What eventually happens to sediments? Soil is formed! Soil- a mixture of weathered rock (bits of rock, silt, clay, sand), mineral fragments, water, air, and humus (decayed plants and animals) The soil profile is in layers called horizons. The law of superposition states that older stuff will be found in the bottom layer of soil. Soon after a plant dies, it breaks down and becomes part of the soil. The upper layer of soil has the most humus and is affected the most by weathering and erosion.

Horizons = layers

Horizon O The Organic Layer The top layer Referred to as “humus” Made of dead plant matter

Horizon A Smallest Layer Contains some humus. Examples of organic material: worms, stems, or roots Very rich in nutrients due to humus Most plants grow here (small roots) Topsoil found here

Horizon B Largest Layer Contains silt and mostly clay Contains small pieces of weathered rock Large roots grow here

Horizon C Also called “Regolith” Lies on top of Bedrock Bottom layer of soil Contains mostly large pieces of weathered rock

Bedrock SOLID rock layer beneath all layers of soil Inorganic Material (NON-Living) Parent material of soil. Because soil formation begins when broken down by weathering.

The Process of Soil Formation - How Soil Forms The Process of Soil Formation Soil forms as rock is broken down by weathering and mixes with other materials on the surface. Soil is constantly being formed wherever bedrock is exposed.

Graphic Organizer Soil Horizons Bedrock begins to weather The C horizon forms as bedrock breaks apart. The A horizon develops from the C horizon where plants grow in it. The B horizon develops as clay and minerals wash down from the A horizon.

**Mature soil will have all 4 layers. Soil development starts in the C Horizon, where the bedrock has started to weather. **Mature soil will have all 4 layers. **Immature soil may be missing the B horizon because that is the last to develop.