266 NITRITE- AND NITRATE-BACTERIA 



for their supply of nitrogen, would, owing to their ready solu- 

 bility, soon become depleted, were it not that they are continually 

 being reinforced by the action of Bacteria. The decay of plant 

 and animal bodies, which is likewise due to bacterial agency, 

 leads to the production of a large number of waste products, 

 of which one of the most important is ammonia. The latter 

 combines with the calcium carbonate to form ammonium car- 

 bonate, and this is oxidised to a nitrite by the so-called Nitrite- 

 Bacteria which belong to the genus Nitrosomonas. The nitrites 

 in their turn are converted into nitrates by the Nitrate-Bacteria 

 (Nitrobacter), and in this way the ammonia, liberated by the 

 decay of dead organisms, again becomes available to living plants. 



By means of these oxidative processes the Bacteria in ques- 

 tion gain the energy necessary for their vital activities. It has 

 been found possible to cultivate them only in the absence of 

 organic matter, but in nature the presence of the latter in the 

 soil seems even to be beneficial. Nevertheless these organisms 

 appear to be capable of building up organic substance from 

 simple compounds, utilising for this purpose the energy obtained 

 in the oxidative processes which they carry on. These nitri- 

 fying Bacteria cannot flourish in acid soils, or in such as contain 

 an appreciable amount of free ammonia. This probably explains 

 their paucity in many soils which are rich in humus (e.g. moor- 

 lands) or poor in lime. Where the decay of organic matter takes 

 place on a large scale, as in the guano-fields of Chile, so large a 

 quantity of nitrates may be formed that they accumulate as an 

 efflorescence on the surface of the soil. This is the mode of 

 origin of Chile saltpetre (potassium nitrate). 



The beneficial action of these organisms is to some extent 

 countered by the breaking down of nitrates with the evolution 

 of nitrogen by such Bacteria as Bacterium denitrificans (denitrifi- 

 catiori). There are, however, others (the nitrogen- fixing Bacteria), 

 which actually have the power of fixing the free nitrogen of the 

 air, with the formation of organic nitrogen compounds, although 

 the chemical processes involved are obscure. The most impor- 

 tant of the organisms concerned is Azotobacter (Fig. 145, d), an 

 aerobic form which obtains its energy by the breakdown of 

 carbohydrates, a process setting free a considerable amount of 

 carbon dioxide, The amount of nitrogen fixed is proportional 



