,A Study of Colorado Wheat 
41 
brought up to about the optimum quantity. Even at this minimum 
rate of fixation, the amount of nitrogen added to the soil in one 
month would be sufficient, if nitrified, to nourish several crops of 
wheat. So it is by no means necessary that even this minimum ob¬ 
served rate of fixation should be maintained throughout the year in 
order that this process should form a factor demanding our con¬ 
sideration. This amount of fixation means the addition of 192.8 
pounds of nitrogen to an acre-foot of soil in one month, equivalent 
to the addition of about 1,100 pounds of sodic nitrate. We have no¬ 
where intimated that we hold this nitrogen to be available in this 
form, but have taken, tacitly it may be, the extreme view that only 
nitric nitrogen is available to the wheat plant. 
It is* because of the importance and immediate connection of 
this latter form of nitrogen, with the growth, yield and, as I be¬ 
lieved when I began this work, with the character of the wheat pro¬ 
duced, that we have given in some detail the moisture and nitric 
nitrogen in the soil throughout the two seasons of 1913 and 1915. 
The moisture is not only necessary to the growth of the wheat plant, 
but also to the biological factors in the soil. We determined the 
moisture in the soil for the latter reason quite as much as for the 
former, and endeavored to determine the effects of irrigation upon 
the distribution of the nitrates. In 1913, we found a very great dif¬ 
ference in the amount of, and also in the distribution of, the ni¬ 
trates on the different dates, 29 April, before irrigation, and on 27 
June, thirteen days after irrigation. These differences were so big 
that the usual considerations of variability due to differences in 
samples from place to place within limited areas, are wholly ex¬ 
cluded. In the top four feet of one sample we found on 29 April, 
nitric nitrogen equivalent to 1,908 pounds of sodic nitrate; in an¬ 
other on the same date, for the same depth of soil, the equivalent of 
471 pounds; but in this latter sample we find in the 7th, 8th and 
9th foot, taken together, the equivalent of 721 pounds. On 27 June, 
after irrigation, we found for the top four feet of three different 
sections of soil, 162, 91 and 156 pounds, respectively, and below the 
depth of four feet, usually from six to eight pounds per acre-foot; 
but this amount varies somewhat; the maximum found below four 
feet in three borings taken to a depth of 12 feet was 30 pounds to 
the acre-foot, the minimum was nil. In the cropped land the nitric 
nitrogen, especially during the actively vegetative period of the 
plants, usually falls to less than an equivalent of 20 pounds of sodic 
nitrate per acre-foot of soil, except in the top three or perhaps six 
inches, where it is usually a little higher, provided the sample be not 
taken immediately after a rain. The surface portions of land cropped 
to wheat may occasionally show nitric nitrogen equivalent to as 
much as 200 pounds of sodic nitrate to the acre-foot, but in most cases 
we found from 30 to 40 pounds in such samples. In 1913, the second 
