1. Soils, Chapter 10
© Brooks/Cole Publishing Company / ITP

2. Chapter Overview Questions
What is soil?
How is soil formed?
What are the characteristics of soil?
What is the importance of soil as a resource?

3. SOIL: A RENEWABLE RESOURCE
Soil is a slowly renewed resource that provides most of the nutrients needed for plant growth and also helps purify water.
Soil formation begins when bedrock is broken down by physical, chemical and biological processes called weathering.
Mature soils, or soils that have developed over a long time are arranged in a series of horizontal layers called soil horizons.

4. Weathering
Weathering- when rocks are exposed to air, water, certain chemicals or biological agents that degrade the rock.
Physical weathering- the mechanical breakdown of rocks and minerals.
Water gets into cracks in rock, where it expands when it freezes. Growing plant roots can pry rock sections apart.

5. Weathering
Chemical weathering- the breakdown of rocks and minerals by chemical reactions.
Rain water contains carbonic acid, which wears away limestone

6. Erosion
Erosion- the physical removal of rock fragments from a landscape or ecosystem. Wind, water, ice transport and living organisms can erode materials.
Deposition- the accumulation or depositing of eroded material such as sediment, rock fragments or soil.
Soft rocks, such as sedimentary rocks, erode more easily than metamorphic and igneous rocks.

7. Soil Resources
Soil- weathered rock mixed with organic material (humus), mineral nutrients, microorganisms, water and air
Soil is important because it
Is a medium for plant growth
Serves as a filter for water
Is a habitat for living organisms
Serves as a filter for pollutants

8. Ecosystem Services of Soil

9. Factors That Determine the Formation of Soil
Parent material- what the soil is made from influences soil formation
Climate- what type of climate influences soil formation
Topography- the surface and slope can influence soil formation
Organisms- plants and animals can have an effect on soil formation
Time- the amount of time a soil has spent developing can determine soil properties.

11. Wood sorrel Oak tree Organic debris builds up Lords and ladies
Dog violet
Rock
fragments
Grasses and
small shrubs
Earthworm
Millipede
Fern
Moss and
lichen
Honey
fungus
O horizon
Mole
Leaf litter
A horizon
Topsoil
B horizon
Bedrock
Subsoil
Immature soil
Regolith
Young soil
C horizon
Figure 3.23
Natural capital: soil formation and generalized soil profile. Horizons, or layers, vary in number, composition, and thickness, depending on the type of soil. (Used by permission of Macmillan Publishing Company from Derek Elsom, Earth, New York: Macmillan, Copyright © 1992 by Marshall Editions Developments Limited)
Pseudoscorpion
Mite
Parent
material
Nematode
Root system
Actinomycetes
Red Earth
Mite
Fungus
Mature soil
Bacteria
Springtail
Fig. 3-23, p. 68

12. Soil Horizons
O horizon – surface litter layer. Contains organic material in various stages of decomposition (leaves, twigs, etc.)
A horizon – top soil. Contains humus and mineral particles. Where overlying organic material mixes with underlying inorganic material
B horizon – subsoil. Zone of accumulation of metals and nutrients
C horizon – least weathered portion of profile, same composition as parent material
Bedrock or parent material – the rock material from which soil is derived.

13. Soil Horizons
As soils form, they develop characteristics layers.

14. Layers in Mature Soils
Infiltration- the downward movement of water through soil.
Leaching- dissolving of minerals and organic matter in upper layers carrying them to lower layers.
The soil type determines the degree of infiltration and leaching.

15. Animation: Soil Profile
PLAY
ANIMATION

16. Soil Profiles of the Principal Terrestrial Soil Types
Figure 3-24

17. Physical Properties of Soil
Texture- the percentage of sand, silt and clay the soil contains.
Soils vary in the size of the particles they contain, the amount of space between these particles, and how rapidly water flows through them.

18. Soil Triangle
Soil texture can be determined using the soil triangle

19. Physical Properties of Soil
Soil texture determines porosity, permeability and structure of soil
Porosity- measure of volume of pores per volume of soil
Permeability- rate at which water and air move from upper to lower soil layers
Structure- way in which soil particles are organized and clumped together

20. Physical Properties of Soil
Porosity- the ratio of the volume of pore space in a unit of material to the total volume of material.
Which has the highest porosity?

21. Physical Properties of Soil
Permeability- how quickly the soil drains (which depends on its texture)
Sand has large, loosely-packed particles and drains quickly. Clay, with its platy particles, drains much more slowly.

22. Chemical Properties of Soil
pH- measure of how acidic or basic a substance is. It determines plants’ ability to take up nutrients from soil
Cation exchange capacity- the ability of a soil to adsorb and release cations (positively charged mineral ions)
Soil bases- calcium, magnesium, potassium and sodium
Soil Acids- aluminum and hydrogen

23. Biological Properties of Soil
Many organisms are found in the soil including fungi, bacteria, protozoans, rodents and earthworms. 23

24. Soil type affects vegetation: trees are growing on soil formed from sandstone that holds water

25. Differences in rocks, soil and microclimates determine the plants that live on two sides of the same mountain.

26. Soil Degradation
Soil degradation- loss of some or all of soil’s ability to support plant growth
Causes:
Soil erosion
Soil compaction (machines, foot traffic or overgrazing)
Salinization- excess salts added to soil through incorrect irrigation practices
Desertification- transformation of arable land into desert or unproductive land

27. Review Questions How do soils form?
What are the roles of soils in ecosystems?
How do a soil’s physical and chemical properties influence its role as a medium for plant growth?