162 SOILS AND MANURES 



The nitrogen from all these sources undergoes nitrifica- 

 tion more or less quickly according to the nature of the 

 compounds, but the nitrates are quickly taken up by plants 

 and converted into organic matter. The bulk of the nitro- 

 gen is, therefore, present in the soil in an inactive form. 

 It gradually becomes available as the organic matter 

 decays, but the proportion present, in an available state, 

 at any time is comparatively small. In this respect it is 

 in exactly the same position as the other elements of plant 

 food. 



But there is another reason why nitrates do not accumu- 

 late in the soil. They are all very soluble in water, and are 

 peculiarly liable to loss by drainage. The loss can be 

 largely diminished by growing " catch crops " in the 

 autumn. The nitrates formed during the summer are 

 picked up, converted into organic compounds, and so 

 preserved. The crops may be either fed off, or ploughed in 

 green, in order to restore the nitrogen to the soil. 



