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LANDSCAPING | GARDENING | PROBLEM SOLVING

Urban soil problems

Anyone who has landscaped a home and discovered a layer of clay subsoil on top of topsoil or discarded siding from the construction of the home has experienced typical problems associated with urban soils. An urban soil is one that has been disturbed or contaminated through urban development, and no longer has characteristics like the natural landscape surrounding the developed area.

Gardening practices that worked for native soils may not prove satisfactory for urban soils. Urban soils are more likely to need modification of their structure to prevent undue stress for plants growing in them.

A number of characteristics of urban soils should be considered.

  1. Most urban soils have a high degree of variability in soil horizons. Gardeners may not necessarily find the topsoil at the soil surface. It may have been removed by developers, or it may have been buried under sub-soil excavated from the basement or crawl space.
    Distinct layering of soil can impede water movement through the soil profile. If possible, mix the layers by tilling or digging. Add organic matter to loosen clay soils.

  2. Compaction is likely present. Construction equipment driven over soil, even if it wasn’t excavated will compact the soil. Compaction may be broken up by deep tillage and the addition of organic matter.

  3. Compaction also leads to restricted aeration and water drainage. Inadequate air and water availability affects soil microbes and plant root development. The same techniques used to alleviate compaction usually will improve aeration and water drainage at the same time.

  4. Urban soils often develop a surface crust that repels water. Again, addition of organic matter will facilitate water absorption and help prevent crusting by loosening clay soils.

  5. The pH of sub soils is usually higher than that of topsoil. When subsoil is brought to the surface, the root zone for planting will have an elevated pH.

  6. Greater soil temperature fluctuations will be found in new urban landscapes than in forested or prairie areas. Dense sod and shade from trees help modify the soil temperature, providing some degree of insulation from hot sun in the summer and cold winter winds.

    Roots of most plants are more sensitive to temperature fluctuations than above-ground plant segments, so variability in soil temperatures can lead to plant stress. In addition, masses of concrete and paving in urban environments can lead to excessively high soil temperatures.

    Mulching plantings in urban landscapes, and maintaining adequate levels of soil moisture will help modify soil temperatures.

  7. The natural cycles of nutrient recycling are usually no longer present in urban landscapes. Fallen leaves in the forest gradually decay and recycle their nutrients back to the plant. Decaying prairie grasses added organic matter and built up prairie soils.

    In the landscape, however, most gardeners remove fallen leaves, mow the grass, and remove excess thatch. Removal of these “natural organic fertilizers” necessitates addition of supplemental fertilizers to maintain vigorous plant growth.

Making amendments

The above tells what happened in the past wherever housing was constructed and why it's difficult to get things to grow. Now we must look at ways of improving what we have.

Organic matter

A much-overlooked necessity in garden soils is the incorporation of organic matter. Humus is the end product of organic matter breakdown and is usually associated with the dark color in topsoil's. Humus is needed to maintain a loose, well-drained soil that does not compact easily. Soil containing good quantities of humus will cultivate easily and will promote good root growth.

Humus is also useful in supplying nutrients to plants, particularly nitrogen. A very important point is that you must keep adding partially decomposed plant material to the garden each season in order to improve its tilth and fertility. Good materials to use include compost, well-rotted manure, and peat moss. For lawns, adding the above amendments is called top-dressing and is done to an existing lawn, but at a much lower rate of application compared to what you might add to a new garden bed.

Also, aeration treatments for the lawn helps open the soil's surface and is especially helpful when combined with a top-dressing.