SOIL AND SEWAGE 6l 



CHAPTER IV 



SOIL AND SEWAGE 

 SOIL 



Surface soil contains a vast number of organisms. With 

 increased depth the number rapidly diminishes, and below a 

 metre but few bacteria are to be found in undisturbed soil. 



The rapid diminution in the number of bacteria in soil was 

 first clearly established by Fraenkel and has since been confirmed 

 by numerous observers. The following figures will give a good 

 idea of the number of bacteria in soils. 



Houston 1 , working with soil in the grounds of Morningside 

 Asylum, Edinburgh, on a plot of land which was formerly a 

 vegetable garden, but which had lain untouched for some time, 

 found, as the result of a large number of experiments, that the 

 average number of germs in I gramme of soil was on the 

 surface about 1,688,000; at a depth of I foot, 1,100,000; 2 feet, 

 900,000; 3 feet, 174,000; 4 feet, 25,000; 5 feet, 920; and 

 6 feet, 410. These figures deal only with the numbers which 

 will develop on gelatine media, and do not give any true 

 idea of the total number of bacteria actually present, excluding, 

 as they do, for example, the vast number of nitrifying organisms. 

 They, however, illustrate the rapid decrease with increased soil 

 depth in the number of organisms which grow on gelatine media 

 under aerobic conditions. In Fraenkel's researches anaerobic 

 bacteria were also found to be absent, or relatively absent, in 

 the deeper layers. 



The number also varies with the kind of soil, particularly 

 whether virgin or cultivated. Thus, Houston 2 , who examined 

 twenty-one samples of surface soil from different sources, found 

 that the virgin sandy soils gave less than 100,000 bacteria per 

 gramme, the other virgin soils about 1,000,000, the garden soils 

 from 1,000,000 to 2,000,000, and two grossly polluted soils, 



1 Edinburgh Med. Journ. 1893, xxxviil, part ii, p. 1122. 



2 Local Gov. Board Med. Officer's Report, 1897-8, p. 251. 



