4 6 



PHYSICAL CONSTITUTION OF SOIL 



TABLE IV 



WEIGHT OF SOIL PEK ACRE 



i. Old Pasture, Rothamsted, Loam with Clay Subsoil 



2. Arable Land, Rothamsted } Loam with Clay Subsoil 



3. Arable Land, Woburn, Sandy Soil 



with wider interspaces, than the subsoil. In the case of arable 

 land, the mechanical action of rain just described is much 

 aided by the loosening of the surface soil by tillage, and this 

 is itself a direct agent in producing a looser and lighter soil 

 at the surface. 



In heavy soils under arable culture, a so-called ' hard pan ' 

 or ' clay pan ' is often formed immediately under the surface 



