CHAPTER 9 Focus Points

• Soil composition: By volume, soils are composed mostly of weathered fragments of rocks and minerals. Healthy soils are filled with living organisms and their organic remains.

• Soils and climate: Soils develop most quickly where it is warm and wet and most slowly where it is cold and dry. On a small (broad) geographic scale, climate is an important soil-forming factor.

• Anthropogenic soils: People can form soils intentionally by composting.

• Soils and time: In nature, a few centimeters of soil take centuries or more to form.

• Soil erosion: Human activity is causing soil erosion to happen faster than soils form naturally.

• Soil taxonomy: The soil taxonomy classification system groups soils into 12 orders, based mainly on horizon development and color.

• Importance of soils: Soils provide food and other ecosystem services, such as new medicines, climate change mitigation, and water purification.

• Surface water: The Palmer Drought Severity Index provides a measure of water deficits.

• Groundwater: About half the water used in the United States comes from the ground.

• Permeability: Water flows through rocks or sediments at a rate of centimeters to meters per day, depending on the permeability of those materials.

• Surface water: Bodies of surface water, such as lakes and streams, occur where the water table reaches Earth’s surface.

• Water table fluctuations: The water table fluctuates over time as a result of seasonal changes in precipitation, long-term climate change, and withdrawals of groundwater by people.

• Wells: The tops of most wells lie above the potentiometric surface of the groundwater recharge area, and water must be pumped out of them. Water gushes out of some artesian wells.

• Groundwater overdraft and mining: Groundwater overdraft and groundwater mining withdraw water from aquifers more rapidly than it can be replenished.

• Groundwater pollution: Contaminants that enter groundwater by infiltration come from a variety of sources, including gasoline tanks, sewage, and landfills.

• Water footprints: Food crops and material goods, such as computers and clothing, require water to produce.

• Virtual water: Global virtual water flow exceeds 900 trillion L (237 trillion gal) each year.