Slope Steep slopes often have little or no soil on them because of gravity. Runoff from precipitation tends to erode the slope also. Moderate slopes and valleys may encourage the formation of deep soils.

Depth Some soils are very shallow (like in some places in San Antonio). It can be only two inches of soil and then you hit rock. Other areas can have soil 36 inches deep or more.

Color Dark soil is rich with lots of organic matter. Light soil (like sand) is not so rich with very little organic matter.

Soil Horizons

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.

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.

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.

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.

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

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.

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)

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

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

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

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

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

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.

6.4 billion tons of soils are eroded from the U.S. each year; this would fill 320 million average-sized dump trucks that, if parked end-to-end, would extend to the moon and ¾ of the way back! Erosion

Definition Erosion is the movement of soil components, especially surface litter and topsoil, from one place to another.

Importance In undisturbed ecosystems, the roots of plants help anchor the soil, and usually soil is not lost faster then it forms. But, farming, logging, construction, overgrazing by livestock, off-road vehicles, deliberate burning of vegetation etc. destroy plant cover and leave soil vulnerable to erosion. This destroys in a few decades what nature took hundreds to thousands of years to produce.

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.

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. Figure 13-9

Global Outlook: Soil Erosion Soil is eroding faster than it is forming on more than one-third of the world’s cropland. Figure 13-10