Soil Formation and CompositionChapter 7 Section 2

Soil Formation Soil Composition

Soil- the loose, weathered material on Earth’s surface in which plants can grow

Soil forms as rock is broken down by weathering and mixes with other materials on the surface

Bedrock- the solid layer of rock beneath the soil

Soil is a mixture of rock particles, minerals, decayed organic material, air, and water

Type of rock depends on the bedrock that was weathered to form the soil and the type of weathering

Soil Formation

Soil Texture Soil Horizons

Soil texture depends on the size of individual soil particles

Largest to smallest:

gravel > sand > silt > clay Loam- soil that is made up

of about equal parts of clay, sand, and silt

Soil texture is important for plant growth

Soil horizon- a layer of soil that differs in color and texture from the layers above or below it

Soil horizons are made of topsoil, subsoil, and weathered rock

What are soil horizons???Soil horizon is a layer of soil that

differs in color and texture from the layers above or below it

The Rate of Soil FormationThe rate at which soil forms

depends on the climate and type of rock

Weathering occurs more quickly in warm, wet climates

Weathering in occurs slowly in hot, dry climates

Life in SoilSome soil organisms mix the soil and make spaces in it

for air and waterSome soil organisms make humus, the material that

makes soil fertileLitter- a loose layer formed by plants as they shed

leavesDecomposition- occurs as organisms that live in soil turn

dead organic matter into humusDecomposers- organisms that break the remains of

dead organisms into smaller pieces and digest them with chemicals

Fungus, protists, bacteria, and worms are decomposers

Soil TypesSoils are classified into groups based

on the climate in a regionSoils can be classified based on their

soil composition- rocky, sandy, or rich in clay

Major soil types of North America: forest, prairie, desert, mountain, tundra, and tropical soils

ReviewWhat role does weathering play in the formation of soil?

Soil forms as rock is broken down by weathering and mixes with other materials on the surface

What are the different materials that make up soil?Rock particles, minerals, decayed organic material, air, and water

How do plants and animals affect the formation composition of soil?Some soil organisms mix the soil and make spaces in it for air and water. Other soil organisms make humus.

How do forest soils differ from prairie soils?Prairie soils have topsoil that is thick and rich in humus, while northern and southern forest soils may be low in humus

Earthworms breathe by absorbing air in the soil through their skin. Why do you think earthworms crawl to the surface when it rains?When the soil becomes full of water, there are no spaces for air in the soil. Worms climb to the surface for air.