1. SOIL: A RENEWABLE RESOURCE Soil is a slowly renewed resource that provides most of the nutrients needed for plant growth and also helps purify water. Soil formation begins when bedrock is broken down by physical, chemical and biological processes called weathering. Mature soils, or soils that have developed over a long time are arranged in a series of horizontal layers called soil horizons.

2. SOIL: A RENEWABLE RESOURCE

3. Layers in Mature Soils Infiltration: the downward movement of water through soil. Leaching: dissolving of minerals and organic matter in upper layers carrying them to lower layers. The soil type determines the degree of infiltration and leaching.

4. Layers in Mature Soils Infiltration: the downward movement of water through soil. Leaching: dissolving of minerals and organic matter in upper layers carrying them to lower layers. The soil type determines the degree of infiltration and leaching.

5. Mosaic of closely packed pebbles, boulders Weak humus- mineral mixture Dry, brown to reddish-brown with variable accumulations of clay, calcium and carbonate, and soluble salts Alkaline, dark, and rich in humus Clay, calcium compounds Desert Soil (hot, dry climate) Grassland Soil semiarid climate)

6. Fig. 3-24b, p. 69 Tropical Rain Forest Soil (humid, tropical climate) Acidic light-colored humus Iron and aluminum compounds mixed with clay

7. Fig. 3-24b, p. 69 Deciduous Forest Soil (humid, mild climate) Forest litter leaf mold Humus-mineral mixture Light, grayish- brown, silt loam Dark brown firm clay

8. Fig. 3-24b, p. 69 Coniferous Forest Soil (humid, cold climate) Light-colored and acidic Acid litter and humus Humus and iron and aluminum compounds

9. Some Soil Properties Soils vary in the size of the particles they contain, the amount of space between these particles, and how rapidly water flows through them.

10. SOIL EROSION AND DEGRADATION Soil erosion lowers soil fertility and can overload nearby bodies of water with eroded sediment. Sheet erosion: surface water or wind peel off thin layers of soil. Rill erosion: fast-flowing little rivulets of surface water make small channels. Gully erosion: fast-flowing water join together to cut wider and deeper ditches or gullies.

11. SOIL EROSION AND DEGRADATION Soil erosion is the movement of soil components, especially surface litter and topsoil, by wind or water. Soil erosion increases through activities such as farming, logging, construction, overgrazing, and off-road vehicles.

12. Global Outlook: Soil Erosion Soil is eroding faster than it is forming on more than one-third of the world’s cropland.

13. Case Study: Soil Erosion in the U.S. – Some Hopeful Signs Soil erodes faster than it forms on most U.S. cropland, but since 1985, has been cut by about 40%. 1985 Food Security Act (Farm Act): farmers receive a subsidy for taking highly erodible land out of production and replanting it with soil saving plants for 10-15 years.

14. Very severeSevereModerate Desertification