BACTERIA IN THE SOIL 



117 



rPooO 



water derived directly from flowing streams and rivers is 

 most likely to contain these organisms in greatest num- 

 bers. The most dangerous water for drinking pur- 

 poses is that of rivers which have been contaminated in 

 any way by sewage material, a condition of things true of 

 the water used in some cities. 



Soil. Soil on the surface of the eartlj is usually filled 

 with bacteria (Fig. 56). They are usually abundant in 

 the superficial layers, decreasing 

 rapidly as we pass below the sur- 

 face, until at the depth of a com- 

 paratively few feet they practi- 

 cally disappear. They are more 

 abundant in some kinds of soil 

 than in others. Where the soil 

 is dry and sandy the number is 

 comparatively small; where it is 

 moist and loamy they are more 

 abundant. They are found pro- 

 fusely around buried bodies of 

 animals, or in soil that contains 

 decaying roots of ordinary plants. 

 They are immensely numerous 

 in the vicinity of earth closets 

 or privies, and the soil near sink drains and manure heaps 

 is filled with them. Indeed, any soil which contains the 

 remains of animal or vegetable matter and a considerable 

 amount of moisture will have bacteria in inconceivable num- 

 bers, while in cleaner soils they will be much less abundant. 



They are sure to be abundant in all dirt which accu- 

 mulates in a household, for nearly all such dirt contains 



Fig. 56. A group of soil 

 bacteria. 



u, a., nitrate organism ; b, b. nitrite 

 organism ; c, B. graveoleus ; d, B. 

 fusifortnis ; e, B. stibtilis ; _/*, Clos- 

 terium pasieuriana. 



