BACTERIA IN NATURAL PROCESSES 67 



may have been reduced to free nitrogen (N), to ammonia (NH 3 ), 

 or to nitrites (-NO 2 ), must be built up into nitrates (-NO 3 ) before 

 the plants can use them. 



Here it is that other species of bacteria E intervene, and this 

 time their work is one of construction rather than destruction. 

 In the soil everywhere there exists a class of bacteria, known as 

 nitrifying bacteria, which have the power of uniting oxygen with 

 these simple compounds. There are apparently two distinct 

 steps in the process. In the first stage ammonia is oxidized to 

 nitrous acid or to nitrites by the nitrosobacteria or nitrosomonas. 

 These nitrites are somewhat unstable and are quickly oxidized 

 into nitrates by still another species known as nitrobacter. Whether 

 in the form of nitric acid or nitrates the nitrogen is now ready to 

 be absorbed by the roots of the plant and to start once more on 

 its journey around the food cycle. Thus plants by a constructive 

 process form the connecting link between the soil and animal life, 

 and bacteria, by a reducing process sometimes followed by one 

 of synthesis, complete the cycle of returning to the soil again the 

 substances originally derived from it. 



The nitrogen cycle is not quite complete at this point. When- 

 ever putrefaction takes place some of the nitrogen escapes into the 

 air as gas and is dissipated. Apparently this portion has escaped 

 from the cycle since plants cannot extract free nitrogen from the 

 air. There are other sources of apparent loss also, such as the 

 gradual draining of the soil into streams, bodies of plants and 

 animals that fall into rivers and are carried to sea, the sewage 

 disposal of cities which often discharge vast quantities of nitrog- 

 enous material into the great lakes or the sea, the use of nitrog- 

 enous compounds as explosives. It would seem in this way that 

 large amounts of nitrogen must be irretrievably lost from the 

 cycle. Fortunately, however, other bacteria are unceasingly 

 at work to redeem this supply which has apparently flown off at 

 a tangent. It has been found that soil entirely free from plants, 

 but containing certain species of bacteria, will slowly but surely 

 gain in the amount of nitrogen that it contains. That the com- 

 pounds are manufactured by bacteria is certain because they do 



