2 52 ORGANIC AGRICULTURAL CHEMISTRY 



These reactions are the basis of new and important commercial 

 processes for the conversion of atmospheric nitrogen into com- 

 pounds useful as soil food for plants. In the Birkelund-Eyde 

 process, as carried out in Norway, calcium nitrate is thus made 

 and used as a fertilizer. Such fixation of atmospheric nitrogen 

 is not only a commercial process, but it is probably a means by 

 which soil nitrogen is continually supplied. 



(3) By the Action of Nodule Bacteria. — As just recently 

 discussed, bacterial nodules occurring on leguminous plants 

 fix atmospheric nitrogen in the plant and indirectly in the soil, 

 probably in the form of protein compounds. In the total this 

 method is possible of fixing large amounts of free nitrogen and 

 is continually going on at present. 



It is possible that in one or perhaps all of these three ways 

 atmospheric nitrogen originally entered the soil and became food 

 for plants. The last two methods are constantly going on in 

 nature under present conditions, tending to preserve nitrogen 

 equilibrium. Whatever the original process was, it is true that 

 in the soil there is present a large amount of nitrogen in the form 

 of nitrate salts, ammonium salts or amino compounds, and this 

 nitrogen is the source of all protein nitrogen in plants, except as 

 stated in connection with the nodule bacteria. By means of 

 this nitrogen, together with the products of photosynthesis, 

 the plant is able to metabolize protein compounds which serve 

 as its own food supply or as reserve food. This reserve protein 

 food, intended for the plant's offspring, becomes, however, 

 animal food and in herbivorous animals is the sole source of 

 the nitrogen for animal protein. In the metabolic processes 

 of animals this plant protein nitrogen, built up into animal 

 protein, is again torn down to furnish energy to the animal. 

 The nitrogen products of protein katabolism in animals are 

 urea and the other nitrogen compounds of the urine. In the 

 form of these compounds in animal manure the original nitrogen 

 again reaches the soil. Here they are decomposed by bacterial 

 action, yielding ammonia, and this by similar action is converted 

 into nitric acid and the cycle of the nitrogen is completed. 



