CHAPTER 1 6 



LEGUMINOUS ROOT TUBERCLES AND THE ACCUMULATION 

 OF NITROGEN; NITROGEN-CONSUMING PLANTS 



Loss of Soil Nitrogen. It is important before considering the accumu- 

 lation of soil nitrogen, to briefly state how the soil may lose this valuable 

 chemical substance. The nitrogen in the form of ammonia gas may be 

 diffused into the atmosphere, and there is no doubt, but that a very con- 

 siderable amount of nitrogen is thus dissipated. The soluble ammonia, 

 nitrites and nitrates may be removed from the superficial layers of the 

 soil by drainage into the subsoil, where they usually find their way by 

 percolation of rain-water from above. This loss by drainage is greatest 

 during the late summer and autumn, when the process of nitrification is 

 excessive. This process of nitrification in soils is preceded by putrefac- 

 tion where the organic materials of the soil are broken down by bacteria 

 and fungi into various end products among them ammonia, which is also 

 formed by the fermentation of the urine of herbivorous animals, according 

 to the reaction: 



CO(NH 2 ) 2 + 2 H 2 = CO 3 (NH 4 )s. 



Nc less than sixty species of bacteria are said to occur in manure and sew- 

 age and a considerable number are able to cause the ammoniacal fermen- 

 tation of urine. 



Nitrification. Now an entirely different set cf organisms come into 

 play. The ammonia is converted by the activity of several species of 

 Nitrosococcus and Nitrosomonas into nitrous acid, or the corresponding 

 nitrite. The next step in the process of nitrification is the conversion of 

 nitrous acid, or nitrite, into nitric acid, or the corresponding nitrate. 

 This is accomplished by the nitrate bacteria (Nitrobacter), which con- 

 vert the nitrous acid or nitrite into nitric acid, or nitrate. 



Here, we have the explanation why Indian corn does not exhaust the 

 soil, as rapidly, as some other crops. The growth of corn extending much 

 further into the late summer and autumn, the plant acts as a catch crop 

 in the utilization of part of the nitrates formed during the active processes 

 of nitrification. 



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