Soil Formation and Conservation
Soil is one of Earth’s most valuable natural resources
because everything that lives on land, including humans,
depends directly or indirectly on soil. Soil is a limited
supply because it takes a long time to form. It can take
hundreds of years for just a few centimeters of soil to
form. The thick, fertile soil of the prairies took many
thousands of years to develop.
Soil: a loose mixture of rock fragments, organic material, water, and air that
can support the growth of vegetation. Not all soil is the same because soils are
made from weathered rock fragments, they type of soil that forms depends on
the type of rock that weathers. The rock formation that is the source of
mineral fragments in the soil is called parent rock, and bedrock is the layer of
rock beneath the soil. The bedrock is the parent rock because the soil above it
formed from the bedrock below. The soil that remains above its parent rock is
called residual soil and soil that is blown, washed away, or moved by glaciers
are called transported soil.
* Although weathered rock is the basic component of soil, the
composition and texture of soil and its fertility and resistance to
erosion are greatly influenced by plants and other organisms.
Soil Composition: Soil is not just particles of weathered bedrock. Soil is
actually a mixture of rock particles, minerals, decayed organic material,
water, and air.
1. Humus is a dark-colored substance that forms as plant and animal
remains decay. Humus forms in a process called decomposition. During
decomposition, organisms that live in soil turn dead organic material
into humus. Decomposers are the organisms that break the remains of
dead organisms into smaller pieces and digest them with chemicals.
Soil decomposers include fungi, bacteria, worms, and other organisms.
Humus helps create spaces in soil for the air and water that plants must
have. Humus also contains substances called nutrients. Plants need
nutrients in order to grow. As plants grow they absorb nutrients from
the soil.
2. Fertile soil: Fertile soil is rich in the nutrients that plants need to grow.
The fertility of soil is a measure of how well the soil supports plant
growth. Soil that is rich in humus has high fertility. Sandy soil
containing little humus has low fertility. Fertile soil is valuable because
there is a limited supply. Less than one eighth of the land on Earth has
soils that are well suited for farming.
Soil Texture: Soil texture is the soil quality that is based on the proportions
of soil particles. Soil is made up of different-sized particles. These particles
can be as large as 2 mm, such as sand, and other particles can be too small to
see without a microscope. Soil texture depends on the size of individual soil
particles. Soil texture is important for plant growth. Soil that is mostly clay
has a dense, heavy texture. Some clay soils hold a lot of water, so plants grown
in them may “drown” for lack of air. In contrast, sandy soil has a coarse
texture. Water quickly drains through it, so plants may die for lack of water.
Soil that is made up of about equal parts of clay, sand, and silt is called loam. It
has a crumbly texture that holds both air and water. Loam is best for growing
most types of plants.
Soil Horizons: Soil is constantly being formed whenever bedrock is
exposed, and this process of soil formation continues over a long period of
time. Gradually, soil develops layers called soil horizons. Soil horizon is a
layer of soil that differs in color and texture from the layers above or below it.
Soil horizons form in three steps:
1. Horizon A (Topsoil with humus): a crumbly, dark brown soil that is a
mixture of humus, clay, and other minerals. Horizon A develops as
plants add organic material to the soil and plant roots weather pieces of
rock.
2. Horizon B (Subsoil): usually consists of clay and other particles washed
down from the Horizon A, but little humus. Horizon B develops as
rainwater washes clay and minerals from the A horizon to the B
horizon.
3. Horizon C (Rock Fragments): Contains only partly weathered rock.
Horizon C forms as bedrock weathers and rock breaks up into soil
particles.
Soil Mixers: Earthworms and burrowing animals, such as mice, moles,
prairie dogs, and gophers break up hard compacted soil and mix humus
through it. This helps to add water and to aerate, or mix air into, the soil.
Earthworms do most of the work of mixing humus with other materials in soil.
As earthworms eat their way through the soil, they carry humus down to the
subsoil and subsoil up to the surface. Earthworms also pass out the soil they
eat as waste. The waste soil is enriched with substances that plants need to
grow, such as nitrogen.
Soil Damage and Loss: Besides supporting the growth of plants, soil plays
other life-sustaining roles. Soil helps purify, or clean water as it drains
through the ground into rivers, lakes, and oceans. Soil also provides a home
for a variety of living things, from one-celled organisms to small mammals.
The way people use land can affect the levels of nutrients and pollution in soil.
Any activity that exposes soil to wind and rain can lead to soil loss. Farming,
construction, and development, and mining are among the main activities that
impact soil resources. The value of soil is reduced when soil loses its fertility
and when topsoil is lost due to erosion. Soil erosion is a serious problem. Soil
can be damaged from overuse by poor farming techniques or by overgrazing.
Overused soil can lose its nutrients and become infertile. Plants can’t grow in
soil that is infertile. Without plants to hold and help cycle water, the area can
become a desert. This process, formally known as desertification, is now
called land degradation. Without plants and moisture, the soil can be blown
or washed away. Also, when plants and trees are cut down to clear land, it can
lead to soil erosion. Plants and trees are like anchors to the soil. Roots keep
the topsoil from being eroded. Therefore, plants and trees protect the soil. By
taking care of the plants and trees (vegetation), you also take care of the soil.
Soil Conservation: Soil conservation is very important, because soil can be
difficult or impossible to replace once it has been lost. Soil takes a long time to
form. A soil with well-developed horizons may take thousands of years to
form. Most soil conservation methods are designed to hold soil in place and
keep it fertile. Some of the methods soil can be conserved is through contour
plowing, conservation plowing (tillage), crop rotation, terraces, and wind
breaks.
1. crop rotation: is a practice of planting different crops on the same field
in different years or growing seasons. Grain crops, such as wheat, use
up a lot of the nitrogen—a necessary plant nutrient—in the soil. The
roots of bean crops, such as soybeans, contain bacteria that restore
nitrogen to the soil. By rotating these crops, farmers can help maintain
soil fertility.
2. Conservation plowing (tillage): includes several methods of reducing
the number of times fields are tilled or plowed, in a year. The less soil is
disturbed by plowing, the less likely it is to be washed or blown away.
In one method of conservation tillage, fields are not plowed at all. The
remains of harvested crops are simply left on the fields to cover and
protect the soil. New seeds are planted in narrow bands of soil.
3. Terraces: are flat, steplike areas built on a hillside to hold rainwater
and prevent it from running downhill. Crops are planted on the flat
tops of the terrace.
4. Contour plowing: is the practice of plowing along curves or contours, of
a slope. Contour plowing helps channel rainwater so that it does not
run straight downhill, carrying away soil with it. A soil conservation
method called strip-cropping is often combined with contour plowing.
Strips of grasses, shrubs, or other plants are planted between bands of a
grain crop along the contour of a slope. These strips of plants also help
slow the runoff of water.
5. Windbreaks: are rows of trees planted between fields to “break,” or
reduce, the force of winds that can carry off soil.