Soils
© Pamela J. W. Gore, 1995, 2009
Georgia Perimeter College
Objectives
1. Know the three major types of soils (pedalfer, pedocal, laterite)
2. Recognize the soil profile.
Soil consists of:
- minerals and weathered rock fragments
- organic matter
- gases
- water
- living organisms
Regolith is minerals and weathered rock fragments
Humus is decayed organic matter.
How does soil form?
There are five factors influencing soil formation:
- Climate
- Parent rock
- Organisms
- Relief (slope, topography)
- Time
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
- Subsoil.
- 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?
- Pedalfer
- These soils are rich in Al and Fe.
- They form in humid climates, such as the southeastern U.S.
- Pedocal
- 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.
- Laterite
- These soils have been depleted of nearly all elements except iron and aluminum
oxides.
- 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
the soil.
- 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.
- Bauxite
- Bauxite has also been depleted of nearly all elements due to intense tropical
weathering.
- It is rich in Al oxides, which are all that remains after the more soluble Ca, Na, K,
and Si have been leached out.
- Bauxite is derived from the weathering of granite (sialic parent rock).
- This Al-rich soil is the principle ore of aluminum.
- Found in: Brazil, Jamaica, also locally in Georgia.
Have you ever had your soil analyzed?
A soil analysis can be performed by your county extension office. A soil anaalysis will give you
information on:
- 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
Soil erosion
Soil may be eroded due to the action of water (runoff) or wind.
(Please note that weathering and erosion are two different things).
Soil is transported by streams, and most will eventually be deposited on a floodplain or at the
mouth of the river in a delta. (Today, much eroded soil will be trapped behing dams across
rivers, filling reservoirs. Loss of sediment transport to the coast also depletes beaches of sand,
and can lead to accelerated beach ersoion.
The Dust Bowl of the 1930's was due to wind erosion of soil following extended drought and
over-tilling of the soil.
How can we slow soil erosion?
- Contour plowing
- Terraces
- Planting groundcovers; roots hold the soil. (What is kudzu?)
- Windbreaks
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