CHAPTER XIII. 



BACTERIOLOGIC EXAMINATION OF SOIL. 



ALMOST all soil contains bacteria in its upper layers. 

 Their number and character, however, depend some- 

 what upon the surrounding conditions. Near the hab- 

 itations of men, where the soil is cultivated, the ex- 

 crement of animals, largely made up of bacteria, is 

 spread upon it to increase its fertility, this being a treat- 

 ment which not only adds new bacteria to those already 

 present, but also enables those present to grow very much 

 more luxuriantly because of the increased 

 amount of organic matter they receive. 



The researches of Fliigge, C. Frankel, 

 and others show that the bacteria of the 

 soil do not penetrate very deeply that 

 they gradually decrease in number until 

 the depth of a meter is reached, then 

 rapidly diminish until at a meter and a 

 quarter they rather abruptly cease to be 

 found. 



Many of the soil-bacteria are anaerobic, 

 and for a careful consideration of them 

 the reader must be referred to monographs 

 upon the subject. The estimation of their 

 number seems to be devoid of any dis- 

 tinct practical importance. C. Frankel 

 has, however, originated a very accurate 

 method of determining it. By means 

 of a special boring apparatus (Fig. 43) 

 earth can be secured from any depth without digging and 

 without danger of mixing that secured with that of the 

 superficial strata. With sterile liquefied gelatin a definite 



147 



FIG. 43. Fran- 

 kel's instrument for 

 obtaining earth from 

 various depths for 

 bacteriologic study. 



