I50 IMPORTANCE OF BACTERIA 



(c) Economic Importance of Bacteria. — Insignificant and simple 

 as are the forms and structures of bacteria it is safe to say 

 that no plants are so vitally related to our welfare. A small 

 number of species construct necessary food products; others form 

 certain mineral compounds; some remove the remains of ani- 

 mals and plants through putrefaction and decay; and others pro- 

 duce a form of decay called fermentation. It is also a familiar 

 fact that many diseases are due to these plants. 



Attention has already been called to certain bacteria of the first 

 group mentioned above as very important plants in keeping up 

 the fertility of the soil. These bacteria form tubercles in the 

 roots of plants belonging to the bean family where they combine 

 the free nitrogen of the air into compounds that can be absorbed 

 by the plants, page 60. There are also other very important 

 kinds of bacteria in the soil that have the power of fixing nitrogen. 

 Some of these utilize the free nitrogen of the air and it is believed 

 that they play an important part in maintaining the soil nitrates. 

 Their life history and vital processes are very imperfectly known, 

 but they probably fix the nitrogen and build up complex com- 

 pounds, very likely of a proteid nature, which are subsequently 

 decomposed and eventually converted into available nitrates. 

 Other kinds of bacteria oxidize the ammonia derived from the 

 decay of plant and animal life into nitrates. There are two 

 classes of these forms: one oxidizing the ammonia into a nitrous 

 form or nitrite, the other adding another atom of oxygen, changes 

 the nitrous compound to a nitric form or to a nitrate. These 

 two organisms work together, each doing its special work, and 

 they are so efficient that a solution of ammonia poured upon 

 soil will show no trace of the ammonia in the solution after it has 

 percolated through the soil. Fully 65 per cent of the nitrification 

 is effected in the upper twelve inches of the soil and very little 

 below three feet. It is probable that the Chilian saltpeter beds, 

 previously referred to, are the remains of inland seas where great 

 growths of seaweeds accumulated. Owing to the drying out of 

 these bodies of water and the decay of the vegetation ammonia 

 was formed which became changed to nitrates as outlined above. 

 Bacteria having the power of fixing nitrogen are called nitrifying 



