134 MANUAL OF BACTERIOLOGY. 



work done by nitrifying bacteria in making nitrates from 

 sewage, manure and the like is indispensable to most plant 

 life. Bacteria have also been credited with the assimilation 

 of free, atmospheric nitrogen, resulting in the addition of 

 a valuable proportion of nitrogen compounds to the soil. 

 This is spoken of as nitrogen -fixation. Inasmuch as a large 

 part of the excrementitious products of animals containing 

 nitrogen are not retained in the soil, where they may be 

 employed as food by plants, but are washed directly or in- 

 directly into the sea by means of sewage and the rivers, it 

 will be seen that the supply of nitrogen compounds might 

 be in a way to suffer gradual exhaustion. Furthermore, it 

 has already been noticed (page 130) that one of the products 

 of decomposition by bacteria is nitrogen, which is not avail- 

 able to animals and most plants as food. These facts have 

 met with practical recognition by agriculturists in the adop- 

 tion of various methods of fertilizing the soil. It appears 

 that the roots of peas, beans, clover, alfalfa and some other 

 plants frequently present minute tubercles. These tubercles 

 are pathological growths, caused by the development of 

 microorganisms related to the bacteria. The organisms 

 appear to have the power of assimilating atmospheric nitro- 

 gen and of converting it into nitrogen compounds. Experi- 

 ments show that these observations may be destined to be of 

 great value to the farmer. 1 



The bacteria of the soil may easily be studied in plate- 

 cultures made from small portions of soil collected with the 

 necessary precautions to avoid contamination, or plate-cul- 

 tures may be made from sterilized water with which a por- 

 tion of the soil has been properly mixed. Anaerobic bac- 

 teria must be cultivated by the special methods adapted to 

 them. 



1 For simple experiments to illustrate these phenomena see Buxton, 

 Journal of Applied Microscopy, September, 1902. 



