Chap 7, lesson 2 Soil
Before You Read
Think of the plants that grow where you live. Do you think
your area would be a good place to grow vegetables? Explain
why or why not on the lines below.
Formation of Soil
Soil science is called pedology. What is soil? Where does
it come from? As you read in the last section, weathering
slowly breaks rocks into smaller and smaller fragments.
A layer of weathered rock and mineral fragments covers
Earth’s surface. But these fragments don’t become good
quality soil until plants and animals live in them. Plants and
animals add organic matter, the remains of once living
organisms. Organic matter can include leaves, twigs, roots,
and dead worms and insects. Soil is a mixture of weathered
rock, decayed organic matter, mineral fragments, water,
What factors affect soil formation?
Soil can take thousands of years to form. In some places
soil is 60 m thick, but in other places it is only a few
centimeters thick. Five factors—climate, slope of the land,
types of rock, types of plants, and the amount of time that
rock has been weathering—affect soil formation. For
example, different types of soil develop in tropical areas
than in polar regions. Soils that form on steep slopes are
different from soils that develop on flat land.
How does soil form?
Over time, soil can form from rock. Natural acids in
rainwater begin to weather the surface of rock. Water seeps
into cracks in the rock and freezes, causing the rock to
break apart. Then, plants start to root in the cracks. As the
roots of the plants grow, they continue breaking down the
rock. A thin layer of soil begins to form. Organisms like
grubs and worms live among the plant roots, adding organic
matter to the soil. As organic matter increases, the layer of
soil gets thicker. Over time, a rich layer of soil forms that
can support trees and plants with larger roots.
Composition of Soil
Soil is made up of rock and mineral fragments, organic
matter, air, and water. The rock and mineral fragments
come from rocks that have been weathered. Most of the
pieces of rock and mineral are small particles of sediment
such as sand, silt, and clay.
Where does organic matter come from?
Most of the organic matter in soil comes from plants.
Animals add organic matter to soil when they die. After
plant and animal material gets into the soil, bacteria and
fungi help it decay. The decayed organic matter turns into a
rich, dark-colored material in soil called humus
(HYEW mus). Humus is a source of nutrients for plants.
Animals are important in making good soil. As worms,
insects, and rodents, like mice, burrow in the soil, they mix
the humus with the fragments of rock. Good soil is made up
of equal amounts of humus and weathered rock material.
There are many small spaces between the sediment and
humus particles in soil that are filled with water or air. The
spaces in moist soil hold water that plants need to grow.
During a drought, the spaces in the soil are filled with air.
When water soaks into the ground, it moves into the pores.
If you’ve ever seen a deep hole, you’ve probably seen
different layers of soil. Most plant roots grow in the top
layer of soil. The top layer is usually darker than the layers
below it. The different layers of soil are called horizons. All
the horizons of a soil form a soil profile. Most soils have
three horizons that are referred to as A, B, and C horizons.
What is found in the A horizon?
The top layer of soil is called the A horizon. In a forest, the
A horizon might be covered with organic plant litter. Litter
consists of twigs, leaves, and other organic matter that will
become humus. Litter traps water and keeps the A horizon
moist. Litter also prevents erosion. When litter decays and
turns into humus, it provides nutrients for plant growth.
Another name for the A horizon is topsoil. Topsoil has more
humus and fewer rock and mineral fragments than other layers.
The A horizon is usually dark-colored and rich, or fertile.
What is found in the B horizon?
The layer below the A horizon is the B horizon, also
known as subsoil. The B horizon is lighter in color than the
A horizon because it contains less humus. As a result, the
B horizon is not as fertile. Sometimes material from the
A horizon moves down into the B horizon through a
process called leaching.
Leaching is the carrying away of minerals that have been
dissolved in water. Water seeps through the A horizon. The
water reacts with humus and carbon dioxide to form acid.
This acid dissolves some of the minerals in the A horizon.
The acid seeps deeper into the soil, carrying the minerals
into the B horizon. The figure below shows how leaching
occurring in forest soil.
What is found in the C horizon?
The C horizon is the bottom horizon in a soil profile. It is
often the thickest soil horizon. The C horizon contains partly
weathered rock and little organic matter from plants and
animals. Leaching generally does not affect the C horizon.
At the bottom of the C horizon lies rock—the rock that
formed the soil above it. This rock is called the parent
material of the soil. Because the C horizon is nearest to the
parent material, it is most like the parent material.
What is soil structure?
Soil structures can be granular, platy, blocky, or prismatic.
Granular structures are common in surface soils with high
organic content that glues minerals together. Platy structures
are often found in subsurface soils that have been leached or
compacted by animals or machinery. Blocky structures are
common in soils with high clay content. Prismatic structures,
found in B horizons, are dense and difficult for plant roots
If you travel around the country and look at soils, you
will notice that they are not all the same. Some soils are
thick and red. Others soils are brown and rocky. The figure
below shows some of the different soil types in various areas
in the United States.
How does climate affect soil?
Different regions of Earth have different climates.
Different climates produce different types of soil. Deserts
are dry and have few plants. Because desert soil contains
little organic matter, desert soil is thin and light. On the
grasslands of the prairie, the soil contains plenty of organic
matter from grasses. Lots of organic matter gives prairie
soils thick, dark A horizons. Other regions, such as
temperate forests and tropical areas, have their own
particular types of soil.
What other factors affect soil type?
Parent material affects the kind of soil that develops
from it. Clay soils often form on parent rocks like basalt.
The minerals in basalt weather to form clay. If the parent
material is sandstone, it weathers into sand, producing
Plants Rock type also affects the types of plants that grow
in an area. Different rocks provide different nutrients that
plants need to grow. Soil pH controls many chemical and
biological activities in soil.
Time Soil development is affected by time. If rock has been
weathering for a short time, the sediment will be similar to
the parent material. If rock has been weathering for a long
time, the soil is less like the parent material.
Slope The slope of the land is a factor in soil profiles. On
steep slopes, soils are poorly developed because sediment
does not have time to build up before it is carried downhill.
In bottomlands, sediment and water are plentiful. As shown
in the figure below, the soils of bottomlands and valleys are
usually thick, dark, and full of organic material.
horizon: layer in the soil
humus: dark-colored material formed by decaying
leaching: process in which dissolved minerals are carried
through soil by water
litter: twigs, leaves, and other organic matter that covers the
forest floor and will become humus
soil: mixture of weathered rock, decayed organic matter,
mineral fragments, water, and air
soil profile: all the horizons of a soil