Chapter 4 Outline: Weathering and Soil Formation
I. Mechanical and chemical forces break down rocks.
A. Weathering breaks rocks into smaller pieces
B. Mechanical Weathering produces physical changes in rocks.
1. ICE WEDGING- expansion of water as it freezes in cracks
2. PRESSURE RELEASE (exfoliation)- rocks formed under pressure –surface expands
3. PLANT ROOT GROWTH- roots enlarge in cracks
4. ABRASION- friction (in water, air, etc)
C. Chemical weathering changes the mineral composition of rocks.
1. DISSOLVING- H2O (acidic) breaks down minerals
2. RUSTING- oxygen & H2O cause iron to form oxides
(iron+oxygen= iron oxide, called “oxidation”)
D. Weathering occurs at different rates
1. SURFACE AREA- more exposed surface area = faster weathering
2. ROCK COMPOSITION- softer rock weathers faster weathering
3. CLIMATE- wetter=faster weathering,
chemical weathering goes faster warm climates,
mechanical weathering goes faster in freezing temperatures
II. Weathering and organic processes form soil.
A. Soil is a mixture of weathered rock particles and other materials.
1. soil composition
a. weathered rock particles: This is the main ingredient
b. organic matter- called humus
c. water- 20-30%
d. air– 20-30%
4. air – 202. soil horizons (name and describe)
a.Horizon A: this is the topsoil, contains the most humus, dark in color
b.Horizon B: contains little organic matter, contains clay and minerals that
leach (wash down) from the A horizon, brown or reddish color
c.Horizon C: deepest layer, contains the largest and least weathered rock
particles, light yellowish brown in color.
B. Climate and landforms affect soil.
1. tropical – Warm, rainy regions. Heavy rains wash topsoil away;
Soils not good for growing crops
3. temperate- moderate temperatures & rainfall; Soils are rich in organic matter and
minerals; good for growing crops
4. arctic— cold/dry regions; soils are rocky with thin topsoil
C. The activities of organisms affect soil.
1. Plants: most organic matter in humus comes from plants
2.Microorganisms: decomposers such as bacteria and fungi
3. Animals: loosen and mix the soil, improving aeration and pore space
D. Properties of soil can be observed and measured.
1. Texture
a.sand: largest particle, usually light in color, not sticky
b.silt: smaller; too small to see without microscope, a little sticky
c.clay: smallest particle: absolutely microscopic!, very sticky
2. Color
a.Red color is from iron and humus (iron gives reddish color)
b.Dark color (brown/black) is from high amounts of humus
c.Light color is from sand and lack of humus
Light colored soils tend to drain well. Why?_sandy soils drain well. Why? (pore space)
3. Pore space: this means space between soil particles. Range from 25-60%
50% pore space is the best for growing plants
4. Chemistry: this means nutrients dissolved in the soil and the acidity (pH) of
soil. Acidity affects how easily nutrients can dissolve in soil.
III. Human activities affect soil
A. Soil is a necessary resource used for:
1.Growing food
2.Cleaning water
B. Land-use practices can harm soil.
1. Farming harms by:
a.fertilizing b. overgrazing/clearcutting
2. construction and development harms by: clearing trees
3. mining harms by: clearing trees/pollution
C. Soil can be protected and conserved. KNOW THE FOLLOWING:
1.Crop Rotation1
4. Contour plowing
.2. Conservation tillage
5. Wind breaks
3. Terraces
Chapter 4
2. desert– dry regions, soils are shallow and contain little organic matter