The) Fixation of Nitrogen in Colorado Soils 41 
The only condition that seems indispensible to their development is a 
constant and fanly abundant supply of moisture and a chemically 
alkaline soil. We have recorded samples taken from a cultivated 
field on the same date which showed the presence of from 5.0 to 
150 p. p. m., and again samples taken from the same field on dif¬ 
ferent dates which gave from 65 to 100 and even 333 p. p. m. of 
nitric nitrogen. 
