1. Section 5.2

2. Soil is an important product of weathering.
All life depends on a dozen or so elements that come from Earth’s crust.
Weathering produces a layer of rock and mineral fragments called regolith.
Soil is the part of the regolith that supports the growth of plants.
Three important characteristics of soil are its:
Composition
Texture
Structure

3. Soil has four major components:
Soil Composition:
Soil has four major components:
Mineral Matter (Broken-down rock)
Organic Matter (Humus – decayed organisms)
Water
Air
The percentages of the four major components varies greatly.
In most soils, organic matter (humus) is an essential component for plants nutrients and the soil’s ability to retain water.

5. 2. Soil Texture:
Most soils contain particles of different sizes.
Soil texture refers to the proportions of different particles sizes.
To classify soil texture, the U.S. Department of Agriculture has established categories based on the percentages of clay, silt, and sand in soil.
Texture strongly influences a soil’s ability to support plant life.

7. 3. Soil Structure:
Soil particles usually form clumps that give soils a particular structure.
Soil structure determines how easily a soil can be cultivated and how susceptible it is to erosion.
Soil structure also affects the ease with which water can penetrate the soil and thus influences the movement of nutrients to plant roots.

8. Which of the following is not a major component of soil?
Mineral matter
Air
Humus
Earthworms

9. A soil’s texture is determined by
Mineral composition
Type of humus
Water content
Particle size

10. The main source of organic matter in soil is
Water
Plants
Fungi
Bacteria

11. Soil forms through the complex interaction of several factors.
The most important factors in soil are:
Parent material
Time
Climate
Organisms
Slope

12. The source of the mineral matter in soil.
Parent Material:
The source of the mineral matter in soil.
May be either bedrock (residual soil) or unconsolidated deposits such as those in a river valley (transported soil).
The nature of the parent material influences soils in two ways:
It affects the rate of weathering and the rate of soil formation.
The chemical makeup of the parent material affects the soil’s fertility.
Fertility influences the types of plants the soil can support.

14. 2. Time:
The longer a soil has been forming, the thicker it becomes.
The parent material largely determines the characteristics of young soils.
As weathering continues, the influence of the parent material can be overshadowed by the other factors, especially climate.

15. 3. Climate:
Climate has the greatest effect on soil formation.
Variations in temperature and precipitation influence the rate, depth, and type of weathering.
The influence of climate is so great that soil scientists have found that similar soils can be produced from different parent materials in the same climate.
Dissimilar soils can be produced from the same parent material in different climates.

16. Burrowing animals mix the mineral and organic matter in soil.
4. Organisms:
The types of organisms and how many there are in a soil have a major impact on its physical and chemical properties.
Scientists name some soils – such as prairie soil, forest soil, and tundra soil – based on the soil’s natural vegetation.
Plants are the main source of organic matter in soil (animals/microorganisms are other sources).
Microorganisms (fungi, bacteria, and single-celled protozoans) play an active role in decomposing dead plants and animals (nitrogen gas into nitrogen compounds).
Burrowing animals mix the mineral and organic matter in soil.
Example: Earthworms – can mix thousands of kilograms of soil each year in a single hectare (10,000 square meters).

17. On steep slopes, erosion is accelerated.
The slope of the land can vary greatly over short distances, which results in different soil types.
Many of the differences are related to the amount of erosion and the water content of the soil.
On steep slopes, erosion is accelerated.
Little water soaks in, little to no plants, which results in thin or nonexistent soils.
In flat areas, there is little erosion and poor drainage.
Resulting in waterlogged soils that are typically thick and dark (large amounts of organic matter).

18. The direction the slope faces also affects soil formation.
In the temperate zone of the Northern Hemisphere, south-facing slopes receive much more sunlight than do north-facing slopes.
Soils on south-facing slopes are usually warmer and drier, which influences the types of plants that grow in the soil.

20. The factor that has the greatest effect on soil formation is
Climate
Parent material
Time
Slope orientation

21. In which of the following areas will soil formation be greatest?
A steep slope in a warm, wet climate.
A flat area in a cold, wet climate.
A flat area in a warm, wet climate.
A north-facing area on a steep slope.

22. Soil that forms on unconsolidated deposits is called
Transported soil
Humus
Residual soil
Bedrock

23. The processes that form soil operate from the surface downward.
Soil varies in composition, texture, structure, and color at different depths.
These variations divide the soil into zones known as soil horizons.
A vertical section through all of the soil horizons is called a soil profile.
In some soil profiles, the soil horizons blend gradually from one to another, in others, they are quite distinct.
Mature soils usually have three distinct soil horizons (A, B, and C Horizons).

24. A Horizon:
Commonly known as topsoil.
Upper part consists mostly of organic matter.
Full of insects, fungi, and microorganisms.
Lower part is a mixture of mineral matter and organic matter.
2. B Horizon:
Commonly known as subsoil.
Contains fine clay particles washed out of the A Horizon.
In some soils, the clay that accumulates in the B Horizon forms a compact, impenetrable layer called hardpan.
Is the lower limit of most plant roots and burrowing animals.
C Horizon:
Between the B Horizon and the unaltered parent material.
Contains partially weathered parent material.
Resembles parent material.

