2.3 Soil porosity and bulk density

Soil porosity and movement are crucial aspects of soil physics. They determine how water and air flow through soil, affecting plant growth and nutrient cycling. Understanding these properties helps us manage soils for optimal agricultural and environmental outcomes.

Bulk density and porosity are interconnected soil properties. As bulk density increases, porosity decreases, impacting water retention and root growth. Measuring these properties helps assess soil health and guides management decisions to improve soil structure and function.

Soil Physical Properties

Soil porosity and movement

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• Soil porosity fraction of soil volume occupied by pores or voids typically expressed as percentage calculated as $Porosity = (1 - \frac{Bulk Density}{Particle Density}) \times 100\%$
• Water movement determines infiltration rate affects holding capacity influences hydraulic conductivity (permeability, drainage)
• Air movement facilitates gas exchange supports root respiration enables microbial activity (nutrient cycling, decomposition)

Macropores vs micropores

• Macropores (>0.08 mm diameter) formed by root channels earthworm burrows and cracks allow rapid water movement promote aeration and root growth quickly lose water under gravity (preferential flow paths, rapid drainage)
• Micropores (<0.08 mm diameter) found within soil aggregates hold water against gravity through capillary action provide water storage for plant use contribute to soil water retention (field capacity, wilting point)

Bulk density and porosity relationship

• Bulk density mass of dry soil per unit volume expressed in g/cm³ or Mg/m³ calculated as $Bulk Density = \frac{Mass of Dry Soil}{Total Soil Volume}$
• Inverse relationship as bulk density increases porosity decreases ideal bulk density varies by soil texture (sand: 1.3-1.7 g/cm³, clay: 1.1-1.5 g/cm³)
• Factors affecting bulk density:
  • Soil texture coarser textures generally have higher bulk density
  • Organic matter content reduces bulk density improves soil structure
  • Soil structure well-structured soils have lower bulk density
  • Management practices compaction increases bulk density reduces porosity (tillage, heavy machinery)

Methods of porosity measurement

• Core method:
  1. Use metal cylinder of known volume to collect undisturbed soil sample
  2. Dry sample in oven at 105℃
  3. Weigh dried sample
  4. Calculate bulk density using mass and volume
  5. Determine porosity using particle density
• Excavation method:
  1. Dig hole and line with plastic
  2. Measure volume of water needed to fill hole
  3. Collect and dry all excavated soil
  4. Weigh dried soil
  5. Calculate bulk density using mass and volume
  6. Estimate porosity using particle density
• Particle density determination pycnometer method or assumed average of 2.65 g/cm³ for mineral soils (quartz density)