1. Soil Horizons

2. Organic Layer (O-horizon) The uppermost layer; it is rich in organic material. Plant litter accumulates in the O- horizon and gradually decays. In desert soils the O-horizon is completely absent, but in certain organically rich soils it may be the dominant layer.

3. Topsoil (A-horizon) It is dark and rich in accumulated organic matter and humus. It has a granular texture and is somewhat nutrient-poor due to the loss of many nutrient minerals to deeper layers and by leaching.

4. Subsoil (B-horizon) The light-colored subsoil beneath the A- horizon; it is often a zone of illuviation where nutrient minerals have leached out of the topsoil and litter accumulate. It is typically rich in iron and aluminum compounds and clay.

5. Parent Material (C-horizon) This contains weathered pieces of rock and borders the unweathered solid parent material. Most roots do not go down this deep and it is often saturated with groundwater.

7. Fig. 3-23, p. 68 Fern Mature soil Honey fungus Root system Oak tree Bacteria Lords and ladies Fungus Actinomycetes Nematode Pseudoscorpion Mite Regolith Young soil Immature soil Bedrock Rock fragments Moss and lichen Organic debris builds up Grasses and small shrubs Mole Dog violet Wood sorrel Earthworm Millipede O horizon Leaf litter A horizon Topsoil B horizon Subsoil C horizon Parent material Springtail Red Earth Mite

8. 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.

10. Fig. 3-24a, p. 69 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)

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

12. 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

13. 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

14. Case Study: Industrialized Food Production in the United States Industrialized agriculture uses about 17% of all commercial energy in the U.S. and food travels an average 2,400 kilometers from farm to plate. Figure 13-7

15. Fig. 13-7, p. 277 4% Food production Food distribution and preparation Food processing LivestockCrops 5%6%2% 17% of total U.S. commercial energy use

16. Traditional Agriculture: Low Input Polyculture Many farmers in developing countries use low- input agriculture to grow a variety of crops on each plot of land (interplanting) through: Polyvarietal cultivation: planting several genetic varieties. Intercropping: two or more different crops grown at the same time in a plot. Agroforestry: crops and trees are grown together. Polyculture: different plants are planted together.