2 - 3 Soil Physical Properties and Water

Introduction to Soil Physical Properties and Hydrology

• The lecture covers the fundamental properties of soil that determine its suitability for plant growth, bioremediation, and construction.

• Significance of Soil Choice: Soil properties underpin its role as a medium for agriculture, a tool for mitigating pollution, or a foundation for infrastructure.

• Recommended Resources:

  • Textbook: Chapters 8 and 9 cover general soil properties, soil water flow, and water capture.

  • Advanced Reading: Soil Physics by Marshall and Holmes for detailed technical information.

  • Video Lectures: Links to Selker and Orr on My Aberdeen provide deep-level detail for enthusiasts.

Global Importance and Sustainability

• Land Degradation: Approximately half of the world's land is currently degraded, necessitating improved management.

• Water Scarcity: Agriculture consumes the majority of available fresh water; effective irrigation and soil management are essential for efficiency.

• Climate Change Interactions:

  • Waterlogged or compacted soils produce higher levels of greenhouse gases.

  • Crop breeding now focuses on resilience to climate change.

• Regenerative Agriculture: A growing global movement (dominant in North and South America, Europe, and parts of Asia) focused on sustainable soil health.

• Policy Frameworks:

  • European Commission: Recently released a framework on sustainable soils as the precursor to a Soil Protection Law.

  • Scotland: Released soil protection frameworks in 2009; currently looking at more effective protections.

  • Policy Gaps: Current policies target agriculture heavily while often ignoring the construction industry. For example, it is currently cheaper for house builders to dump soil in landfills than to reuse it due to tax structures.

The Scope of Soil Physics

• Hydrology: Concerned with how water is stored and how it moves through the soil. This interacts closely with gas exchange (oxygen is vital for roots and microbes).

• Mechanical Properties: Concerned with soil hardness and stability.

  • Stability: Resilience against erosion and wash-off during rain.

  • Plant Interaction: How easily roots can penetrate the soil and how well plants are anchored (preventing tree blowdown).

  • Management impacts: Tillage (breaking up soil) versus compaction (tractor use).

Applications in Food Production and Environmental Science

• Food Production Concerns:

  • Mechanical stress: Resistance to root expansion.

  • Hydrological stress: Drought impact on growth rates.

  • Aeration: Oxygen availability.

  • Habitats: Environments for beneficial microbes.

  • Structure: Resilience of seedbeds to collapse and erosion.

• Environmental Science Concerns:

  • Water Protection: Tracking nutrient movement into surrounding water bodies.

  • Flood Prevention: Determining the water storage capacity of the soil.

  • Carbon Sequestration: Storing carbon to mitigate greenhouse gases.

  • Slope and Foundation Stability: Influencing regional engineering and safety.

• Parallel Processes: Plowing a field for nitrogen (Agriculture) is functionally similar to mixing soil and adding nutrients to stimulate microbial "overdrive" to clean up pollutants (Bioremediation/Phytoremediation).

Soil Structure and Tilth

• Tilth: Defined as the physical condition of soil as related to its ease of tillage, fitness as a seedbed, and its impediment to seedling emergence and root penetration.

  • Good Tilth: Aggregated, crumbly structure with high porosity.

  • Bad Tilth: Compacted, massive structure that resists root growth.

• Natural Systems vs. Management:

  • In nature, biology (roots, fungal hyphae, earthworms) creates structure through exudates and burrowing.

  • In agriculture, humans use mechanical agitation (tillage) to mimic this structure.

Tillage Practices and Systems

• Conventional Tillage:

  • Historically dating back to Neolithic times; commercialized by John Deere in 1837.

  • Primary Tillage: Plowing (20–30 cm depth) to flip the soil, bury residue, and control weeds.

  • Secondary Tillage: Using discs, chisels, or harrows to break clods into smaller aggregate sizes for better seed-soil contact.

  • Rolling: Pressing the soil to stabilize the bed and ensure even germination.

• Reduced and No-Till Systems:

  • Minimum Tillage: Shallow cultivation (5–10 cm depth) with fewer machinery passes.

  • One-Pass Tillage: Using modern implements to perform all steps in a single pass.

  • No-Till (Zero Tillage): Cutting a narrow slot for seeds, leaving the rest of the soil undisturbed to allow natural biological recovery.

• Energy Consumption: In Sweden, tillage moves 4.5 billion tons of soil annually (compared to 15 million tons for the Euro Tunnel construction).

• Advantages of Tillage: Increases yield via nutrient mineralization, homogenizes the soil for even ripening, levels rough surfaces, and controls disease.

• Disadvantages of Tillage: Accelerates carbon mineralization (CO2 release), disrupts soil fauna (slicing earthworms and hyphae), increases erosion risk for bare soil, and requires high fuel/labor costs.

Case Study: Northeast China (The Li Shu Method)

• Focuses on the Phaeozem soils (Black soil region) which are highly productive but vulnerable due to high evaporation and monoculture (Maize) degradation.

• Professor Li Bao Xiu (Bao Guo): Developed the Li Shu method at China Agricultural University, implementing reduced tillage and effective residue return to stabilize soil quality and ensure food security.

