54 Elementary Principles of Agriculture 



removed by flowing water. Such soils are referred 

 to as residual soils. 



86a. Weigh a fruit jar and fill with the muddy water flowing 

 from the field after a heavy rain. Let stand until the water is clear, 

 and note the amount of soil in the bottom of the jar. 



86b. Weigh the jar again, pour off the clear water, leaving the 

 thick sediment. Dry and weigh the sediment, and calculate the 

 per cent of sediment in the muddy water. 



87. Other Classifications. A convenient and natural 

 classification of soils is often made according to the 

 color, texture and structure of the soil layers. We com- 

 monly speak of a soil as consisting of a surface soil and 

 a subsoil. 



The surface soil includes the top layer of soil "that 

 which is moistened by the rains, warmed by the sun, 

 permeated by the atmosphere, in which the plant ex- 

 tends its roots, gathers its soil-food, and which, by the 

 decay of the subterranean organs of vegetation, ac- 

 quires a content of humus." The surface soil may be 

 subdivided further into surface soil and sub-surface soil; 

 the surface soil proper, or soil mulch, including the layer 

 of top soil that is moved about by the ordinary operations 

 of tillage, and the sub-surface soil, referring to the layer 

 of surface soil that is just beneath the soil mulch, thus 

 being a part of the surface soil and yet is not stirred 

 by ordinary inter-tillage. 



The subsoil is the layer just below the surface soil, 

 and in all soils it is taken to mean the second layer, 

 showing characteristic differences from the surface 

 soil. Sometimes the subsoil, or a layer just beneath the 

 top layer of the subsoil, may consist of a hard, stiff layer 

 of clay or other compacted material, impermeable to 

 water and air. This is spoken of as hard-pan. It is often 

 absent altogether, or it may be at various depths. It 



