Learning Objectives
After reading and studying this chapter, students should
• Understand soil terminology and the processes responsible for the development of soils.
• Understand soil fertility and the interactions of water in soil processes.
• Become familiar with soil classification.
• Understand the primary engineering properties of soils.
• Understand relationships between land use and soils.
• Know what sediment pollution is and how it can be minimized.
• Understand how soils affect land-use planning.
• Understand how we can sustain soil resources.
Chapter Summary
This chapter addresses numerous aspects of soil properties, soil-related processes, and the relationship
between soils and land use. The chapter opens with an overview of soils, their importance, and their
general characteristics. Subsequent sections address important soil properties, soil fertility, the behavior
of water in soils, and classification of soils. The heart of the chapter consists of an extensive discussion
of engineering properties of soils and the influences of those properties on land use. The engineering
properties are subsequently applied to discussions of soil erosion, the relationship between land use and
soil problems, soil pollution, and the importance of soil surveys in land-use planning.
Chapter Outline
I. Introduction to soils
A. Big question with soil: can agricultural systems maintain and improve soil fertility?
B. Soil
C. Study of soil properties and land use is increasingly important
1. land capability
3. natural hazards
4. historical records of climate changes
II. Soil profiles
A. Weathering
2. residual soil
4. soils are open systems
B. Soil horizons
2. soil profile consists of soil horizons
a. O horizon
b. A horizon
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c. E horizon
d. B horizon
III. Soil properties
A. Color
1. O and A horizons tend to be dark
3. B horizon colors vary from yellow-brown to dark red to light colored
5. soil color may indicate how well soil drains
B. Texture
2. commonly identified by estimation in the field
C. Structure
2. important indicator of development and age
D. Relative soil profile development
1. the level of development in a soil profile
a. weakly developed
E. Soil chronosequences
1. a series of soils arranged from youngest to oldest on the basis of their relative profile
development.
IV. Soil fertility
A. Soil as a complex ecosystem
1. soil fertility refers to capacity of a soil to supply nutrients
B. Soil fertility depends on setting and use
C. Soil is a basic component of an ecosystem
1. soil processes interact with biochemical processes and organisms
V. Water in soil
A. Water exists in pore spaces between soil particles
B. Amount and movement of water through soils are important research topics
VI. Soil classification
A. No soil taxonomy exists that includes engineering, physical, and chemical processes
B. Soil taxonomy
1. emphasizes physical and chemical properties
1. unified soil classification system
2. major divisions are coarse-grained, fine-grained, and organic
VII. Engineering properties of soils
A. Overview
2. soil usefulness is greatly affected by variations in proportions and structure of the three phases
B. Soil strength
2. strength is a function of cohesive and frictional forces
3. vegetation may influence soil strength
C. Soil sensitivity
1. changes in soil strength resulting from disturbances such as vibrations and excavations
3. abundance of fine materials influences sensitivity
D. Soil compressibility
2. partly a function of the elastic nature of soil particles
E. Erodibility
1. ease with which soil materials can be removed by wind or water
3. cohesive, cemented or gravel-rich soils have low erosion factor
F. Hydraulic conductivity
2. higher in sand and gravel, declines with abundance of fine particles
4. very important in many soil environmental problems
G. Corrosion potential
2. depends on soil chemistry, the buried material, and available water
H. Ease of excavation
2. three categories of excavation techniques
I. Shrink-swell potential
2. expansive soils
J. Soil properties determine potential uses
1. clay soils should be avoided for heavy structures
2. soils with high corrosion potential or that do not yield to common excavation should be avoided
VIII. Rates of soil erosion
A. Measurement
2. rates vary with properties, land use, topography, climate
B. Approaches to measurement
2. reservoir siltation analysis
3. Universal Soil Loss Equation (see A Closer Look: Universal Soil Loss Equation)
IX. Sediment pollution
Soils and Environment
2. fills lakes, reservoirs, etc.
4. creates nuisance that is difficult to remove
B. Sources
2. overgrazed land
4. land recently logged or burned
5. land disrupted by mining
C. Solutions (see Case History: Reduction of Sediment Pollution, Maryland)
X. Land use and environmental problems of soils
A. Overview
2. land use and sediment yield in Piedmont region has varied with land use
B. Agriculture
2. traditional agriculture is particularly damaging
3. practices to sustain soils
a. contour plowing
C. Urbanization
1. conversion of land for urbanization causes dramatic changes in land and streams
a. soil disturbed or removed
D. Off-road vehicles
2. problems associated with use are common
4. off-road bicycles and foot traffic also have impacts
XI. Soil pollution
A. Overview
1. occurs when detrimental materials are applied to soils
XII. Soil surveys and land-use planning
A. Soil survey
1. soil descriptions
3. test results
4. provides information for determining potential problems prior to construction
5. can be helpful in land-use planning
Answers to Review Questions and Critical Thinking Questions
Review Questions
1. To a soil scientist, soil is an altered solid Earth material that can support rooted plant life, whereas an
4. Soil texture is the relative proportions of sand, silt, and clay.
8. Shrink-swell potential is the tendency of a soil to gain or lose water, and its concomitant potential to
10. Sediment pollution is a significant environmental problem because it has widespread effects on water
bodies and their uses, on vegetation, and on general land use.
14. A soil survey is a compilation of soil descriptions, soil maps, and soil properties (determined by
testing) and is important in land use planning through rating of soils for specific uses.
Critical Thinking Questions
1. A defense of this statement will depend on a discussion of rates of soil erosion, in comparison with
2. Clear-cut logging and off-road vehicle use can lead to loss of soil fertility in two ways: by enhancing
3. Possible conflicts in this situation largely revolve around enhancement of soil erosion processes (see
answer to question 1), and the concomitant impacts to the local land surface and to impacts on
Soils and Environment
4. A plan to evaluate soil characteristics on this plot of land for organic farming should include a survey
of soil types, their chemical and engineering characteristics, their fertility, the potential for soil