Soil physical and chemical properties

Important Properties of Soil

  • Physical Properties: Influence soil function and management.

    • Key properties:

      1. Colour

      2. Texture

      3. Structure

      4. Density

      5. Porosity

  • Chemical Property: pH

Soil Colour

  • Colour helps in estimating other soil characteristics, although it has little direct impact on functioning. Useful for classification.

  • Examples:

    • Podzolic B: Black or reddish hues, yellower with depth.

    • Chernozemic A: Darker than 5.5 dry, 3.5 moist.

  • Measured using the Munsell colour chart: Hue, Value, Chroma.

Factors Affecting Soil Colour

  • Organic matter content

  • Mineral composition

  • Drainage conditions (affects iron (Fe) and manganese (Mn)):

    • Well-oxidized conditions yield rust colors (hematite)

    • Reducing conditions yield grey or ashy colors.

Soil Texture

  • Defined by grain size distribution (sand, silt, clay).

    • Sand: 0.05mm – 2mm

    • Silt: 0.002mm – 0.05mm

    • Clay: < 0.002mm

  • Example calculation: Soil with 30% clay and 10% silt needs little sand to determine its total composition.

Influences of Soil Texture

  • Affects many soil functions, including:

    • Porosity: Spaces for air/water.

    • Aeration and Water Retention.

    • Nutrient retention and Pollutant Leaching.

    • Resistance to erosion and compaction.

Soil Structure

  • Arrangement of particles into aggregates (peds).

    • Influenced by biological and abiotic processes.

    • Types:

      • Unaggregated (structureless)

      • Aggregated (most common)

Importance of Soil Structure

  • Affects porosity and connectivity of voids.

    • More resistant to erosion and compaction.

    • Indicates soil health through stability of aggregates.

Density of Soil

  • Defined as mass per unit volume (g/cm³).

    • Bulk Density vs Particle Density.

  • Typical bulk density ranges:

    • Clayey soil < Silty soil < Sandy soil

Importance of Bulk Density

  • Indicates porosity and aeration, affecting water retention and root penetration.

  • Measurement methods: Core method and Excavation method.

Soil Porosity

  • Estimated from bulk density:

    • Porosity = volume of voids/volume of soil

    • Typical range: 30 – 60%

  • Factors affecting porosity:

    • Texture, Organic matter content, Depth, Disturbance.

Soil pH

  • Measures H+ ion activity; logarithmic scale (0 - 14).

    • Most favorable for plants: pH 5.5 - 7.

  • Influences nutrient mobility and contaminant availability.

Influences on Soil pH

  • Climate, vegetation, parent material, soil age, and human management practices (fertilizers, atmospheric deposition).

Conclusion

  • Key properties influencing soil function:

    • Colour

    • Density

    • Porosity

    • Texture

    • Structure

    • pH

  • Understanding these properties is crucial for effective soil management.