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Soils and Their Preservation
> WHAT IS SOIL?
WHAT IS SOIL?
Soil is the ground
underfoot, a thin layer of the Earth's crust that has been modified by
the natural actions of agents such as weather and organisms. It is easy
to cake soil for granted. We walk on and over it throughout our lives,
but rarely stop to think about how important it is to our survival.
Vast numbers and kinds of organisms
inhabit soil and depend on it for shelter and food. Plants anchor
themselves in soil, and from it they receive essential minerals and
water. Thirteen of the 16 different elements essential for plant growth
are obtained directly from the soil (Table 14-1). Terrestrial plants
could not survive without soil, and because we depend on plants for our
food, humans could not exist without soil, either.
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How Soils Arc Formed
Soil is formed from rock (called
parent rock) that is slowly broken down into smaller and smaller
particles by chemical and physical weathering processes in nature.
It fakes a very long time, sometimes thousands of years, for rock to
disintegrate into finer and finer mineral particles. Time is also
required for organic material to accumulate in the soil. Soil
formation is a continuous process that involves interactions between
the Earth's solid cruet and the biosphere (Figure 14-1). The
weathering of parent rock beneath soil that has already formed
continues to add new soil. |
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Living organisms and
climate both play essential roles in weathering, sometimes working
together. For example, soil organisms produce acids that etch tiny
cracks in the rock. In temperate climates, water from precipitation
seeps into these cracks, which enlarge when the water freezes. Over many
seasons, alternate freezing and thawing cause small pieces of the rocks
to break off.
Topography, a region's surface
features—such as the presence or absence of mountains and valleys—is
also involved in soil formation. Steep slopes often have very little or
no soil on them because soil and rock are continually transported down
the slopes by gravity; runoff from precipitation tends to amplify
erosion on steep slopes. Mod-crate slopes, on the other hand, may
encourage the formation of deep soils.
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