CHAPTER XII 
SOILS 
As the life of all ordinary plants is dependent upon the soil, 
every practical cultivator of flowers, fruits, vegetables, or 
field crops must give attention to this all-important factor 
in their prosperity. 
The soil is the loose surface layer of the earth’s crust in 
which plants grow. Its depth varies from an inch in certain 
poorly favored regions to several feet in many of the prairie 
sections of the Northwest. It is more fertile and is usually 
darker in color than the ground immediately below it, which 
is called the sub-soil, but there is no hard and fast line divid- 
ing the one layer from the other. . 
Composition. — The chief part, or the basis, of the soil 
consists of small particles of rock or mineral matter. To 
this basis are added the remains of plants and sometimes 
of animals, this organic matter being called humus. Thus 
the nature of any soil depends simply upon the kind of rock 
from which it was formed, the size of the rock particles, and 
the amount of humus it contains. 
The mineral matter in any given soil is practically perma- 
nent, varying only in a slight degree as some of the more 
precious elements are temporarily exhausted by growing 
plants. The mineral matter in one soil may differ from that 
in another soil (1) in respect to chemical composition, de- 
pending upon the kind of rocks from which the two were 
originally derived. The chemical composition, of course, 
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