WEIGHT OF SOILS 



31 



3. Arable land, Woburn, sandy soil 



These tables show: (1) That each of these classes of soil is lighter 

 at the surface ; (2) that in each case the weight increases with an in- 

 crease in depth. This increase in weight of the lower zones is due: 



(1) to the increase of pressure to which the lower zones are subjected; 



(2) to the fact that the surface soil is more loose and porous; (3) 

 to coarser texture of subsoil. This condition is brought about by the 

 removal of the finest soil particles from the surface into the sub-soil 

 by the action of rain ; by the accumulation of organic matter in the 

 surface soil ; and, in the case of arable soils, by tillage. 



The specific gravity of a soil indicates its weight as compared with 

 the weight of an equal volume of water. An English authority has 

 published the following table, which gives the specific gravity of the 

 more common soil constituents : — 



Schone gives the following for the specific gravity of soils : — 



Clay soil 2.65 



Sandy soil 2.67 



Fine soil 2.71 



Humus soil 2.53 



The true specific gravity of an arable soil varies from about 2.5 to 

 2.7. 



