L17_urban soils

Urban Soil Overview

• Urban Soil Mosaic: Consists of various soil types, including managed, transported, disturbed, and remnant soils.

Role of Soil

• Support: Acts as a growth medium for roots.
• Water: Retains moisture necessary for plant survival.
• Aeration: Provides necessary oxygen (O2) for root respiration.
• Nutrients: Supplies essential nutrients to plants.

Volumetric Composition of Soil

• Typical Volumetric Composition:
  • Water: 20-30%
  • Air: 20-30%
  • Organic Matter: 5%
  • Mineral: 45%

Soil Properties

• Structure: Arrangement of soil particles and their aggregation.
• Texture: Composition and size of soil particles.
• Other Properties: pH, density, organic matter, and nutrients.

Soil Structure

• Definition: Grouping/arrangement of soil particles into aggregates.
• Influences: Root penetration, water movement, aeration.
• Creation: Biotic activities, root growth, freeze/thaw cycles.
• Changeability: Soil structure can be altered.

Soil Structure Types

• Single Grain: Rapid water/gas movement.
• Blocky: Moderate water/gas movement.
• Granular: Moderate movement.
• Prismatic: Rapid water/gas movement.
• Platy: Slow movement.
• Massive: Slow movement.

Urban Soil Alterations

• Impact of Urban Systems: Changes in structure may affect root development and water retention, impacting overall ecosystem health.

Soil Texture

• Definition: Relative proportions of sand, silt, and clay.
• USDA Classifications:
  • Sand: 0.05 – 2.0 mm
  • Silt: 0.002 - 0.05 mm
  • Clay: <0.002 mm

Soil Texture Example

• Texture Composition: Sand 50%, Silt 20%, Clay 30%.

Importance of Soil Texture

• Influences on Properties:
  • Water Holding Capacity: Varies with texture.
  • Aeration: Affected by pore size.
  • Drainage Rate: Fast for sand, slow for clay.
  • Other Properties: Decomposition rates, compactability, erosion susceptibility, nutrient storage.

Soil Pores

• Definition: Spaces between soil particles.
• Pore Size Importance: Affects water availability and gas exchange.

Pore Size Effects

• Macro-Pores: Important for gas/water transport; >50 µm.
• Micro-Pores: Water binding; <0.2 µm is unavailable to plants.

Soil Aeration

• Significance: Essential for root respiration and microbial life.
• Gas Movement: Occurs primarily in larger pores; oxygen levels decrease with depth.

Bulk Density and Compaction

• Definition: Ratio of mass to volume of soil, affected by composition.
• Effects of Compaction:
  • Decrease in total porosity (especially macropores).
  • Increased bulk density and decreased aeration.

Soil pH

• Definition: Measure of acidity/alkalinity; scale of 0-14.
• Optimal Level: 6.0-6.5 is generally favorable for trees.
• Urban Soil pH: Elevated due to various factors like deicing and construction.

Organic Matter in Soil

• Importance: Enhances soil structure, nutrient availability, and water retention.
• Sources: Plant litter and decomposing roots.

Issues with Urban Soils

• Common Problems:
  • Structure loss and compaction leading to reduced aeration, infiltration, and water holding capacity.
  • Increased resistance to root penetration.
  • Pollution and changes in chemical composition due to anthropogenic activities.

Summary of Soil Properties

• Understanding six key soil properties is vital for recognizing alterations in urban systems and their effects on vital processes like nutrient cycling and hydrology. Key issues involve urban soil fertility and structural integrity.