SOIL 471 



determines the period at which such an area may with 

 propriety and safety be utilised for building purposes ; 

 or to the agriculturalist in informing him of the suita- 

 bility of any given area for the growth of crops. 



The surface of the ground, exposed as it is to the 

 bactericidal influence of sunlight and to rapid alterna- 

 tions of heat and cold, rain and wind, contains but 

 few micro-organisms. Again, owing to the density 

 of the molecules of deep soil and lack of aeration on the 

 one hand, and the filtering action of the upper layers 

 of soil and bacterial antagonism on the other, bacterial 

 life practically ceases at a depth of about 2 metres. 

 The intermediate stratum of soil, situated from 25 to 

 50 cm. below the surface, invariably yields the most 

 numerous and the most varied bacterial flora. 



Collection of Sample. A small copper capsule 6 cm. 

 high by 6 cm. diameter, with " pull-off" cap secured 

 by a bayonet catch, previously sterilised in the hot- 

 air oven, is the most convenient receptacle for samples 

 of soil. 



The instrument used for the actual removal of the 

 soil from its natural position will vary according to 



FIG. 217. Soil scoop. 



whether we require surface samples or soil from vary- 

 ing depths. 



(a) For surface samples, use an iron scoop, shaped 

 like a shoe horn, but provided with a sharp spine 

 (Fig. 217). This is wrapped in asbestos cloth and 

 sterilised in the hot-air oven. When removed from the 

 oven, wrap a piece of oiled paper, silk, or gutta-percha 

 tissue over the asbestos cloth, and secure it with 

 string, as a further protection against contamination. 



