4.2

Answer in your science notebook

• Which type of weathering changes the makeup of a rock? • What is the main cause of chemical

weathering? • What three things affect the rate of

chemical weathering?

Weathering and Soil Formation4.2 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

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

SOIL COMPOSITION• Weathered rock particles (main ingredient)• Water (20-30%)• Air (20-30%) • Humus (5%)

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

• 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.

Types of SoilThe 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.

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.

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.

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.

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.

ORGANISMS AFFECTING THE SOIL

• 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.

ORGANISMS AFFECTING THE SOIL

• 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.