EXTENT OF NITRIFICATION. 65 
tion occurs in the upper twelve inches of soil, 30 per cent, more in the 
layers from twelve to thirty-six inches lower, and little or none be- 
low this. Surface soils alone are thus highly fertile. 
EXTENT OF NITRIFICATION. 
The production of nitrates in ordinary soil is very vigorous. 
\Yhile 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 care- 
ful 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. This is 
particularly true when wheat is grown, wheat being an especially 
exhausting crop. To furnish this amount of nitrate a proper amount 
of organic nitrogen must be added in the form of manure or other- 
wise. 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 infre- 
quently that soil may contain large amounts of nitrogen and yet 
fail to produce good crops, the plants seeming to be insufficiently 
supplied with nitrogen in spite of its abundance. These barren 
soils will not yield good crops unless supplied with a considerable 
amount of nitrogen as a fertilizer. Upon an open hillside or a 
meadow we may find the land very poor for supporting vegetation, 
and yet its soil, when analyzed, may yield a considerable quantity 
of nitrogen. In such a soil the nitrogen is simply locked up in the 
humus in a form useless to plants. At the end of decomposition, 
a large part of the nitrogen may be held in a form not available for 
ordinary vegetation, so that plants growing in such soil will be 
nitrogen-starved, although growing in the midst of plenty of nitrogen 
compounds. Such soils might become highly fertile if some agency 
for unlocking these nitrogenous compounds could free the nitrogen 
from its stable relations, thus producing compounds of a nature to be 
assimilated by plants. A nitrification is evidently what is needed to 
make these soils productive. If a comparatively small amount of 
6 ' 
