1. How Do Soils Form? Chapter 7 Lesson 4 pp. 290-292

2. How Soil Forms  Weathering  Weathering is the process by which rocks are broken down into smaller and smaller pieces.  Weathering is the most important factor in the process of soil formation

3. 2Types of Weathering  Physical Weathering  This is caused by wind, water and gravity.  Plants can also cause physical weathering. (Tree roots)

4. 2Types of Weathering  Chemical Weathering  This occurs when chemicals in water break up rocks by dissolving parts of them, such as the cement that holds them together.

5.  Humus  Topsoil  Subsoil  Bedrock

6. Composition of Soil  Bedrock  Very Bottom  Bedrock is mostly solid rock.  Minerals in bedrock determine what type of soil is formed.

7. Composition of Soil  Subsoil  Subsoil is mostly made of small rocks.  Temperature and tree roots break up rocks.  Contains little if any organic matter.  The upper part of subsoil is clay-rich.  It contains minerals that have filtered down from the ground above.

8. Composition of Soil  Topsoil  Topsoil is made partially from broken pieces of lower layers of rock.  It is more crumbly and the mixture is more varied than lower layers.  Topsoil contains bits of rock, air, water, and humus.  Topsoil contains minerals and living organisms.

9. Composition of Soil In Topsoil-  Humus  Humus is decayed plants and animal matter.  It contains nitrogen and phosphorus.  This rich mixture is good for growing plants.

10. Color of soils  Soil color gives clues about its minerals and the amount of humus.  Red soils may have lots of iron.  Dark soils may have lots of humus.  Gray soil may be poor soil that has little iron or oxygen.

11. Classifying Soil  Soil is classified by the size of its particles.  Sandy  Silty  Mostly Clay

12. Classifying Soil  Soil is classified by the size of its particles.  Sandy  Sandy is coarse, has large bits of rocks, and feels gritty.  Water runs through quickly.  Farmers add humus to sandy soil to help it hold water.  Silty  Silty soil has smaller bits of rock and feels like flour.  Mostly Clay

13. Classifying Soil  Sandy  Silty  Mostly Clay  Clay soils have the smallest particles.  Clay soils are sticky and muddy when wet.  Farmers add humus to clay soil to break up the soil and improve drainage.

14. How the Dust Bowl Formed Conserving Soil p. 292  April 14, 1935  Severe dust storms in the central United States removed all the topsoil. Farmers could no longer farm.  Farmers plowed deep rooted grasses under and planted shallow rooted crops like wheat.  Livestock overgrazed the land.  Ranchers and farmers did not understand conservation.  Years of drought followed by severe windstorms resulted in the soil being blown away.

17. What Farmers do today to conserve soil  Strip farming. Farmers plant in strips, alternating plants with shallow roots with plants with deep roots.  Contour farming. Farmers plow across slopes to prevent water and soil flowing downhill.

18. What Farmers do today to conserve soil  Terrace farming. Farmers use "steps" that are built into the side of a mountain or hill. Crops are planted on each level. When it rains, instead of washing away all of the nutrients in the soil, the nutrients are carried down to the next level.

19. What Farmers do today to conserve soil  Windbreaks. Farmers plant lines of trees to stop the wind from blowing away the soil.  Crop rotation. Farmers change the crop planted in fields to avoid robbing particular nutrients from that field.

20. What Farmers do today to conserve soil  Modern farming practices protect and preserve the soil for future generations.