SCSC 301 Chapter 4

Porosity

  • Macropores (> 0.08 mm)

    • Act as highways for water and air

    • Water drains out via gravity

    • Filled with water only when saturated

  • Micropores & Mesopores (< 0.08 mm)

    • Small pores that retain water tightly, drying last

    • Common in fine-textured soils (e.g., clay)

    • Slow movement of water and air

    • Water moves by capillary action against gravity

% Pore Space

  • Use 2.65 g/cm³ for particle density (PD)

  • Bulk density (BD) needed for calculation:
    (1−BDPD)⋅100=% pore space\left(1-\frac{BD}{PD}\right)\cdot100=\%\text{ pore space}

  • BD = total soil volume / mass of dry soil​

Effects of Tillage on Bulk Density and Porosity

  • Short-term: Decreases BD and increases porosity

  • Long-term: Increases BD and decreases porosity, especially macroporosity

    • Weakens soil structure

    • Depletes organic matter that helps bond aggregates

Practices to Reduce Tillage Impact

  • Conservation Tillage: Minimize tillage and maintain surface residues

Soil Color

  • Hue Value/Chroma

    • Hue: Color indication

      • Reds: oxidized and dry (hematite)

      • Yellows: oxidized and moderately wet (goethite)

      • Grey/green/blue: reduced and very wet (reduced Fe)

    • Value: Darkness to lightness (0-10)

      • 4 or less indicates possible organic matter presence

    • Chroma: Color intensity (0-10)

      • 6 or more with R or YR hue suggests red or orange

Mottles

  • Spots indicating differing Fe redox environments, reflecting changing moisture conditions.