Notes on soils

Chapter 18 in the textbook is concerned with the geography of soils. Many students have difficulty imagining why anyone would find soil interesting. Consider, however, that the soil is the earth's skin, almost a living organism, and that it mediates many of the environmental processes that are essential to the survival of life on earth and that affect other important earth systems. It is the very thin layer of weathered material, altered by physical, chemical, and biological processes, that we find at the earth's surface and at the boundary between the atmosphere and the underlying solid earth. It plays a role in the hydrologic cycle; in the weathering of rock to produce sediment and dissolved ions; in cycling of nutrients, carbon and other elements; in the accumulation and breakdown of organic matter, which in turn may affect the composition of the atmosphere; in the production of microbial and plant life, on which so much of the rest of the biosphere depends. There is so much to the story that we can't possibly do more than scratch the surface, and we won't even try to do so in our coverage of this topic.

Before we can begin discussing the properties of soil, we need first to understand some basics about weathering processes, which are responsible for the transformation of rock into the weathered parent material from which soil is made. This requires that we look through a short reading from chapter 13, inclulding pages 403-409.

Next read chapter 18 through page 567. Most of the remainder of the chapter focuses on a review of the 11 major soil orders. This is an important classification scheme, but there is no way we can keep it all straight unless we spend more time on it, so we will focus on description of a "typical" soil, its properties, and the dominant processes involved in soil formation. We will not have time to study the different soil orders and will simply point out why there are so many different kinds of soils.

Topics:

    Chapter 13, p. 403-409

• Regolith - the loose weathered mantle at the earth's surface
• Weathering as a set of processes occurring at or immediately beneath the earth's surface
• Factors influencing weathering rates and processes: frequency of joints and fractures, slope angle, vegetation, climate
• Types of weathering processes: physical and chemical weathering
• Physical weathering processes: frost action, pressure release (also known as sheeting or exfoliation),  thermal expansion, salt crystallization, hydration
• Chemical weathering processes: carbonation and solution, hydrolysis, and oxidation
• acidic pH of natural rain water and  the role of atmospheric CO2 in formation of carbonic acid
• How chemical weathering affects rocks: disintegration of rocks and o mineral grains by alteration of chemical composition and weakening of chemical bonds; the pervasive importance of water

    Chapter 18, p. 557-567

• Definition of soil and the difference between soil and regolith
• Soil profiles and basic units of analysis: pedons and soil horizons; solum as distinct from underlying weathered parent material
• Properties of the O, A, E, B, and C horizons
• Composition of soils (mineral grains, organic matter, water, gases) and processes of soil formation:
• biological processes
• organic-matter accumulation and formation of humus
• percolation of water through the soil; leaching and eluviation; chemical precipitation and illuviation
• Characteristic properties of soils:
• color
• texture (including definition of particle-size classses); sand-silt-clay ternary diagram
• structure
• consistence/cohesion
• porosity and moisture status (discussed in chapter 9)
• soil chemistry:
• cations and anions
• importance of base cations as plant nutrients; cation exchange capacity (CEC) and the role of colloids in the soil
• soil pH and acidity-alkalinity status
• Factors of soil formation
• parent material
• climate and moisture availability
• topographic position
• time/age
• human activities and their impacts on soil properties and soil degradation
• Soil taxonomy: the basis for classification
  • relationship between soil types, vegetation types and climatic factors, e.g.
    • highly weathered lateritic soils in tropical wet environments
    • acidic podzolic soils in northern coniferous forests
    • base-rich soils in grassland environments with calcified layers in B horizon
    • arid-region soils: thin, affected by salinization, low organic-matter content

Soil Systems by Michel Ritter at University of Wisconsin-Stevens Point. This is part of his introductory Physical Geography course.

Visit the on-line set of course notes from the introductory soils course by Terry Cooper at the University of Minnesota. The Table of Contents for Soils Laboratory Units and the
Lecture Notes for Basic Soils
contain some excellent material that is easy to read and will give you a good sense of what soils are all about, how they form, and how soil characteristics are mapped for purposes of land management and planning. Units 1 through 4 from this site are required in conjunction with your reading of chapter 18 in our textbook.

Soil Survey Manual
Soil Landscapes of Canada
Soil Geography
Soil characterization field guide
Soil characterization protocols: a step by step guide (read the section on "Horizon Properties")
Soils lab table of contents
Natural Resources Conservation Service Data Resources
The Twelve Soil Orders  from University of Idaho
Keys to Soil Taxonomy from the Natural Resources Conservation Service