1. Soil Formation and Composition

2.  I. Soil Formation –A. When bedrock is exposed, it weathers. –B. Particles of rock mix with other material. –C. Soil forms - the loose, weathered material on Earth ’ s surface in which plants can grow.

3.  II. Soil Composition –A. Soil is a mixture of: 1. Rock particles 2. Minerals 3. Decayed organic material (humus) 4. Air 5. Water

4.  III. Soil Texture –A. Rock particles come in 4 sizes: 1. Gravel (2 mm and up) 2. Sand (less than 2 mm) 3. Silt (less than 1/16 mm) 4. Clay (less than 1/256 mm) microscopic view

5. –B. Permeability 1. Clay soils are dense, heavy and hold a lot of water, so plants may drown. 2. Sandy/gravel soils have larger air spaces, so water drains quickly. 3. Loam is made up of equal parts clay, sand, and silt and is good for growing most plants because it holds both air and water.

6.  IV. Soil Horizons –A. A layer of soil –The O horizon is leaf litter- a thin layer of organic matter. 1. Topsoil (The A horizon) –a. Dark brown soil, mixture of humus, clay and other minerals. Develops from C; plant roots weather rocks mechanically and chemically. 2. Subsoil (The B horizon) –a. Materials washed down from the A horizon, but with little humus. Only roots of very large plants –reach B. 3. C horizon first step – weathering of bedrock; partly weathered rock 4. Bedrock –solid layer of rock

9.  V. Life in Soil –A. Soil organisms: 1. mix the soil –a. burrowing mammals and worms mix soil. 2. create spaces for air and water –a. burrowing mammals and worms do this too. 3. make humus –a. most comes from plant leaves on the surface, called litter. –b. decomposers break the dead matter down. i. fungi, protists, bacteria, worms