THE BACTERIA OF SOIL, AIR, WATER, ETC. 1 23 



in making nitrates from sewage, manure and the like is indis- 

 pensable to most plant life. Certain bacteria found in the soil 

 are also concerned in 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 indirectly into the sea by means of sewage 

 and the rivers, it will be seen that the supply of nitrogen com- 

 pounds might suffer gradual exhaustion. Furthermore, it 

 has already been noticed (page 119) that one of the products 

 of decomposition by bacteria is nitrogen, which is not available 

 as food for animals or for most plants. These facts have met 

 with practical recognition 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. The tubercles are pathological 

 growths, caused by the development of microorganisms 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. Experiments 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 simple experiments to illustrate these phenomena see Buxton. Journal 

 of Applied Microscopy. September, 1902. For practical application to agricul- 

 ture consult G. Moore. U. S. Dept. Agriculture. Bureau of Plant Industry, 

 Bulletin No. 71. Jan. 23, 1905. 



