CHAPTER XIV. 

 BACTERIOLOGY OF THE SOIL. 



THE upper layers of the soil contain bacteria in proportion 

 to their richness in organic matter. Near the habitations of 

 men, where the soil is cultivated, the excrement of animals, 

 largely made up of bacteria, is spread upon it to increase its 

 fertility, this treatment not only adding new bacteria to 

 those already present, but also enabling those present to 

 grow much more luxuriantly because of the increased nour- 

 ishment they receive. 



Where, as in Japan, human excrement is used to fertilize 

 the soil, or as in India, it is carelessly deposited upon the 

 ground, bacteria of cholera, dysentery, and typhoid fever 

 are apt to become disseminated by fresh vegetables, or 

 through water into which the soil drains. In such localities 

 fresh vegetables should not be eaten, and water for drinking 

 should be boiled. 



The researches of Fltigge, C. Frankel, and others show that 

 the bacteria of the soil do not penetrate deeply, but gradu- 

 ally decrease in number until the depth of a meter is reached, 

 then rapidly diminish until at a meter and a quarter they 

 rather abruptly disappear. 



The bacteria of soil are, for the most part, harmless sapro- 

 phytes, though a few highly pathogenic organisms, such as 

 the bacilli of tetanus and malignant edema, occur. Many 

 of them are anaerobic, and it is interesting to speculate upon 

 their biology. Whether they develop and multiply in the 

 soil in intimate association with strongly aerobic organisms 

 by which the free oxygen is absorbed, or whether they re- 

 main latent in the soil and develop only in the intestines of 

 animals, is not known. 



The estimation of the number of bacteria in the soil seems 

 to be devoid of any practical importance. C. Frankel has, 

 however, originated an accurate method of determining it. 

 By means of a special boring apparatus (Fig. 75) earth can 

 be secured from any depth without digging and without 

 danger of mixing with that of the superficial strata. A 



260 



