Lecture 17_18 urban soils and urban ag

Roles of Soil

• Support: Provides physical stability and support for plants.

• Water: Holds and supplies water to plants.

• Aeration (O2): Allows for gas exchanges critical for flora and fauna.

• Nutrients: Supplies essential nutrients for plant growth.

Composition of Soil

• Typically Composed of:

  • Water

  • Air

  • Minerals

  • Organic matter

Soil Properties

• Key Properties:

  • Structure: Arrangement and organization of soil particles.

  • Texture: Relative proportions of different sized soil particles.

  • pH: Acidity or alkalinity of soil.

  • Density: Mass per unit volume.

  • Organic Matter: Decaying material contributing to fertility.

  • Nutrients: Availability and types of nutrients present.

Structure of Soil

• Defines how soil particles are shaped and arranged.

• Created by factors including:

  • Organisms

  • Root growth

  • Freezing/thawing cycles

• Types of Soil Structure:

  • Single grain

  • Granular

  • Blocky

  • Platy

  • Massive

  • Prismatic

• Effect of Urban Systems:

  • Urbanization compresses soil structure, reducing pore space for air and water.

Texture of Soil

• Particle Size Classes:

  • Sand: 0.05 - 2.0 mm, low water capacity, good aeration, high drainage.

  • Silt: 0.002 - 0.05 mm.

  • Clay: <0.002 mm, high water capacity, poor aeration, very slow drainage.

• Soil Types:

  • Loam: Medium-high water holding, medium aeration, slow-medium drainage.

Soil Texture and its Importance

• Cannot be changed easily; influenced by weathering over time.

• Affects:

  • Porosity: Impacts water retention and drainage.

  • Aeration: Gas exchange crucial for plant health.

  • Erosion and Compaction: Affects soil structure and fertility.

  • Percolation: Influenced by pore size; large pores drain faster.

• Field Capacity: Water available to plants after drainage.

• Soil Pores: Differentiate between availability based on pore size.

Compaction of Soil

• Caused by:

  • Loss of soil aggregates

  • Traffic or urban development pressure.

• Compaction results in:

  • Reduced aeration

  • Increased bulk density

  • Messured by resistance to penetration and decreased porosity.

• Silva cells can help in reducing compaction and managing stormwater.

Soil pH

• Measures acidity or alkalinity (0-14 scale).

• Ideal range for trees: 5.0 - 6.5.

• Influences soil organisms and mineral availability.

• Urban soils often have elevated pH due to:

  • Calcium applications

  • Irrigation practices

  • Weathering of urban structures.

Organic Matter

• Crucial for soil health; derived from leaves, roots, and decomposed organisms.

• Enhances nutrient availability and water retention.

• Urban soils face challenges such as loss of structure and lower organic input due to construction and pollution.

Problems with Urban Soils

• Decreased Aeration: Caused by compaction, leading to poor drainage and water holding capacity.

• Pollutants: Urban environments introduce contaminants that disrupt soil health.

• Temperature: Higher soil temperatures due to lack of shade impacting organisms.

Urban Agriculture**

• Motivations include:

  • Education and training programs.

  • Economic viability through reduced transport costs.

  • Improved food access and promoting food justice.

• Common practices:

  • Operate on small plots with varying volunteer help.

  • Younger, innovative farming methods maximize output.

Benefits of Urban Agriculture

• Contributes to:

  • Soil conservation and improvement.

  • Enhanced water drainage, reducing flooding risks.

  • Organic waste use for soil fertility.

Lead in Soil

• Major urban concern due to contamination from historical sources:

  • Leaded gasoline: Millions of tons released.

  • Lead-based paint: Present in many pre-1978 homes.

• Public Health Impact:

  • Lead contamination from soil can affect children significantly.

  • Associated with developmental issues.

Handling Lead Contamination

• Risk Factors:

  • Gardening activities can resuspend lead.

  • EPA standards categorize levels of intervention:

    • 5000 ppm and above: Very high, must be treated.

    • 2000-4999 ppm: High, treatment needed for recreational use.

    • 81-399 ppm: Low; caution advised.

    • 80 ppm or lower: No treatment needed.

  • Advice on exposure and garden management in contaminated areas.

Soil Sampling and Lead Distribution

• Sampling Techniques: Using XRF technology for non-destructive sampling.

• Lead concentration varies by proximity to buildings and beneath tree canopies.

• Garden boxes show lower lead levels compared to direct soil sampling.