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 bac- 

 teria, 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 

 nourishment 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, dystentery, 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 Fliigge, C. Frankel, and others show that the 

 bacteria of the soil do not penetrate deeply, but gradually decrease 

 in number until the depth of a mieter is reached, then rapidly di- 

 minish until at a meter and a quarter they rather abruptly disappear. 



The bacteria of soil are, for the most part, harmless saprophytes, 

 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 aborbed, or whether 

 they remain 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, origi- 

 nated an accurate method of determining it. By means of a special 

 boring apparatus earth can be secured from any depth without 

 digging and without danger of mixing with that of the superficial 

 strata. A measured quantity of the secured soil is thoroughly 

 mixed with liquefied sterile gelatin and poured into a Petri dish or 

 solidified upon the walls of an Esmarch tube. The colonies are 

 counted with the aid of a lens. Fliigge found in virgin earth about 

 100,000 colonies in a cubic centimeter. 



Samples of earth, like samples of water, should be examined as 

 soon as possible after being secured, for, as Giinther points out, the 

 number of bacteria changes because of the unusual dryness, warmth, 

 exposure to oxygen, etc. 



The most important bacteria of the soil are those of tetanus and 



243 



