Page 529 - 9780077418427.pdf

/Users/user-f465/Desktop
tiL12214_ch20_501-520.indd Page 506 9/3/10 6:23 PM user-f465
tiL12214_ch20_501-520.indd Page 506 9/3/10 6:23 PM user-f465 /Users/user-f465/Desktop

20.3 SOILS
Accumulations of the products of weathering—sand, silt, and
clay—result in a layer of unconsolidated earth materials known
as soil. Soil is, however, usually understood to be more than just
loose weathered materials such as sand or clay. Soil is a mixture
of unconsolidated weathered earth materials and humus, which
is altered, decay-resistant organic matter. A mature, fertile soil
is the result of centuries of mechanical and chemical weather-
ing of rock, combined with years of accumulated decayed plants
and other organic matter. Soil forms over the solid rock below,
which is generally known as bedrock.
There are thousands of different soil types, depending on
such things as the parent rock type; climate; time of accumula-
tion; topographic relief; elevation; rainfall; percentage of clay,
sand, or silt; amount of humus; and a number of other envi-
A ronmental variables. In general, soils formed in cold and dry
climates are shallower with less humus than soils produced in
wet and warm climates. This is so because chemical reactions
occur at a faster pace in warmer, wetter soil than they do in
dry, cooler soil. In addition, the wet and warm climate would
be more conducive to plant growth, which would provide more
organic matter for the formation of humus.
An easily crumbled soil that has approximately equal pro-
portions of sand, silt, and clay is called loam. Loam is a great soil
for gardening because it drains well but holds enough moisture
for sustained plant growth. Loam is usually found in the top-
most layers of soil, so it is also referred to as topsoil. Topsoil is
usually more fertile because it is closer to the source of humus.
The soil beneath, referred to as subsoil, often contains more
rocks and mineral accumulations and lacks humus.
As described in the section on weathering, quartz grains of
sand and clay minerals are the two minerals that usually remain
after rock has weathered completely. The sand grains help keep
the soil loose and aerated, allowing good water drainage. Clay
minerals, on the other hand, help hold water in a soil. A good,
fertile loam contains some clay and some sand. The balanced
mixture of clay minerals and sand provides plants with both the
air and the water that they need for optimum root growth.

CONCEPTS Applied

City Weathering
Prepare a display of photographs that show weathering and
erosion processes at work in a city environment. Provide
location, agent of weathering or erosion, and a description of
what could be done to slow or stop each process illustrated.

B 20.4 EROSION
FIGURE 20.6 Limestone caves develop when slightly acidic Weathering has prepared the way for erosion and for some
groundwater dissolves limestone along joints and bedding
agent of transportation to move or carry away the fragments,
planes, carrying away rock components in solution. (A) Joints
and bedding planes in a limestone bluff. (B) This stream has clays, and solutions that have been produced from solid rock.
carried away less-resistant rock components, forming a cave The weathered materials can be moved to a lower elevation by
under the ledge. the direct result of gravity acting alone. They can also be moved

506 CHAPTER 20 Shaping Earth’s Surface 20-6