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Plug Into Any Course Soils Fundamentals Module
Fall and Spring
Contact Information
Email:mcgahan@tarleton.edu
Office:Joe W. Autry (Agriculture) Building, 203A
Phone:254-968-9701

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Preparing for Your Class

Module: "How to start thinking soil science"

This module is designed to offer a firm set of facts that every student involved in agronomic, or environmental sciences should know, by heart.

The information offered below is presented in a condensed manor suitable for flash card study and examinations can be drawn from every bit of this.

What is a-soil?

  • A dynamic (i.e.,continuously changing) natural body composed of mineral materials, organic materials, gases, liquids and living organisms which can serve as a medium for plant growth
  • Soil is one of our "ultimate" resources, like water and air
  • Soil is a slowly renewable resource
  • In fact it is very slowly renewable. Therefore, it should be treated as a non-renewable resource
  • A soil is both an ecosystem in itself and a critical part of the larger terrestrial ecosystem

Functions of soils in our ecosystem

  1. Medium for plant growth for food, fiber, forage & fuel
    •  source of a) support, b) nutrients, c) water, and d) air
  2. Recycling of raw materials
    • organic wastes are broken down and nutrients are released
  3. Habitat for soil organisms
    • a handful of soil contains billions of organisms from thousands of species
  4. Regulator and purifier of water supplies
    • water storage and cleansing
  5. Engineering medium
    • building material and support for building

What is a typical soil composed of?

Approximately 50% solids and 50% pore space

  1. Mineral (inorganic) materials
    • decomposed rocks (45-50% -percent by volume) 
  2. Organic materials
    • remains of plants and animals (0-5%) 
  3. Water (20-30%)
  4. Air (20-30%)

Communicating soil information

There are about 19,000 different soils identified in the USA alone. In order to compare and contrast them, we must be able to describe them in a systematic, accurate and concise way

  • Soil Profile: A two-dimensional vertical slice of the soil
  • Soil Pedon: A three-dimensional soil body
  • Horizons: Horizontal layers approximately parallel to the earth's surface having differing chemical, physical, biological, and morphological properties (The correct term is Genetic Horizon)

Each soil is characterized by a given sequence of horizons

Soil Morphology

The study of structure or form; properties characterized by sight, feel, smell or sound

  • Color
  • Texture
    • distribution of particle sizes (sand, silt, and clay)
  • Structure
    • how particles are arranged
    • grade, size and type
  • Consistence
    • the ability of particles to stick together (adhesion and cohesion forces)
  • Pores - size and abundance
  • Roots - size and abundance

Description of Soil Horizons

Nomenclature to describe soil horizons

Example = Bt2 horizon

  1. Capital letters - master horizons (A, B, C, O, E, R see below).
  2. Lower case letters - specific characteristics or subdivisions of the master horizon (link to come later).
  3. Arabic numerals - further subdivision of horizons with similar features.

Master Horizons

  • O horizon
    • a surface layer dominated by organic materials (> 20% organic carbon)
    • Oi - slightly decomposed organic matter; can still identify the original plant & animal remains
    • Oe - intermediately decomposed
    • Oa - highly decomposed; can not identify the original source of the organic material
  • A horizon
    • mineral horizon (<20% organic C) which forms at the surface or beneath an O horizon
    • characterized by a darker color than the rest of the profile due to the accumulation of organic matter; high biological activity
    • eluvial horizon (loss of materials such as iron/aluminum oxides and clays)
  • E horizon
    • an intensively leached eluvial horizon in which organic matter along with iron/aluminum oxides and clay have been removed; most commonly found in forest soils
    • typically white or light gray in color due to the lack of coatings on the mineral surfaces
  • B horizon
    • horizon formed beneath an A, E or O horizon and is a zone of accumulation (illuvial horizons). May accumulate clay, iron/aluminum oxides, organic matter, carbonates, etc.
  • C horizon
    • a layer of unconsolidated material showing little weathering (alteration) and biological activity (e.g., beach sand, alluvium deposited by rivers, glacial till deposited by glaciers)
  • R layers
    • consolidated rock that can not be dug with a shovel and shows little evidence of weathering (e.g., granite, sandstone)
  • L layers
    • layer can include organic and limnic (limnic = Of or pertaining tofresh waterfreshwater) materials. deposited in water by precipitation or through the actions of aquatic organisms, such as algae and diatoms, or derived from underwater and floating aquatic plants and subsequently modified by aquatic animals. They are used only in Histosols (organic soil)
  • M layer
    • root limiting subsoil layer consisting of nearly continuous, horizontally oriented, human-manufactured materials
  • W layer
    • indicates water layer(s) within or beneath the soil. [Wf if frozen]

Transition horizons

horizons that contain properties of two types of master horizons

Example =AB horizon - has a dark color due to organic matter (A-like), plus red color due to accumulation of iron (B-like)

  • Common transition horizons: AB, BA, BC, CB
    • The dominant horizon is listed first
  • Not every soil contains all of the master horizons.
  • O horizons form preferentially under forest vegetation; often absent under grass vegetation
  • Soils that have been eroded may be missing their O or A horizon
  • A horizon may be missing in some forest soils (e.g., O-E-B-C)
  • B horizon may be missing in young soils (e.g., A-C); B horizons take a long time to form

Additional Terminology

  • Solum - the zone of active soil formation; comprised of the A, E, and B horizons
  • Topsoil - the surface layer of the soil -the layer manipulated by tillage; typically the upper 10-25 cm
  • Subsoil - the soil layers beneath the topsoil (does not include the C horizon)

Note: Topsoil is a term that is depreciated in soil science

  • A soil manipulated by tillage typically has an A horizon. Call it an A horizon (or A horizon material)
  • Topsoil purchased at a materials yard may not be soil at all!

This page was lasted edited on February 27, 2012.

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This is a soils module for the "Fundamentals of Crop Production" course. This information is applicable to any agronomic endeavor or environmental studies that deal with land and soils.

Course Fees: $45

Laboratory Fees: $5

Required Text(s):

  • not for this module