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. 

 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 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 

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