26. In a well-developed soil profile, which horizon is the uppermost layer?
The C Horizon.
The B Horizon.
The A Horizon.
The Parent Horizon.

27. What kind of material is found in the C horizon of a soil profile?
Partially weathered parent material.
Clay Particles.
Hardpan.
Mineral and organic matter.

28. How are soil horizons ordered from the top of the profile to the bottom?
A, C, B
A, B, C
C, B, A
B, A, C

29. The B horizon is also called the
Topsoil
Unaltered parent material
Partially altered parent material
Subsoil

30. Climate has a major effect on the type of soil that forms.
Three common types of soil are:
Pedalfer
Pedocal
Laterite
Pedalfer:
Usually forms in temperate areas that receive more than 63-cm (25 inches) of rain each year.
Present in much of the eastern half of the U.S., mostly in forested regions.
The B Horizons in pedalfers contain large amounts of iron oxide and aluminum-rich clays, giving it a brown to red-brown color.

31. 2. Pedocal:
Found in the drier western U.S. in areas that have grasses and brush vegetation.
Generally contains less clay than pedalfers.
Contain abundant calcite, or calcium carbonates, and are typically a light gray-brown.
3. Laterite:
Form in hot, wet tropical areas.
Are usually deeper than soils that develop over a similar period in temperate areas.
The large quantity of water that filters through these soils removes most of the calcite and silica.
Iron oxide and aluminum oxide are left behind.
The iron oxide gives laterite a distinctive orange to red color.
When dried, laterite becomes very hard and practically waterproof (bricks).

33. Laterite contains almost no organic material.
With the lack of organic material, the soil cannot support agriculture for more than a few years.
The nutrients that the soil does have are quickly washed out by the plentiful rainwater that filters through the soil.

34. A soil that is characteristic of the humid eastern U.S. is
Laterite.
Pedalfer.
Pedocal.
Humus.

35. The soil associated with the hot and wet tropics is
Laterite
Pedocal
Pedalfer
Bedrock soil

36. Laterite soils contain high amounts of
Organic material
Iron oxide
Calcite
Calcium carbonate

37. Pedalfer soils would most likely be found
On an island close to the equator.
In a tropical rainforest.
In the dry areas of the western U.S.
In the eastern half of the U.S.

38. Which of the following is not true of laterite soils?
They form in the wet tropics.
They are red in color.
They are enriched in iron oxide.
They are very productive agriculturally.

39. Soils are among our most abused resources.
The loss of fertile topsoil is a growing problem as human activities disturb more of Earth’s surface.
Water erodes soil every time it rains (tiny bombs).
When water flows across the surface it then carries away dislodged particles, which is called sheet erosion.

41. After flowing as a thin sheet for a short distance, the water forms tiny streams called rills.
As more water enters the rills, they erode the soil further, creating trenches known as gullies.

43. Human activities that remove natural vegetation, such as farming, logging, and construction, have greatly accelerated erosion.
Without plants, soil is more easily carried away by wind and water.
Scientists can estimate the rate of erosion due to water by measuring the amount of sediment in rivers.
These estimates indicate that before humans appeared, rivers carries about 9 trillion kilograms of sediment to the oceans each year.
The amount of sediment currently transported to the sea by rivers is about 24 trillion kilograms per year.

44. Wind generally erodes soil much more slowly than water does.
During a long drought, strong winds can remove large quantities of soil from unprotected fields.
Example: 1930’s Great Plains Dust Bowl.
The rate of erosion depends on soil characteristics and on factors such as climate, slope, and type of vegetation.
In many regions, including about one-third of the world’s croplands, soil is eroding faster than it is being formed.
This results in lower productivity, poorer crop quality, and a threatened world food supply.

45. Another problem caused by erosion is the deposition of sediment.
Rivers that accumulate sediment must be dredged to remain open for shipping.
As sediment settles in reservoirs, they become less useful in storing water, controlling floods, and generating electricity.
Some sediments are contaminated with agricultural pesticides.
Sediments also contain soil nutrients, which may come from natural processes and from added fertilizers.
Excessive nutrient levels in lakes stimulate the growth of algae and plants, which accelerates a process that eventually leads to the early death of the lake.

46. We can significantly slow erosion by using soil conservation measures:
Preserve environments
Protect the land.
These measures include planting rows of trees (windbreaks), plowing along the contours of hills, and rotating crops.
Preserving fertile soil is essential to feeding the world’s rapidly growing population.

47. Compared to the past, rates of soil erosion are
About the same.
Faster.
Slower.
More unpredictable.

48. Which of the following human activities has caused an increase in soil erosion?
Clear-cut logging.
Clearing land for construction.
Plowing land for farming.
All of the above.

49. The rate of soil erosion depends on
Climate.
Slope steepness.
The type of vegetation.
All of the above.

50. Since humans have appeared, the amount of sediments carried by rivers has
Increased dramatically.
Increased slightly.
Stayed about the same.
Decreased by about half.

51. What is the correct order for water eroding soil?
Gullies, rills, sheet erosion.
Sheet erosion, rills, gullies.
Sheet erosion, gullies, rills.
Rills, sheet erosion, gullies.