Soil Density

Density is defined as mass per unit volume. It is a critical property for understanding soil compaction and pore space. There are two primary types:

Particle Density (True Density):

• Definition:The weight per unit volume of the solid portion of soil only (excluding pore space).
• Typical Value:For normal mineral soils, it is approximately 65 g/cm³.
• Influencing Factors:
  • Increaseswith the presence of heavy minerals (e.g., magnetite, limonite, hematite).
  • Decreaseswith an increase in organic matter content or due to tillage operations.
• Examples:Silt (~2.79 g/cm³), Clay (~2.83 g/cm³).

Bulk Density:

• Definition:The oven-dry weight of a unit volume of soil including its pore space.
• Formula:Bulk Density = Oven Dry Weight / Total Volume.
• Typical Values:
• Mineral soil: ~33 g/cm³
• Sandy soil: ~6 g/cm³
• Organic soil: ~5 g/cm³
• Key Relationship:Bulk density is always smaller than particle density. It is often about half the particle density.
• Influencing Factors:Decreases with tillage operations and increased organic matter.

Soil Porosity and Pore Space

• Pore Space:The volume in the soil mass not occupied by solid particles, which is filled with air and water.
• Porosity: The percentage of the total soil volume that is occupied by pore space.
• Formula: Porosity = 100 – [(Bulk Density / Particle Density) × 100]

Types of Pore Space:

• • Macro-pores (Non-capillary pores):Large pores that allow free movement of air and water. Predominant in sandy soils.
  • Micro-pores (Capillary pores):Small pores where the movement of air and water is restricted. Predominant in clay soils.
  • Ideal Condition:A healthy soil should have a balance of both macro and micro pores.

Soil Temperature and Thermal Properties

• Sources of Soil Heat:
  1. Solar Radiation (primary source)
  2. Conduction
  3. Biological and Chemical Reactions
  4. Rain

Thermal Conductivity: Definition:The amount of heat flowing through a unit cross-section of soil in unit time.

Influencing Factors:

• Higher in sandy soils compared to clay soils.
• Increases with soil moisture content.
• Decreases with the addition of organic matter and tillage.

Specific Heat: Definition:The amount of heat required to raise the temperature of a unit mass by one degree.

Values:

• Water: 1 cal/g
• Organic Matter: 0.5 cal/g
• Soil: 0.2 cal/g

4. Soil Plasticity and Consistency

• Soil Plasticity:

• Definition: The ability of soil to deform without cracking and retain its new shape.
• Measurement: Determined using a liquid limit device.
• Occurrence: Exhibited by soils containing more than 15% clay.
• History: The plasticity index was defined by Atterberg in 1911.

• Soil Consistency:

• Definition: The resistance of soil to mechanical stress at different moisture levels.
• Order of Resistance: Dry soil > Moist soil > Wet soil.
• Soil Consistence (Forces of Attraction):

• Cohesion:Attraction between substances of the same kind (e.g., water molecule to water molecule).
• Adhesion:Attraction between unlike substances (e.g., soil particle to water molecule).

5. Soil Colour: Soil colour is a simple but important indicator of soil composition.

• Indicators:
  • Black/Dark Grey:High organic matter
  • Red:Presence of ferric oxide (iron rust).
  • White:Presence of silica, lime, or other salts.
• Measurement:Determined using a Munsell Colour Chart.

• Colour Components:
• Hue: The dominant spectral colour (e.g., Red, Yellow).
• Value: The lightness or darkness of the colour.
• Chroma: The purity or intensity of the colour.