Unit 24

PHYSICAL PROPERTIES OF SOIL

Unit Overview

This unit examines the different physical properties of soil. These properties vary over space and time. The main sections are as follows:

  • Sol and ped
  • Soil texture
  • Soil structure
  • Soil colour
  • Soil acidity and alkalinity
  • Soils of hills and valleys
  • The soil-development system

"Sol" and "ped" are two common terms encountered when studying soils. Texture, which includes the size of the particles in the soil, alters the porosity and permeability. For example, a "clay" soil has a low permeability.

Regarding structure, a soil can have either a platy, prismatic, blocky/angular, or spheroidal/granular structure. The structure can greatly affect a soil's resistance to erosion. Soils can also have a range of different colours. The acidity of a soil is controlled by the presence of hydrogen, and the acidity, in turn, affects the fertility of soils. In addition, topography plays a large role in soil formation. Finally, the soil-development process results in soils composed of the following ingredients: organic matter, resistant residue (often silica, such as quartz particles), altered chemical compounds, and soil solution.

Teaching Objectives

  • To introduce terminology used to describe soil characteristics
  • To define some important properties that result from a soil's physical characteristics
  • To illustrate the likely arrangement of soil characteristics in a hypothetical landscape


Glossary of Key Terms

Blocky (angular) structure Involves irregularly shaped peds with straight sides that fit against the flat surfaces of adjacent peds, thereby giving a soil considerable strength.
Field capacity The ability of a soil to hold water against the downward pull of gravity; also the maximum amount of water a soil can contain before becoming waterlogged.
Loam A soil containing grains of all three texture size categories- sand, silt, and clay; however, refers not to a size category, but to a certain combination of variously sized particles.
Pedon A column of soil drawn from a specific location, extending from the 0 horizon (if present) all the way down to the level where the bedrock shows signs of being transformed into C-horizon material.
Platy structure Involves layered peds that look like flakes stacked horizontally.
Prismatic structure Involves peds arranged in columns, giving a soil vertical strength.
Soil catena Derived from a Latin word meaning chain or series, refers to a sequence of soil profiles appearing in regular succession on landform features of uniform rock type; most frequently associated with hillsides where the same parent material has produced an arrangement of different soil types.
Solum Consists of the A and B horizons of a soil, which are the part of the soil in which plant roots are active and playa role in the soil's development.
Spheroidal (granular) structure Involves peds that are usually very small and often nearly round in shape, so that the soil looks like a layer of bread crumbs.


Unit Outline

  • Sol and ped
    • Sol
      • Russian word for soil
      • solum: the A and B horizons where plant roots are active
    • Ped
      • Greek word for ground
      • pedon: column of soil, extending from the O horizon to the bedrock
  • Soil texture
    • Aspects of soil-particle size
    • Sand-silt-clay size hierarchy
    • Particle-size combinations: loam
    • Soil porosity
    • Soil permeability
    • Field capacity
  • Soil structure
    • Structural typology
      • platy
      • prismatic
      • blocky (angular)
      • spheroidal (granular)
    • Soil consistence
  • Soil Colour
  • Soil acidity and alkalinity
    • Acid-alkaline pH scale
    • Moisture conditions
      • acidic soils-moist climates
      • alkalinic soils-drier climates
    • Soil fertility implications
  • Soils of hills and valleys
    • Topography and soil formation
    • Slope and soil horizon
    • Soil catenas
  • The soil-development system
    • Diagrammed and discussed on pp. 314-315


Review Questions

  1. What are the major size categories of soil particles and how do they impact a soil's overall physical properties?
  2. What are the four basic types of soil structure, and how are they distinguished form each other?
  3. What generalizations can you make about slopes and soil formation?