• Research Tools: Use of weighing lysimeters (large metal boxes) to measure absolute inputs/outputs of water and nutrients in a controlled soil profile.

Fundamental Soil Metrics

Mean Weight Diameter (MWD)

• A measure of the size distribution of soil aggregates.

• Method: Sifting soil through a nest of sieves.

• Formula: $MWD = \sum_{i=1}^{n} (x_i \times w_i)$

• (Where $x_i$ is the sieve size and $w_i$ is the proportion of total weight on that sieve).

Bulk Density ($\rho_b$)

• Defines the mass of dry soil per unit volume.

• Formula: $\rho_b = \frac{\text{Mass of dry soil}}{\text{Total Volume}}$

• Standard values range from $1.0\,g/cm^3$ (well-structured/low density) to $1.6\,g/cm^3$ (highly compacted).

• Compacted soil has higher bulk density because the mass remains constant while pore space (air) is reduced.

Particle Density ($\rho_p$)

• The density of the actual mineral particles (excluding pore space).

• Standard Value: Usually taken as $2.65\,g/cm^3$ (the density of quartz).

• Measurement: Using a Pycnometer to measure liquid displacement in a known mass of soil.

Porosity ($f$)

• The proportion of the total volume that is occupied by pores.

• Formula: $f = \frac{\text{Volume of Pores}}{\text{Total Volume}}$

• Interconversion Formula: $f = 1 - \frac{\rho_b}{\rho_p}$

• Pores larger than $75\,\mu m$ are called macropores (vital for aeration/drainage); smaller pores are micropores (vital for water storage).

Soil Water Dynamics

Expressions of Water Content

• Gravimetric Water Content ($\theta_m$): $\theta_m = \frac{\text{Mass of Water}}{\text{Mass of Dry Soil}}$

• Volumetric Water Content ($\theta_v$): $\theta_v = \frac{\text{Volume of Water}}{\text{Total Volume}}$

• Degree of Saturation ($S$): $S = \frac{\text{Volume of Water}}{\text{Volume of Pores}}$

• If $S = 1$, the soil is fully saturated (waterlogged).

Measurement Techniques

• Direct: Oven-drying soil at $105^\circ\text{C}$ for 24–48 hours.

• Gypsum Blocks: Measuring electrical resistance across a buried block.

• Neutron Probes: Using a radioactive source to detect hydrogen atoms; highly effective but expensive and difficult to dispose of safely.

• Dielectric Sensors (Capacitance): Measuring the dielectric constant. Water ($\approx 80$), minerals ($\approx 4$), and air ($\approx 1$). This is the basis for most automated irrigation systems.

• Cosmic Ray Sensors: Measuring neutron bounce-back to estimate moisture over large regional scales.

Water Energy and Suction

• Capillary Rise: Soil acts like a collection of capillary tubes. The height ($h$) water rises is inversely proportional to the radius ($r$) of the pore.

• Formula: $h = \frac{2\gamma \cos(\alpha)}{\rho g r}$

• Smaller pores hold water under higher suction (negative pressure).

• Water Potential (Matrix Potential): The tension under which water is held. Plants must exert a greater suction through transpiration to extract this water.

• Permanent Wilting Point (PWP): The point ($\approx -15\,\text{bars}$ or $-1500\,kPa$) where a plant can no longer exert enough suction to pull water from the soil.

Water Retention Curves and Plant Availability

• Water Retention Curve (WRC): A plot of water content versus suction/tension.

  • Sand: Drains rapidly at low suction due to large pores; poor water storage.

  • Loam: Gradual drainage; large storage capacity; optimal for plants.

• Field Capacity (FC): The moisture level reached after free drainage has ceased (usually 1–2 days after rain).

• Plant Available Water (PAW): The difference between Field Capacity and Permanent Wilting Point.

• Stress Ranges:

  • Waterlogged: Stress caused by lack of oxygen (Anoxia).

  • Intermediate: Optimal balance of water and air.

  • Drought: Stress caused by the inability to overcome soil suction.

Measurement of Water Potential in the Laboratory and Field

• Haines Funnel/Burette: Used for soil samples under low tension ($0$ to $-50\,kPa$).

• Suction Plates (Sand Table): Drains samples at controlled tensions via a hanging water column.

• Pressure Chambers: Applies high air pressure to soil to force water out; used to determine wilting point ($-1500\,kPa$).

• Chilled Mirror Hygrometer: Measures the relative humidity of air equilibrated with dry soil to determine extreme negative water potentials.

• Tensiometers: Field instruments consisting of a water-filled tube with a ceramic tip and a pressure gauge; they measure real-time suction in the root zone.

Questions & Discussion

• Q: What constitutes "residue"?

• A: Any plant material not harvested (leaves, stems, stalks) that is left on or incorporated into the soil.

• Q: What is the format for the Tuesday Practical?

• A: It is an open-book exercise. Students will go to the arboretum for hands-on core sampling, then use a spreadsheet to analyze soil physics data. It requires the use of broad education/principles rather than rote memorization.