20 
Colorado Experiment Station 
very small quantities, we consider the water polluted. It is said that 
natural waters can obtain from sources other than animal matter only 
from one-tenth to two-tenths of a grain per imperial gallon, which 
is io pounds. Taking the higher figure, this would be a little less 
than 50 pounds of sodic nitrate in an acre-foot of water. This amount 
is so good as without influence, but it must come from somewhere. 
It is not stated where it does come from, but it seems to be taken for 
granted that it is formed from the vegetable matter that is de¬ 
composed on the surface or near the surface of the rocks, and soil. If 
there are any more nitrates than this, students of the subject consider 
that the nitrogen in the nitrates originally came from animal excre¬ 
ments. Water from deep wells contains at the very most 30 parts per 
million of nitrates and nitrite together. This would make 81 pounds to 
an acre-foot. This amount would do good and not harm to any crop 
to which it might be applied if it had any effect at all. We have 
never found in any sample of irrigating water more than one-tenth 
of this amount and the largest amount that we have found in return 
waters was in one taken from 1 the Arkansas river at Rocky Ford which 
was one-fifth of the amount here given for deep wells. In the case 
of these deep wells, the nitrates are supposed to have their origin near 
the surface. Nitrates in general are supposed to come from one of 
three sources: They may be washed out of the atmosphere where 
they are formed by electric discharges, or they may be formed as the 
end product of the decomposition of vegetable or animal matter con¬ 
taining nitrogen. The very big quantities of sodic nitrate found in 
Chile and Peru are thought by some to have been formed by the de¬ 
composition of immense masses of sea weeds, which would give them 
a vegetable origin: others have claimed that they were formed by the 
decomposition of dung. This, of course, would give them an animal 
origin. These are the principal suggestions that have been made to 
account for their formation. 
NITRATES NECESSARY TO GROWTH OF PLANTS 
These nitrates constitute the most expensive fertilizers that we 
have and are necessary to the growth of most plants. It is possible 
that plants that grow in stagnant water, for instance, may take up' 
their nitrogen in the form of ammonia compounds, but most plants 
get their nitrogen from the nitrates of the soil. While nitrogen is an 
absolute necessity to the growth of plants without any exception, and 
these nitrates are the most important source of their nitrogen, too 
much of these nitrates will kill the plants. Ordinarily the amount of 
these nitrates in the soil is very small, less than 48 parts per million, 
or less than 200 pounds in the top acre-foot at any one time. The 
crop, as it grows, uses this up* but it is being constantly replaced, not 
so fast, however, as the crop uses it, so that at harvest time the amount 
