SOIL AND SEWAGE 6/ 



The bacteriological examination of soil. 



Soil may be required to be bacteriologically examined for 

 three purposes. 



A. To isolate and study the organisms of nitrification and 

 allied processes. 



B. To detect the presence of special pathogenic bacteria. 



C. To study the degree of pollution with excretal organisms, 

 in relation to water supplies. 



A. The organisms concerned with nitrification are present 

 in all soils, and their functions are of vital importance. They 

 are not readily isolated, while their study is chiefly of value for 

 research purposes. Methods of isolation are not therefore 

 included here. 



B. The pathogenic bacteria which have to be looked for 

 are those enumerated above, and particularly the bacilli of 

 tetanus, malignant oedema, and occasionally B. typhosus and B. 

 anthracis. The methods for their examination are in no way 

 special and are conducted on ordinary lines. No detailed con- 

 sideration is therefore necessary here. 



C. General Soil pollution. The data which give the most 

 valuable information with regard to recent or remote soil 

 pollution are the following : the total number of aerobic 

 organisms, number of spores present, number of B. coli, 

 B. enteritidis sporogenes, and streptococci. 



In collecting soil for bacteriological examination the depth 

 from which it is obtained is of fundamental importance. If the 

 surface soil is to be examined, scrape up with a sterile spatula, 

 and transfer to a sterile receptacle. To obtain soil from a given 

 depth either a fresh cutting must be made and the soil collected 

 at the required depth, or, preferably, some form of borer may 

 be used. For this purpose Fraenkel's borer is convenient, its 

 chief drawback being that it holds only a small quantity of soil. 



If Fraenkel's borer is used, it is advisable to collect at least 

 eight samples from spots about a foot apart, and to mix together 



5-2 



