Chapter 12: Weathering, Erosion, and Soil Resources

Question 1

What is soil?

Answer 1

all unconsolidated material that overlies bedrock, material capable of supporting plant growth

Question 2

How is soil produced?

Answer 2

produced by weathering: the chemical, physical, and biological processes that break down rocks and minerals

Question 3

Mechanical Weathering

Answer 3

(physical weathering) anything that breaks up a rock without altering its composition; (ice, wind, water). Breaks larger chunks of rock into smaller ones, increasing the exposed surface area.

Question 4

Chemical Weathering

Answer 4

Break down of minerals by chemical reactions with water, other chemicals dissolved in the water, and gases in the air.

Question 5

What conditions make weathering more rapid?

Answer 5

Higher amounts of water (wetter climate), higher temperatures (warmer climate): chemical reactions proceed more rapidly at higher temperatures than at low ones

Question 6

How does soil form?

Answer 6

1. chemical and physical weathering of rocks produce debris.
2. mildly acidic rain percolates through the debris and dissolves parts of it, carrying those ions deeper into the ground

Question 7

Zone of Leaching

Answer 7

the layer of soil from which ions have been extracted or removed, this is an upper soil layer

Question 8

Zone of Accumulation

Answer 8

the ions that have been leached out precipitate deeper in the soil column forming new minerals and clays in a region, this is a deeper soil layer

Question 9

Microbes interaction with soil

Answer 9

microbes further weathering and soil breakup, when they die they accumulate in the soil to form an organic-rich soil material (humus) found in the “O” horizon

Question 10

Soil Profile

Answer 10

a cross-section of soil reveals layers consisting of horizons

Question 11

A Horizon

Answer 11

1. The “O” in O horizon stands for “organic layer” and consists of organic matter, or humus.
2. The O and A horizons together make up topsoil, the fertile portion of soil that farmers till for planting crops.
3. Below the A horizon is the E horizon.

Question 12

What zone do O, A, and E horizons form?

Answer 12

form the zone of leaching because rainwater percolates through this initial layer, dissolves soluble minerals, and carries them away

Question 13

B Horizon

Answer 13

Lies in the zone of accumulation, B horizon is called the subsoil, where ions, clay minerals, and iron oxides accumulate. Organic matter from the surface is largely absent from this layer.

Question 14

C Horizon

Answer 14

Consists of material from the underlying bedrock that has undergone some weathering, and consists of mostly broken-up bedrock.

Question 15

What is soil composition influenced by?

Answer 15

1. Climate: large amounts of rainfall and warm temperatures accelerate chemical
   weathering and lead to heavily leached soils. Cooler temperatures and lower rainfalls may inhibit soil formation by slowing down the weathering process.
2. Substrate composition (i.e., what is the bedrock that underlies the composed of): soils can form on basalt, granite, volcanic ash, and quartz sand; leading to soils of different compositions.
3. Time: in temperate regions, it takes approximately 15,000 years to form 1 meter of soil. This estimate is based on glacial evidence.
4. Vegetation: plant roots help prevent soil from washing away

Question 16

Color of Soil

Answer 16

color reflects the minerals which make it up, the soil’s composition. black soils are rich in organic matter, red soils are rich in iron, and white soils are depleted in organic matter

Question 17

What is soil texture and the three components?

Answer 17

The texture is the size of the soil fragments, Components: sand, silt, clay (larger > small)

Question 18

Loam

Answer 18

a soil that has a roughly equal mix of sand and silt and 10-30% clay

Question 19

Soil Structures

Answer 19

How well soil clumps together forming a structure called ped. Soil that forms peds are typically more resistant to erosion.

Question 20

What are the two broad categories of soil classification?

Answer 20

Pedalfer and Pedocal

Question 21

Pedalfer Soils

Answer 21

• reflects soil in more humid regions
• soils are more leached leaving the B horizon - more enriched in aluminum and iron minerals
  usually acidic
• in North America, these soils are found in the southwestern US

Question 22

Pedocal Soils

Answer 22

• reflects soils in drier regions
• soils are less leached, even soluble minerals such as calcium carbonate remain in the B horizon
• usually more alkaline, due to the presence of calcium carbonate
• in North America soils found in southwestern US

Question 23

Lateritic Soil

Answer 23

Heavily leached, very oxidized, an extreme pedalfer

Question 24

Where are Lateritic Soils commonly found?

Answer 24

common in rainforests, laterite soils develop in tropical climates with high temperatures and heavy rainfall

Question 25

Wetland Soil

Answer 25

• contains an abundance of organic matter, the soil is soft
• acts as settling ponds reducing pollution from sediments carried in from water passing through, and also help serve as retention ponds for floodwaters
• many are drained for farmland or development, and others are threatened due to the rising global sea level.

Question 26

Weathering

Answer 26

breakdown of rock or minerals material in place

Question 27

Erosion

Answer 27

physical removal of material from one place to another (can occur through wind, rain, surface runoff, steep slopes, etc.)

Question 28

Dust Bowl

Answer 28

a portion of the American Midwest that experienced a farming crisis in the 1930s brought about by poor farming practices, and the destruction of the natural topsoil due to intensive farming

Question 29

Loss of topsoil in farming leads to several consequences

Answer 29

1. loss of organic matter and nutrients, native topsoil is more permeable and has better structure for crops
2. Lower crop yields (6” of topsoil loss in western TN reduced corn yields by over 40%).
3. Further reliance on fertilizer and other chemicals.
4. persistence of toxic residues of herbicides and pesticides in the soil (microorganisms that would normally break
   these substances down are gone).
5. Increased soil erosion leads to sediment pollution: eroded soil may contain agricultural herbicides and toxic
   chemical residues that ultimately end up in rivers and lakes, decreasing water quality, harming wildlife, and
   ultimately having to be dredged.

Question 30

Soil erosion impact on soil formation

Answer 30

soil erosion is stripping away farmland far faster than the soil is being replaced: soil and high-quality farmland is a nonrenewable resources in many populated areas

Question 31

Strategies for Reducing Erosion

Answer 31

1. Reduce erosion rate: contour plowing to reduce surface runoff
2. protect soil from agents of erosion
3. disadvantages: high short-term cost to the farmer; reduces crop yield, may lose out to competitive practices in other states