado where we could give our personal supervision to col¬ 
lecting the samples, we have exercised every precaution to prevent 
exterior contamination. All of the surface litter was removed before 
opening up the ground. Except where otherwise stated in the de¬ 
scriptions which follow, the samples were taken to a depth of six 
inches, i. e., they included the surface six inches; the soil was re¬ 
moved with sterilized spatulas and placed immediately in double, 
sterilized, paper sugar sacks; each sample contained approximately 
five pounds of the moist soil. All of the samoles were shipped 
by express, as soon after collection as possible, to the bacteriological 
laboratory of the Experiment Station at Fort Collins in order to 
minimize the time in transit, during which, if unduly prolonged, 
the soil flora might undergo changes. This statement is deemed 
necessary since many of the samples were taken more than five 
hundred miles from Fort Collins. As a result of our nitrogen 
fixing and ammonifying studies, we have made it a point in col¬ 
lecting samples where the brown color is present, to avoid getting 
this portion of the soil since experience has taught us that the nitrates 
are apt to be so high that they interfere with the normal develop¬ 
ment of the microorganisms present. 
Preparation of Samples .—From this point, both the Colorado 
and the foreign soils were handled alike. As soon as they reached 
the laboratory, each was emptied on a sheet of heavy, sterilized manilla 
paper and thoroughly mixed. It was next divided into two unequal 
portions, the larger being spread out in the air to dry in diffused light, 
while the smaller was transferred in its original moist condition to a 
sterilized Mason fruit jar and sealed. As soon as air dry, which seldom 
requires more than twenty-four hours in our atmosphere, each soil 
was again mixed and pulverized in a glass mortar. Both the mortar 
and pestle were sterilized carefully between each two different sam¬ 
ples with a 5 per cent, solution of lysol. They were subsequently rinsed 
with hot, boiled water and allowed to dry before the next sample was 
treated. The soils were next passed through a thirty mesh wire sieve 
which was washed between the different samples and sterilized by dry 
heat. 
As containers for the prepared soil, we have used the same style 
of deep culture dish as in our previous experiments. These are similar 
in shape to the ordinary Petri dish except that they are deeper, meas¬ 
uring 10 cm. in diameter and 4 cm. in depth. 
100 grams of soil, prepared as described above, were used for each 
dish. The ammonium sulphate, ammonium chlorid and ammonium car- 
