II] THE SOIL AND PLANT FOOD 19 



into ammonia will go on in substantially the same 

 way in the soil as it would elsewhere : but there is no 

 definite evidence in proof. 



The nitrogen compounds of the soil, however, are 

 not entirely changed into nitrates: a second action 

 takes place that is not in the least degree understood. 

 When protein and other nitrogenous plant compounds 

 are decomposed by micro-organisms in presence of 

 air there frequently appears to be a large amount of 

 gaseous nitrogen given ofi*, especially when much 

 organic matter is present. This change has been 

 very little studied and indeed is commonly confused 

 with another that appears to be wholly distinct — 

 the so-called denitrification, a reduction of nitrates 

 brought about by certain bacteria in presence of 

 organic matter, but in absence of air. Whatever 

 its nature it leads to great losses in rich or heavily 

 manured soils, and is responsible for much of the 

 exhaustion of virgin soils that is now going on at an 

 appalling rate. One of the most pressing problems 

 before the agricultural chemist is to study these two 

 sets of reactions, and in particular to find out 

 whether this wasteful process cannot be suppressed, 

 so that a larger part of the nitrogen compounds 

 shall change into the useful nitrates. In modern 

 fanning nitrogenous manures are by far the most 

 expensive, and profits are cut so low that all sources 

 of loss are to be avoided as far as possible. Both 



2—2 



