136 MANUAL OF BACTERIOLOGY. 



bacteria have similar properties to some extent. The work 

 done by nitrifying bacteria in the formation of nitrates from 

 sewage, manure and the like is indispensable to most plant 

 life. Certain bacteria found in the soil are also concerned in 

 the assimilation of free, atmosphere 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 contain- 

 ing 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 suffer 

 gradual exhaustion. Furthermore, it has already been noticed 

 (page 131) that one of the products of decomposition by bac- 

 teria is nitrogen, which is not available as food for animals or 

 for most plants. These facts have met with practical recogni- 

 tion by agriculturists in the adoption 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 which are caused by the development of microor- 

 ganisms related to the bacteria. These organisms appear to 

 have the power of assimilating atmospheric nitrogen and of 

 converting it into nitrogen compounds. The same property 

 probably belongs to some other bacteria of the soil. Experi- 

 ments show that these observations may be destined to be of 

 great value to the farmer.* 



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-cultures 

 may be made from sterilized water with which a portion of the 

 soil has been properly mixed. Anaerobic bacteria must be 

 cultivated by the special methods adapted to them. 



*For practical application to agriculture consult G. Moore. U. S. Dept. 

 Agriculture. Bureau of Plant Industry, Bulletin No. 71, Jan. 23, 1905. See 

 also various Bulletins of the Department of Agriculture on this subject. 



