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TRANSFORMATIONS OF CARBON AND NITROGKN. 91 



ha\-e such agencies at our command we can understand how 

 soils, now containing useless nitrogen, can be made fertile ; 

 and when we have learned to manipulate them we shall have 

 learned how to restore the fertility to many a barren field. 



What we wish to look for next, then, is some force that can 

 take these end products of decomposition in the soil ammonia 

 salts and the miscellaneous compounds in the humus and 

 build them up into nitrates in which form they are most easily 

 utilized by plants. This brings us to the important problem 

 of nitrification, a term which, as will be noticed, is the opposite 

 of ^nitrification. 



BUILDING UP OF NITROGENOUS COMPOUNDS, NITRIFICATION. 



The process of nitrification is in general an oxidation of the 

 nitrogen compounds of the soil, the final result of which is the 

 formation of nitric acid. The nitric acid thus formed unites 

 readily with the bases present in the soil to form nitrates which 

 are the most useful nitrogenous foods. Our starting point in the 

 consideration of nitrification is the fact that the soil very com- 

 monly contains considerable stores of nitrogen which are not 

 available for plants. Our problem is to notice how this nitro- 

 gen is unlocked from these useless compounds and brought 

 into a condition in which plants can make use of it. 



We must first notice again that ammonia salts are certainly 

 available for plants, though less so than nitrates. From this 

 it follows that the ammoniacal fermentation, which, as already 

 noticed, is the common result of the action of putrefactive 

 bacteria upon proteids, will furnish some food for plants. 

 When the ammoniacal fermentation occurs in the soil in con- 

 nection with the formation of acids which can hold the am- 

 monia in the form of salts, the phenomenon will render 

 available some of the nitrogen which was previously out of 

 the reach of plants. The ammoniacal fermentation is sure to 

 take place wherever high nitrogenous compounds in the soil 



