How Soil Forms

1. How Soil Forms

2. What is soil? • Rocks and sediments might not seem to have much in common with soil, but soil is what they become when they are weathered. • Soil is the loose, weathered material on Earth’s surface in which plants can grow!

3. Bedrock! • One of the main ingredients in soil comes from bedrock. • Bedrock is the solid layer of rock beneath the soil. • Once bedrock is exposed to the surface, it weathers away into small particles that form soil.

4. Soil Composition • Soil is made up of more than just weathered rock. • Soil is a mixture of particles, minerals, decayed organic material, water, and air. • The decayed organic material is called humus.

5. Humus • Humus is a dark-colored substance that forms as plant and animal remains decay. • It helps create spaces in soil for the air and water. • It also contains substances called nutrients (nitrogen, sulfur, phosphorus, & potassium) • These nutrients are needed for plants to grow.

6. Fertility • When a soil has a lot of nutrients, it is fertile. • Fertility of soil is a measure of how well the soil supports plant growth. • Soil that is rich in humus has high fertility. • Sandy soil containing little humus has a low fertility.

7. Soil Texture • Sand feels coarse and gritty, but clay feels smooth and silky. • These differences are known as the soil’s texture. • Soil texture depends on the size of the individual particles. Pg. 187

8. Soil Texture & Plant Growth • Soil texture is very important to plant growth. • If it has too much clay, it will hold too much water and drown the plant. • If it has too much sand, it will not hold very much water, and the plant will die. • The best soil for planting is called loam.

9. Loam • Loam is made up of nearly equal parts of clay, silt, and sand. • The clay allows the soil to retain water. • The sand allows air to get into the soil. • It is for these reasons that loam is the best soil for growing most plant types.

10. The Process of Soil Formation • Soil forms as rock is broken down by weathering and mixes with other materials on the surface. • Soil formation continues over a long period of time.

11. Soil Horizons • Gradually, soil develops layers called soil horizons. • There are three soil horizons. • The A-Horizon, the B-Horizon, and the C-Horizon.

12. A-Horizon • The A-Horizon is made of topsoil. • Topsoil is a crumbly, dark brown soil that contains a mixture of humus, clay, and other minerals.

13. B-Horizon • The B-Horizon is often called subsoil. • Subsoil usually consists of clay and other particles that have washed down from the A-Horizon. • Subsoil contains very little humus (just what washes down from the A-Horizon).

14. C-Horizon • The C-Horizon only contains partly weathered rock.

15. Video of How Horizons and Soil Form • http://www.phschool.com/atschool/phsciexp/active_art/soil_layers/index.html

16. Soil Types • If you looked at the soil in north-central Georgia, the clay would be made of red clay. • In other parts of the country, soils can be black, brown, yellow, or gray. • In the U.S. alone, there are thousands of different types of soil. • Scientists classify the different types of soil into major groups based on climate, plants, and soil composition.

17. Soils of North America Pg. 189

18. Living Organisms in Soil • Soil is teeming with living things! • Some soil organisms make humus, the material that makes soil fertile. • Other soil organisms mix the soil and spaces in it for air and water. Pg. 190-191

19. Forming Humus • Plants contribute most of the organic remains that form humus. • As plants shed their leaves, they form a loose layer called litter. • The plants themselves and their roots become litter after the plant dies. • To form humus, the organic remains go through a process called decomposition.

20. Decomposition • Organisms called decomposers are the ones who break down the remains of dead organisms into smaller pieces and digest them with chemicals. • Soil decomposers include fungi, bacteria, worms, and other organisms.

21. Fungi • Fungi are organisms such as mold and mushrooms. • Fungi grown on, and digest, plant remains.

22. Bacteria • Bacteria are microscopic decomposers that cause decay. • Bacteria attack dead organisms and their wastes in soil. Soil bacteria

23. Other Living Organisms in Soil • Very small animals, such as mites and worms, also decompose dead organic material and mix it with the soil.

24. Mixing the Soil • Earthworms do most of the work of mixing the soil. • As earthworms eat their way through the soil, they carry humus down to the subsoil and subsoil up to the surface. • Earthworms also pass out the soil they eat as waste. • The waste soil is enriched with substances that plants need to grow, such as nitrogen.

25. Mixing the Soil (continued) • Many burrowing mammals such as mice, moles, prairie dogs, and gophers break up hard, compacted soil and mix humus through it. • These animals also add nitrogen to the soil when they produce waste, and they add organic material to the soil when they die and decay. • Soil is aerated for roots thanks to these living organisms.