Weathering and Soil Formation

4.2 Weathering and organic processes form soil.

VOCABULARY

• humus- decayed organic matter (dead leaves) in soil
• soil horizon- a layer of soil with properties that differ from those of the layer above or below it
• soil profile- horizons in a specific location

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Soil is a mixture of weathered rock particles and other materials.

SOIL COMPOSITION

• Weathered rock particles (main ingredient)
• Water (20-30%)
• Air (20-30%)
• Organic matter (5%)

Organic means, “coming from living organisms.” Organic matter in soil comes from the remains and waste products of plants, animals, and other living organisms.

*Soils differ depending on what types of rock the rock particles came from.

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• Humus comes from decayed organic matter.
• Different soils are made up of different ingredients and different amounts of each ingredient.
• The black humus on the left contains much more plant material and water than the red soil on the right.

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Weathering.

Weathering is the fracturing or disintegration and decomposition of rocks by natural agents. It is a static process. There is no movement of weathered material except where the pieces fall due to the force of gravity.

Weathering is classified into three types:-

1. Physical or Mechanical Weathering
2. Chemical Weathering
3. Biological Weathering

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Mechanical Weathering.

is the physical disintegration of rocks due to factors like heat, water and frost. Small pieces or particles of rock break off from the rock surface. There is no change in the�minerals constituting the�rock. The end result is�soil. Mechanical �weathering is more �intense when the rock �layers are exposed on�the surface.

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Types of Mechanical Weathering.

• Block Disintegration: Alternate expansion and contraction over a long period of time results in cracks and joints. The rocks then start to split along the cracks and joints. Slowly the whole mass of rock splits into blocks. This is known as block disintegration.
• Exfoliation: The sharp edges of the blocks�are slowly rounded by another process of�mechanical weathering termed exfoliation �or onion peeling.
• Frost shattering: The volume of ice is 1/10�more than the volume of water and the pressure ice exerts on the cracks and crevices is very intense. Slowly angular fragments of rocks break off along the crack. Repeated freezing deepens and widens the cracks. This process is known as frost shattering.

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Chemical Weathering.

Chemical weathering is the rotting or decomposition of rocks, under the influence of atmosphere and water. This type of weathering is most common in equatorial and tropical regions where there is enough moisture and heat to start off chemical reactions within rock masses.

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Types of Chemical Weathering.

There are four types of chemical weathering:-

1. Oxidation: Oxygen present in the air or water reacts with certain minerals present in the rock. Oxides form and the rocks slowly crumble, for example, iron present in rocks combines with oxygen to form iron oxide or rust.
2. Carbonation: Carbon dioxide present in the atmosphere dissolves with rain water to form a weak acid called carbonic acid. This makes rain water acidic. The acidic rain water reacts with rocks like limestone. Calcium and bicarbonate ions form which are carried away by the water.
3. Hydration: This occurs when hydrogen present in water combines with minerals present in rocks and decomposes it for example, feldspar, a mineral, is converted to kaolin as a result of hydration.
4. Solution: Rain water is able to dissolve certain minerals and chemically weather rocks. Rock salt and gypsum are removed by solution.

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Biological Weathering.

• Biological weathering is a form of weathering caused by growth of roots and burrowing of animals. Plants roots are the most efficient agents of the biological weathering as they give off acids that contributes to break down of rocks. Biological weathering increases with soil thickness until the optima for biotic activity is reached.

Trees put down roots through joints or cracks in the rock in order to find moisture. As the tree grows, the roots gradually pries the rock apart.

Types of Soil

The kind of soil that forms in an area depends on:

• The kind of rock in the area
• The area’s climate (overall weather pattern)
• The landforms in the area (mountains, valleys)
• The plant cover in the area
• The animals and other organisms in the area
• Time

*The composition of the soil determines what you can grow in it, what you can build on it and what happens to the rainwater that falls on it.

Climate and landforms affect soil

• Different kinds of soils form in different climates.
• Soil that forms in hot, wet climates is different than those that form in cold, dry climates.
• The shape of the land affects soil development.
• Mountain soils (cold climate) are different than nearby valleys.

Soil horizons

• Soil develops in a series of horizontal layers called horizons.
• Deeper soil looks different than that on top.
• Further down you will find larger, less weathered rock particles and less organic matter.

Main horizons are labeled A, B, C

• The A Horizon- The upper layer of soil commonly called topsoil. Often includes more organic matter (humus) and, therefore, is darker in color.

• The B Horizon- Just below the A horizon. It has little organic matter and is usually brownish or reddish in color. Contains clay and minerals that wash down from above.

• The C Horizon- The deepest layer of soil. It contains the largest and least-weathered rock particles. Typically they are light yellowish-brown.

Activities of organisms affect soil.

• There is a whole world alive below your feet!

ORGANISMS AFFECTING THE SOIL

• Plants- Trees/ grasses provide much of the organic matter that forms humus.
• Microorganisms- Include decomposers (fungi, bacteria). A spoonful of soil may contain 1 million microorganisms. They change nitrogen in soil and air to compounds that plants can absorb. Bacteria can produce acids that break down rocks.
• Animals- Earthworms, ants, termites, mice and groundhogs all live in the soil. They add to the air content of the soil by loosening and tunneling the soil. They help to improve drainage also. Return nutrients to the soil when they die.

Observable, measurable properties of soil.

• Texture- Determined by the size of the weathered rock particles that it contains.
• Color- Comes from iron compounds and humus.
• Pore Space- Spaces between particles. Soils range from 25-60% pore space. Ideal growing soil is about 50% pore space.
• Chemistry- Acidity of water in soil determines how well nutrients dissolve.