EXTENT OF NITRIFICATION 71 
the necessary oxygen for a vigorous nitrification, thus furnishing 
the crops with a larger supply of the easily assimilated nitrates. 
In this fact, too, we find a partial explanation of the fact that only 
the upper layers of the soil are fertile since nitrification will go on 
only in the layers where oxygen readily penetrates. About 65 
per cent. of the total nitrification occurs in the uppei twelve inches 
of soil, 30 per cent. more in the layers from twelve to thirty-six 
inches lower, and little or none below this. Surface soils alone are 
thus highly fertile. 
Crop.—Some crops stimulate nitrification, while others may 
even depress it. Among the crops which stimulate it, corn seems 
to be the most important. It has been observed not only that 
more nitrates are present in soil upon which corn has been grow- 
ing than upon fallow soil, but also that nitrates are most abundant 
under corn while it is making its most active growth. The fact 
may be partly explained by the ability corn seems to have of using 
nitrogen in other forms besides nitrates, but this cannot be the 
whole explanation. Evidently in some unknown manner the 
nitrifiers are affected by the kind of crop growing on the soil. 
EXTENT OF NITRIFICATION 
The production of nitrates in ordinary soil is very vigorous. 
While in some soils it does not occur at all, from a lack of some of 
the conditions already mentioned, in other soils nitrates are rapidly 
formed. In fact, a much larger amount of nitrates is produced in a 
cultivated soil, ordinarily, than is used by the crops. In some 
careful tests it has been shown that twice as much nitrate is formed 
as is used by the crop, the rest being lost to the soil by drainage. 
To furnish this amount of nitrate a proper amount of organic 
nitrogen must be added in the form of manure or otherwise. 
With plenty of such material as a source, nitrification is very 
vigorous during all seasons, except when the soil is actually frozen. 
The Unlocking of Soil Nitrogen.—It happens not infrequently 
that soil may contain large amounts of nitrogen and yet fail to 
