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Soil Formation
Notes & Focus Questions
Directions: Read pp. 138 140 in the Heath
Earth Science book, and pp. 295 297 in the Environmental
Science book. Fill the names of the appropriate layers in the
blanks provided. Soil is a mixture of
organic matter and inorganic sediments. The organic content of soil
comes from the decomposition of the predominant plant growth. The
higher the organic content of soil, the richer it is. Deserts have
little plant growth, and so have very little organic matter in their
soils, and will continue to be unable to support much plant growth in
the future. Forest soils vary. Coniferous forests drop
Ό of their
needles every spring and fall, and their decomposition makes the soil
more acidic and better able to absorb and retain moisture than other
forest soils. This is necessary, since they grow in areas that get
less rainfall. Deciduous forests drop their leaves annually, but they
can take two or three years to decompose, while new growth begins a
few months later, so soils can be less rich. Rainforests support such
a huge biomass that nearly all soil nutrients are tied up in living
things, and surprisingly tend to have the poorest soils of all
forests. Grasses decompose quickly, returning their nutrients to the
soil readily, and are enriched by the manure of the herbivores that
graze on them, yielding a high organic content. This makes them ideal
for farmland. Climate affects soils too; the higher annual rainfall in
forests tends to leach nutrients deeper into the soils, and the lower
rainfalls in deserts and grasslands leaves the nutrients near the
surface. The inorganic content of the soil is usually dependent on the
parent rock it formed from. Oklahomas red soils come largely from the
high iron content of the clays that form from the bedrock below, and
our soils in this state tend to be limey, since 90% of the rock near
the surface is limestone. Our grasses are adapted to it, and near
urban areas, acidic rainfall (caused by air pollution) makes the
addition of lime necessary to bring the soils back to the proper pH.
Sometimes soils end up far from where they originally formed.
Floodwaters erode topsoil from lands near their headwaters, where they
are still flowing quickly, and deposit them in lands where they slow
down, near their mouths. Loess is a type of soil that is moved by wind
or glaciers to areas far from its parent rock, too.
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A Horizon Also called topsoil, this layer is
found below the O horizon and above the E horizon. Seeds
germinate here and plant roots grow in it. |
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B Horizon Also called subsoil, or
illuviation
layer, this layer is below the E Horizon and above the C
Horizon. It contains clay and minerals like iron and calcium
carbonate that it receives from layers above it through
leaching. |
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C Horizon Also called regolith: the layer below
the B Horizon and above the R Horizon. It consists of slightly
broken-up bedrock. |
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E Horizon Also called eluviation layer, this
layer is below the A Horizon and above the B Horizon. It is
mostly sand and silt, having lost most of its minerals and clay
as water drips through the soil (in the process of leaching.) |
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O Horizon Also called humus, the top layer of
soil, made up mostly of partly decomposed leaf litter. This
layer is present in dense forest soils, and may be absent in
others. |
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R Horizon Also called bedrock, the layer that
is beneath all the other layers., the parent rock of the soil. |
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