138 



NITRIFICATION 



coccus forms I to 2 ^ in diameter. A very minute 

 variety, ,5 //, in diameter, from Java possesses an 

 extremely long flagellum. 



Associated with the nitrite organisms in the soil are the 

 nitrate bacteria, which oxidize the nitrites into nitrates 

 and are unable to attack ammonium compounds. They 

 are non-motile rod-shaped bacteria, about I ^ long, .3 

 to .4 //, broad, and are included in the genus Nitrobacter 

 (Fig. 30) ; probably several forms exist, but owing to 

 the difficulty of isolating and cultivating them little is 

 known of their morphology. Nitro- 

 bacter can be grown on nitrite agar, 

 but the colonies are minute and 

 develop very slowly. In suitable 

 media, containing a small amount 

 of sodium nitrite, the formation 

 of nitrates by Nitrobacter begins 

 in forty-eight hours or less, but 

 goes on slowly for five or six days, 

 after which oxidation proceeds 

 more rapidly until all the nitrite is completely changed 

 in twelve or fourteen days. In nature both groups of 

 organisms are associated and live together. They are 

 abundant in well and river water, and are distributed 

 through the upper layers of the soil, being most abundant 

 from 4 to 9 inches below the surface, and not usually 

 found at a greater depth than 1 8 or 20 inches. When 

 both kinds of bacteria are allowed to act upon solutions 

 of ammonium salts the latter are completely changed 

 into nitrites before tte formation of nitrates commences, 

 and nitrites can be readily recognized in the solution 

 by the usual tests. Under the conditions which prevail 

 in the soil, however, only nitrates the end products of 



FIG. 30. Nitrobacter, sp. 

 (X6oo). 



