How Soil Forms

What is Soil? Soil is the loose, weathered material on Earth’s surface in which plants grow. One of the main ingredients of soil is: Bedrock which is the solid layer of rock beneath the soil.

Soil Composition Soil is comprised of a mixture of rock, particles, minerals, decayed organic material, water and air. The decayed organic material in soil is called humus. Humus provides plants with the nutrients they need to grow. Soil that is rich in humus has a high fertility rate supporting plant growth.

Soil Texture Soil texture depends on the size of the individual particles. The largest particle size is gravel. The smallest particle size is clay. Loam is the best type of soil for growing plants since it is made up of about equal parts of clay, sand and silt.

Soil Particle Size

The Process of Soil Formation Soil forms as rock is broken down by weathering and mixes with other materials. 3 horizons (layers) A horizon: topsoil B horizon: subsoil C horizon: partly weathered rock

Horizon A Horizon A is made up of topsoil. Topsoil is a crumbly, dark brown soil that is a mixture of humus, clay and other materials. This soil horizon is most important for growing food.

Horizon B Horizon B is consists of clay and other particles washed down from Horizon A; very little humus. Horizon B is often called subsoil. Horizon B forms last as particles wash down from the A horizon.

Horizon C Horizon C is weathered rock. The C horizon is most similar to bedrock. It consists mostly of pieces of bedrock.

Rate of Soil Formation The rate of soil formation depends on the climate and type of rock. Weather occurs most rapidly in areas with a warm, rainy climate so soil will form faster in these areas. Soil forms more quickly from limestone than granite since limestone weathers faster.

Soil Types Classified based on climate, plants and soil composition

Types of Soil Fertile soil forms in regions with hot, wet climates. Thin soil forms in mountains and polar regions with cold and dry climates. The thickest and most fertile soil forms in moderate temperatures with moderate rainfall.

Major soil types in North America Forest Prairie Desert Mountain Tundra Tropical

Living Organisms in Soil Two ways organisms contribute to soil: A. Forming Humus B. Mixing the Soil

Forming Humus Plants contribute most of the organic remains that form humus. As plants shed leaves, they form a loose layer called litter. Humus forms in a process known as decomposition where organisms that live in the soil turn dead organic material into humus. Decomposers are the organisms that break down the remains. These include fungi, bacteria, worms, and other organisms.

Mixing the Soil Earthworms mix humus with other materials in soil. They also help move air and water into the soil through a process called aeration.