80 



SOILS: PROPERTIES AND MANAGEMENT 



and the lower subsoil, the upper subsoil being considered 

 to extend to about the depth of three feet below the sur- 

 face. Usually, especially in humid regions, there is a 



Fig. 10. — Soil and subsoil, (a), Surface soil with many plant roots 

 (b), subsoil ; (c), country rock. 



sharp line of demarcation between the soil and the sub- 

 soil, due to differences in the humus content. As organic 

 matter accumulates faster at the surface, the soil there 

 tends to assume a darker color. Whether the land has 

 been tilled or not, this line of separation is fairly marked 

 and can usually be located with little difficulty. On 

 tilled land, where the surface soil extends to about the 

 depth of plowing, the plow line marks the separation of 

 surface and subsoil. Where soil samples are being taken 

 for soil survey or soil analysis, some arbitrary depth, 

 depending on circumstances, is usually established for 

 the surface soil. This depth varies from six to twelve 

 inches. 



61. Soil and subsoil of humid regions. — In humid cli- 



