AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/4/4-2)
Unit 4 · Topic 4.2
4.2 Soil Formation and Erosion
Soil forms over long periods as rock weathers and mixes with organic matter from living things, building distinct layers called horizons. It forms slowly but can be lost quickly to wind and water erosion, which also harms water quality.
Key terms
- parent material
- weathering
- soil horizon
- topsoil
- humus
- erosion
How soil forms
Soil is a mix of weathered rock particles, organic matter, water, air and living things like bacteria, fungi and earthworms. It starts with parent material, the rock or sediment that soil develops from. Parent material can be bedrock right below the soil or material moved in by rivers, glaciers or wind.
Weathering breaks parent material down into smaller pieces.
- Physical (mechanical) weathering breaks rock without changing its chemistry, like water freezing and expanding in cracks, roots wedging rock apart or sand blown against rock.
- Chemical weathering changes the minerals, like acidic rainwater dissolving limestone or oxygen rusting iron-bearing rock.
- Biological weathering is done by living things, like lichens releasing acids or burrowing animals exposing rock.
Factors that shape soil
Soil forms slowly. It can take hundreds of years or more to build just a few centimeters of topsoil. That's why soil is effectively a nonrenewable resource on a human timescale once it's lost.
- Parent material: sets the minerals and texture the soil starts with.
- Climate: warm, wet climates weather rock and decompose organic matter fastest.
- Organisms: plants add organic matter; decomposers turn it into humus; earthworms mix and aerate soil.
- Topography: soil is thinner on steep slopes, where it erodes, and thicker at the bottom of slopes, where it collects.
- Time: older soils are deeper and have more distinct layers.
Soil horizons
Humus is dark, stable organic matter left after decomposition. It holds water and nutrients and makes topsoil fertile. The A horizon is the most valuable layer for farming, and it's the first layer lost to erosion.
| Horizon | Name | What's there |
|---|---|---|
| O | Organic layer | Leaf litter and partly decomposed organic matter (humus) |
| A | Topsoil | Humus mixed with minerals; dark, fertile, full of roots and organisms |
| E | Eluviation (leaching) zone | Light-colored layer where water has washed minerals downward; not found in all soils |
| B | Subsoil | Collects clay, iron and other minerals washed down from above |
| C | Weathered parent material | Partly broken-down rock |
| R | Bedrock | Solid, unweathered rock |
Erosion and why it matters
Erosion is the removal and movement of soil by water or wind. It's a natural process, but human activities speed it up by removing plant cover: clearing forests, overgrazing, tilling fields and leaving them bare, and construction. Without plants, nothing holds the soil or shields it from raindrops and wind. In the 1930s, drought plus the plowing up of Great Plains grasslands led to the Dust Bowl, when huge dust storms stripped topsoil from farms.
Soil also protects water. Healthy soil filters water as it soaks through, trapping sediment and some pollutants. When soil erodes into streams, it makes the water cloudy (turbid), blocks sunlight for aquatic plants, smothers fish eggs, and carries attached nutrients and pesticides with it.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
How long will the topsoil last?
A field has 25 cm of topsoil. Because it's tilled and left bare in winter, it loses an average of 2 mm of topsoil per year, and soil formation adds almost nothing back on this timescale. How many years until the topsoil is gone?
Show the solutionHide the solution
- Step 1: Convert to the same units: 25 cm = 250 mm.
- Step 2: Years = total depth ÷ loss per year = 250 mm ÷ 2 mm/yr.
- Step 3: 250 ÷ 2 = 125.
Answer: About 125 years.
- Example 2
Where does soil form fastest?
Compare how quickly soil would form from the same type of rock in (a) a cold, dry tundra and (b) a warm, wet tropical forest. Explain using two soil-forming factors.
Show the solutionHide the solution
- Step 1: Climate: warmth and moisture speed up chemical weathering, so rock breaks down faster in the tropical forest than in the cold, dry tundra.
- Step 2: Organisms: the tropical forest has far more plants and decomposers adding and breaking down organic matter, while the tundra has few plants and slow decomposition.
- Step 3: So soil forms faster in the tropical forest.
Answer: Soil forms faster in the warm, wet tropical forest because of faster weathering and more organic matter from organisms.
Common mistakes
- Mixing up weathering and erosion. Weathering breaks rock down in place; erosion moves soil and sediment away.
- Listing the horizons out of order. From the top: O, A, (E), B, C, R.
- Treating soil as quickly renewable. Topsoil takes centuries to form but can be lost in a few years.
On the exam
- You may be asked to describe soil horizons or explain how a human activity speeds erosion. Name the activity, explain how it removes plant cover or disturbs soil, and give a consequence.
- When asked about erosion's effects on water, mention turbidity, reduced photosynthesis and sediment smothering habitat.
Connected topics
Videos
Check yourself
4 questions on 4.2 Soil Formation and Erosion. Pick an answer to see if you got it, and why.
In a forest soil profile, which horizon would most likely contain the greatest amount of humus?
Under which conditions would soil most likely form fastest from the same type of parent rock?
A farmer leaves a field bare after harvest. Which outcome is most likely during heavy rains that winter?
As water soaks down through a soil profile, it carries dissolved minerals and fine clay out of the upper layers. In which horizon do these materials mostly build up?
0 of 4 answered