30 AGRICULTURE ON THE PACIFIC SLOPE 



Soil and subsoil. In the humid region (and in the 

 low, wet lands of the arid region) farmers find an im- 

 portant practical difference between soil and subsoil. 

 The soil proper is the surface layer, made dark by humus. 

 Below this there is usually a yellowish subsoil without 

 humus, and almost always containing more clay than the 

 surface soil. The surface soil is seldom more than nine 

 inches thick, and often less than six. As a rule it is only 

 this surface soil that is cultivated, and in it the active 

 feeding roots (see page 12) of plants are chiefly found. To 

 bring up the "raw " subsoil would hurt the coming crop. 



Arid soils. It is quite otherwise in the countries 

 where there are no summer rains, and where irrigation 

 is necessary (arid climates), as in most of California, 

 also in Arizona, eastern Oregon and Washington, 

 southern Idaho, Montana, and generally west of the 

 Rocky Mountains. But little true clay is formed 

 here in rock weathering; the soils are mostly powdery 

 or sandy, and the scanty rains cannot wash clay from 

 the surface down into the subsoil. Water and air, 

 and with them the roots of plants, penetrate to much 

 greater depth in arid than in humid climates; from a 

 few feet to twenty and more. Very little humus is 

 formed except from the decay of these roots, whereas 

 in humid climates the leaf -fall adds greatly to it. Fig- 

 ure 14 shows these points. 



Leaf-fall and humus. In the humid regions the 

 leaves of most plants fall about the same time in the 



