by Priyanwada Atapattu
What are Urban Soils?
Urban soils are often compacted and contain little organic matter, making them less fertile compared to natural soils. Typically, urban soil composition includes 70-80% inorganic materials, with water and air making up 10-20%, and organic matter comprising only about 1%. This low organic content reduces soil health, limiting its ability to retain moisture and support plant growth. Enriching urban soil with organic matter, such as compost or mulch, can improve soil structure, enhance microbial activity, and promote healthier urban green spaces. Understanding urban soil composition is essential for sustainable landscaping, tree planting, and urban agriculture.
Soil Genesis in Cities
Urban soil genesis is a complex process influenced by human activities, construction, and environmental factors. Unlike natural soils, which develop over centuries through weathering and organic accumulation, urban soils form rapidly due to excavation, filling, compaction, and contamination.
Key factors shaping urban soil formation include,
- Parent Material Disruption – Cities alter natural soil layers through construction, grading, and landfill deposits, mixing different soil horizons.
- Compaction & Disturbance – Heavy machinery and foot traffic compact the soil, reducing pore space and limiting water infiltration and root growth.
- Organic Matter Depletion – Urban soils often lack organic material, affecting nutrient cycling and microbial activity essential for soil fertility.
- Chemical Modifications – Pollution, road salts, and industrial waste introduce contaminants, altering soil pH, texture, and biological composition.
- Microclimate Effects – Increased temperatures from urban heat islands impact moisture retention and microbial decomposition rates.
Spatial Structure of Urban Soil
Urban soils exhibit a distinct spatial structure characterized by patchiness, where variations in soil composition occur at multiple scales due to both natural and human influences. At the city scale, geology plays a significant role in determining soil mineral content, with elements like aluminum (Al), magnesium (Mg), manganese (Mn), nickel (Ni), and vanadium (V) varying based on underlying rock formations. Proximity to highways increases concentrations of heavy metals such as lead (Pb), copper (Cu), and zinc (Zn) due to vehicle emissions and road runoff. At the neighborhood scale, the age of developments influences soil contamination, particularly with lead (Pb) from historical use in paints and pipes. At the building scale, soils near older houses often have elevated lead levels due to deteriorating lead-based paint. A global study across five cities in North America, Europe, and Africa ranked sites based on between-site variation in soil pH, carbon (C), and nitrogen (N), revealing that natural vegetation sites had the highest variation, followed by urban fragments of natural vegetation, urban lawns, and urban building sites. This pattern demonstrates that human activities, particularly urbanization and land development, homogenize soils, reducing their natural diversity and altering their ecological functions.
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