Dr. Pamela Gore
Georgia Perimeter College
- Discuss the components of soil.
- Tell how humus forms.
- Explain the factors influencing soil formation.
- Explain the main features of a soil profile.
- Discuss some of the features involved in classifying soil types.
- Discuss soil conservation.
This section addresses, in whole or in part, the following Georgia GPS standard(s):
- S6E5g. Describe soil as consisting of weathered rocks and decomposed organic material.
- S6E5i. Describe methods for conserving natural resources such as water, soil, and air.
This section addresses, in whole or in part, the following Benchmarks for Scientific Literacy:
- Rock is composed of different combinations of minerals. Smaller rocks come from the breakage and weathering of bedrock and larger rocks.
Soil is made partly from weathered rock, partly from plant remains-and also contains many living organisms.
- 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 plant roots and debris, bacteria, fungi, worms, insects, rodents, and other organisms.
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This section addresses, in whole or in part, the following National Science Education Standards:
- Soil consists of weathered rocks and decomposed organic material from dead plants, animals, and bacteria.
Soils are often found in layers, with each having a different chemical composition and texture.
Introduction to soil
Weathering of rocks is required for soil formation.
Weathering is an important part of the rock cycle, which involves changing one type of rock into another through natural processes.
Weathering is the breakdown of rocks, which produces sediment, soil, and dissolved ions in water.
Soil is important for agriculture; different types of soils form in different climates.
The components of soil
Soil consists of:
- minerals and weathered rock fragments
- organic matter
- living organisms
Humus is decayed organic matter. (The decayed remains of animal and plant life, such as decayed leaves.)
Humus is an essential component of soils. It enhances the soil's ability to retain water.
The mineral portion of soil is usually much greater than the organic portion of soil.
Sand is a type of sediment derived from the weathering of rocks such as granite.
The feldspar in granite weathers to clay, and the quartz grains in granite are released as sand particles.
"Sand" is really a SIZE term, referring to grains of sediment ranging from 1/16 mm to 2 mm in diameter.
Silt refers to sediment of a size of 1/256 mm to 1/16 mm. Silt forms form the weathering of rocks.
Clay is both a size term and a mineral term. It refers to grains of sediment less than 1/256 mm in diameter.
It also refers to a group of minerals called the "clay minerals" consisting of minerals such as kaolinite, illite, and montmorillonite.
Clay is produced when certain minerals, such as feldspar, are weathered in the presence of water.
Soils contain particles of different sizes. The texture of a soil influences its ability to transmit and retain water and air, both of which are essential to growth.
Sandy soils drain quickly. Clay-rich soils inhibit drainage, and plant roots may have difficulty penetrating clay.
A loam is a soil in which there is a mixture of sand and silt, with a small amount of clay.
Loam soils are best suited to support plant life. This is due to their moisture characteristics and nutrient storage ability.
How does soil form?
Soil is formed when rocks are broken down by weathering.
There are five factors influencing soil formation:
- Parent rock
- Relief (slope, topography)
What is a soil profile?
A soil profile consists of several soil horizons.
They are numbered roughly alphabetically, beginning at the ground surface, going downward:
- O horizon
- This is the layer of humus on the ground surface.
- A horizon
- Top soil.
- Rich in organic matter. Typically has dark color.
- Also called zone of leaching.
- B horizon
- Also called zone of accumulation.
- May contain soluble mienrals such as calcite in arid climates (caliche).
- C horizon
- Weathered bedrock or saprolite (rotten rock).
- Bedrock lies below the soil profile.
What are the major soil types?
Soils around the world are classified into twelve soil types (or orders) on the basis of the properties of the soils.
The properties are a result of the five factors influencing soil formation (CPORT) acting on the parent rock over time.
Soil classification depends on the presence or absence of diagnostic features including, but not limited to:
- whether there is permafrost within 2 meters of the surface,
- whether the soils are dominantly organics,
- whether the soils are formed on volcanic ash,
- whether the soils are clayey with clays that shrink and swell,
- whether the soils are intensely weathered and leached,
- whether there is a subsurface horizon of calcium carbonate,
- whether they are under grasslands or forests,
- how developed the various layers of the soil horizon are,
- what has accumulated in various soil horizons
The twelve soil orders are as follows:
- Entisol (young soils)
- Ultisol (pedalfer, laterite)
- Inceptisol (young soils)
- Oxisol (only in the tropics - laterite)
- Mollisol (grasslands)
- Aridosol (arid climate - pedocal)
- Alfisol (forested areas)
- Histosol (peat)
- Spodosol (forested areas)
- Vertisol (swelling clays)
- Gelisols (permafrost)
- Andisols (volcanic soils)
What types of soil are present in Georgia?
Click here to see a map showing the type of soil in the southeastern US.
Georgia's soils are mostly Untisols. This map shows some suborders of Ultisols.
Georgia's soils are characteristic of humid climates.
Other more general terms are sometimes used to refer to different types of soils:
- These soils are rich in Al and Fe.
- They form in humid climates, such as the southeastern U.S.
- These soils are rich in Ca.
- They form in arid climates, such as the southwestern U.S.
- These soils commonly contain caliche (or hardpan), a calcium carbonate deposit
which accumulates in the soil.
- These soils have been depleted of nearly all elements except iron and aluminum
- Laterites are derived from the weathering of basalt (mafic parent rock).
- They form in tropical climates with very high rainfall.
- The high rainfall has caused leaching of most of the elements and nutrients from
- This is the soil typical of a tropical rainforest. When used for agriculture, the
small amount of nutrients is quickly depleted, and the soil dries to become as hard
as a brick.
Have you ever had your soil analyzed?
A soil analysis can be performed by your county extension office. A soil analysis will give you
- The pH of your soil.
- Is your soil acidic? (common in Georgia) If so, you will need to add lime.
- Is your soil basic or alkaline? (common in areas underlain by limestone).
- The chemical elements in your soil. Plants require certain nutrients.
- Nitrates (fertilizer) - nitrogen, phosphorus, potassium (in that order) are the numerical
designations of fertilizer, listed as weight percents. (such as 5-10-5 or 10-10-10).
- Organic matter
- Soluble salts
How can we practice soil conservation?
Good soil conservtion techniques include:
- Contour plowing
- Strip planting - different crops in strips
- Cover crops
- Crop rotation
- Planting groundcovers; roots hold the soil ( kudzu)
- Tree planting
An example of severe wind erosion - Dust Bowl of the 1930's - Southern Great Plains: Texas, Oklahoma, Kansas, New Mexico and Colorado.
Poor agricultural practices and years of sustained drought led to the Dust Bowl.
Winds blew the soil away. Dust storms darkened the sky for days.
Farms were buried in drifting dust.
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Page created by Pamela J.W. Gore
Georgia Perimeter College,
Page created March 9, 2005
Modified March 26, 2005
Links updated October 13, 2008