246 AGEICULTUEAL AND INDUSTEIAL BACTEEIOLOGT 



oxidized into nitrates by bacteria belonging to the genus 

 Nitrobacter. These are small, nonmotile rods present in 

 most arable and fertile soils. Like Nitrosomonas they do 

 not require the presence of organic material for their 

 development. They are likewise prototrophic and secure 

 their growth energy by. the oxidation of the nitrites to 

 nitrates in accordance with the following reaction : 



2HN"0, + 0^ = 2HNO3 



Nitrates, therefore, tend to accumulate in the soil. Under 

 certain conditions they may be leached out and concentrated 

 in other places. It is claimed, for example, that the great 

 nitrate deposits of certain arid regions, as in Chile, are due 

 to the concentration brought about in this way. Nitrates 

 are easily taken up by the roots of most higher plants. 

 Consequently it is in this form that nitrogen is assimilated 

 by most growing crops. 



Nitrobacter may be readily cultivated in the laboratory 

 by preparing a solution containing potassium nitrite, car- 

 bonates, phosphates and other salts necessary for growth, 

 but containing no organic material. When such a solution 

 is inoculated with a bit of soil a clouding will soon develop 

 and the nitrites will be found to be changed to nitrates. The 

 oxidation may be materially increased by suitable aeration. 

 The ability of a particular soil to transform nitrites into 

 nitrates may be tested by a manner similar to that already 

 noted for ammonification, but in general it is not differen- 

 . tiated from the process of nitrification as a whole. 



It has already been noted that the process of nitrification, 

 or oxidation of ammonia, starts with the ammonia and ends 

 with nitrates. The ability of soil to bring about this entire 

 change of nitrification may be studied by adding ammonium 

 sulphate or some other compound containing available 

 ammonia to a suitable soil sample in the laboratory, adding 

 the optimum amount of water and analyzing after a suit- 



