Page 112 - Geotechnical Engineering Soil and Foundation Principles and Practice

The Soil Profile
The Soil Profile 107

‘‘o’’ for organisms includes vegetation. A most prominent example is forest versus
grassland: forested soil profiles are thinner and have the gray, ash-like layer that is
acidic, whereas grassland soils typically have a thick black or brown A horizon
and a near-neutral pH. A low pH can contribute to corrosion of metal pipes
buried in soil.

‘‘r’’ designates topography, in particular as it affects the rate of erosion: steep
slopes show little or no weathering to form a soil profile.

‘‘p’’ relates to parent material, whether granite or basalt, sandstone, limestone,
or shale. Basalt as in lava flows contains relatively high-temperature minerals that
are readily weathered compared with granite, which undergoes only moderate
weathering before disintegrating into sand. Shale minerals already are far down
on the weathering scale and show only moderate changes, mainly by disintegrat-
ing into clay. Sandstone disintegrates into sand, and limestone dissolves so that
there is little left to make residual soil. Figure 5.4 shows a generalized map of soil
parent materials in the U.S.

Figure 5.4
Generalized map
of soil parent
materials in the
mainland U.S.
Except at A
alluvium is not
shown. (Modified
from Thompson
and Troeh, 1973.)

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