3 Notes on Soil Structure, Densities, and Porosity Prelab

Soil Structure

  • Definition: Soil structure refers to the arrangement of primary soil particles into clusters known as aggregates, or peds.

  • Importance: The organization into aggregates helps define soil properties such as aeration, drainage, and the ability to support plant roots.

Types of Soil Structure

  1. Granular:

    • Description: Small aggregates that weigh less than ½ inch in diameter, lie loosely, and can be readily shaken apart.

    • Characteristics: Found in many surface soils; includes granular (porous) and crumb (very porous) types.

    • Subtypes:

      • Very Fine: < 1 mm

      • Fine: 1-2 mm

      • Medium: 2-5 mm

      • Coarse: 5-10 mm

  2. Plate-like (Platy, Lenticular):

    • Description: Characterized by thin horizontal aggregates (peds) that may be inherited from parent materials such as sediments.

    • Characteristics: Most prevalent in surface layers of virgin soils. Often found in moist soils with potential frost action.

  3. Block-like (Angular, Sub-angular Blocky):

    • Description: Fragments range from 0.5 to 4 inches, usually located in subsoil B HORIZON.

    • Characteristics:

      • Angular Blocky: Sharp edges and distinct rectangular faces.

      • Sub-angular Blocky: Rounded corners.

  4. Prismatic/Columnar:

    • Description: Vertical aggregates or pillars, which may be 6 inches or more in diameter, typically found in arid regions.

    • Characteristics: Well-developed structures can significantly influence the soil profile.

  5. Wedge:

    • Description: Elliptical units with interlocking lenses, forming acute angles, often are bounded by slickensides.

  6. Structureless Types:

    • Single Grained: Individual sand grains with little to no aggregation.

    • Massive: Continuous and dense solid mass of soil, often seen in certain subsoil horizons, particularly clayey soils.

Factors Affecting Soil Structure

  • Binding Agents: Organic matter, lime, iron and aluminum oxides, and clays contribute to the aggregation of soil particles.

  • Soil Texture: The interaction between soil texture and structure will modify properties such as air and water movement through soils.

Soil Density

  1. Particle Density:

    • Definition: Mass per unit volume of soil solids, unaffected by pore spaces.

    • Typical Range: 2.60 – 2.75 g/cm³, with an average around 2.65 g/cm³.

    • Influences: Higher with heavy minerals like magnetite and decreases with increased organic matter.

  2. Bulk Density:

    • Definition: Mass of a unit volume of dry soil, always smaller than particle density.

    • Typical Range: 1.1-1.4 g/cm³ for surface soils and 1.3-1.7 g/cm³ for subsurface soils.

    • Relationship: Varies indirectly with the total pore space in the soil (looser soils usually have lower bulk densities).

  3. Porosity (E):

    • Definition: The fraction of soil volume occupied by air and water.

    • Range: Generally between 30%-60% for most soils.

    • Calculation: Easily computed as a percentage of bulk soil volume.

Importance of Soil Structure on Water Movement

  • Soil structure significantly influences how water moves through soil. Structured soils facilitate better water infiltration and retention, impacting overall soil health and agricultural productivity.