Soils

Introduction to Soils

• Notion of land as a commodity vs. community

  • Quote from Aldo Leopold: "We abuse land because we regard it as a commodity belonging to us. When we see land as a community to which we belong, we may begin to use it with love and respect."

I. Definition of Soil

• Soil as a natural body composed of:

  • Solids (mineral and organic materials)

  • Liquid

  • Gases

• Characteristics:

  • Occurs on the land surface and occupies space

  • Characterized by horizons/layers

  • Ability to support rooted plants in natural environments

• Soil is teeming with life:

  • Includes bacteria, fungi, animals, plant roots

• Pedon definition:

  • A 3-dimensional soil body large enough to study all physical properties, chemical properties, and all horizons of a soil

II. Soil Profile

• Six major horizons:

  • 1. O horizon:

  • Organic layer above the mineral layer

  • Composed of fresh or partially decomposed organic material

  • Usually absent in cultivated soils

  • Thickest in the fall

  • 2. A horizon (Topsoil):

  • Upper layer of mineral soil with high organic material

  • Accumulates organic material

  • Loss of clays, inorganic minerals, and soluble matter

  • O and A horizons, where most decomposition occurs

  • 3. B horizon (Subsoil):

  • Zone of illuviation (leaching)

  • Accumulates silicates, clay, iron, aluminum, & humus from the E horizon

  • Develops blocky, columnar, or prismatic shapes

  • Claypan: Very hard when dry, stiff when wet

  • Fragipan: A brittle, cementlike horizon low in organic material, high in silt or fine sand.

  • Both claypans & fragipans interfere with root and water penetration

  • 4. C horizon:

  • Contains weathered material

  • Can resemble, or differ from, parent material

  • Little affected by soil formation; some active weathering occurring

  • 5. R horizon:

  • Un-weathered bedrock

III. Physical Properties of Soils

• A. Color:

  • Provides information about the soil

  • Dark soils generally fertile (organic material presence)

  • Red and yellow soils indicate iron oxides (good drainage and aeration)

• B. Texture:

  • Determined by particle size classes:

  • Loam: Equal parts clay, sand, and silt.

    1. Sand: Gritty

    2. Silt: Feels like flour

    3. Clay: Controls plasticity and ion exchange

    4. Soils are negatively charged, tend to collect positive ions: Ca²⁺, Mg²⁺, K⁺, Al³⁺, and Na⁺

  • Ideal soil composition: 50% soil particles, 50% open pore space

  • Pore space includes spaces within and between particles, old root channels, and animal burrows

  • Coarse soil features large pore spaces leading to rapid water infiltration and drainage

  • Very fine-textured soils (clays) can be compacted easily

  • Adding sand or organic material can improve soil texture/structure

Soil Textural Triangle

• Visual representation of the relationship between clay, silt, and sand percentages in soils

• Example: A soil with 60% sand, 30% silt, and 10% clay is categorized as sandy loam.

IV. Soil Depth and Moisture

• C. Soil Depth:

  • Native grasslands typically have several meters deep soils

  • Forests usually have shallower soil profiles

  • Soil depth varies with geographical features such as slopes and floodplains

• D. Moisture:

  • Field Capacity: Maximum amount of water soil retains

  • Wilting Point: The point at which plants can no longer extract water, although water remains in soil

  • Available Water Capacity: Difference between field capacity and wilting point

V. Chemical Properties of Soil

• A. Importance of Clays:

  • Key in soil chemical processes including ion generation

• B. Ion Exchange:

  • Process influenced by clay particle charges and affinity for different ions:

  • Clays predominantly negative
    a) AL³⁺ → H⁺ → Ca²⁺ → Mg²⁺ → K⁺/NH₄⁺ → Na⁺ (Affinity order: AL³⁺ highest, Na⁺ lowest)

  • Feedback loop; roots often secrete H⁺ for nutrient uptake

• C. Cation Exchange Capacity (CEC):

  • Ability of soil particles (clay micelles, organic matter) to absorb positively charged ions (cations)

VI. Soil Formation: Important Factors

• 1. Parent Material: The unconsolidated material on which soil forms

• 2. Climate:

  • Temperature and moisture dictate physical weathering process

• 3. Topography:

  • Influences soil moisture and temperature

  • Slopes facing south/west are drier, north/east more moist

  • Erosion likely on steep slopes

• 4. Biota:

  • Adds organic matter, mixes soils, and supports microbial life which is key to mineralization

• 5. Time:

  • Continuous processes that allow interactions among these factors to form soil

VII. Soil Development Processes

• A. Additions:

  • Organic and inorganic material added to soil body

• B. Losses:

  • Erosion and leaching resulting in material loss

• C. Translocation:

  • Movement of material within soil horizontally or vertically

• D. Transformation:

  • Changes in minerals and organics; includes formation of soil particles into clusters (peds)

    • peds-soil particles held together in clusters of various sizes

VIII. Processes of Soil Formation

• Process List: Examples include eluviation, illuviation, calcification, decalcification, salinization

  • Eluviation -movement of material out of a portion of a soil profile (leaving materials out of soil)

  • Illuviation -movement of material into a portion of a soil profile (entering)

• IX. Soil Orders

• Major Soils:

  • Mollisol: Fertile soils, typically dark in color and rich in organic matter (one of the richest soils)

  • Alfisol: Fertile soils that typically leach less than ultisols

  • Ultisol: Highly weathered, low fertility; usually red in color

  • Oxisol: Old, weathered, typically in tropical regions (oxidized)

  • Histosol: Organic-rich; formed in wet conditions

• Understanding these soil components and properties is vital for sustainable land management, agriculture, and ecosystem health.

• Each major point underlines the intricate and essential role soils play in environmental sustainability and agricultural productivity.