Soils in urban areas are poorly understood. Indeed, until most recently, federal and state efforts devoted to the inventory and classification of soil have been focused on agricultural, rangeland, and forest regions. As the world becomes increasingly urbanized, more attention must be paid to studying urban soil, particularly with respect to urban agriculture and food production.
Urban soils often show evidence of disturbance including cutting, filling and grading to level landscapes for development projects, and filling of areas with construction debris. Despite such disturbances, urban soils, just like their natural counterparts, provide valuable ecosystem services including serving as a medium for plant growth, regulator of water supply, habitat for organisms, modifier of the atmosphere, and recycler of raw materials.
The soil in the vacant lot down the street from your home might be highly disturbed and mixed with fill material. Yet it still sprouts vegetation, even if it is weeds! And with careful management, even these disturbed soils can be made productive for urban dwellers.
So, what is under our feet in Chicago? Is it soil or “landfill”? Unless you are walking on beach sand along the Lake Michigan shoreline, or a paved sidewalk or street, you are most likely walking on soil, both natural and disturbed!
Urban Soils May Be Both Natural and Disturbed
As was discussed in the November 2009 EcoMyths article on wetlands, the city we know as Chicago was once a wetland.
As the meltwaters from the last continental glacier of the “Ice Age” drained from this area, it left behind a flat expanse of poorly-drained lake plain deposits on which developed an extensive network of shallow marshes, sedge meadows and wet prairies. Higher topographic areas were dominated by, with improving drainage, wet mesic, mesic, dry mesic to dry sand prairies, savannas and forests. Over time, soils with unique properties developed in these environments.
But as Chicago grew, the wetlands were drained and filled, the savannas and forests were cleared, and the original soils were altered and many, but not all, were destroyed. In the late 1890s, the city of Chicago began an aggressive lake-filling and land-creation program that over time eventually led to the creation of Northerly Island, Burnham Park, Belmont Harbor, Montrose Harbor, Lincoln Park’s South Lagoon, Grant Park, and Jackson Park.
Notable landfilling occurred in Grant Park, where debris from the 1871 Chicago fire was deposited between Michigan Avenue and the former Illinois Central Railroad tracks, and in the now ritzy neighborhood of Streeterville located just north of the Chicago River, which at one time served as a municipal dump. The urban soil resource in Chicago is a mosaic of natural and disturbed soils.
The manner of site preparation for home construction has changed radically over the decades. Before diesel equipment was readily available, only the actual home site was likely to be disturbed. The soil over the entire property was not stripped as is the practice today. In older neighborhoods, the soil profile around a house is likely to be intact. The presence of some gravel, especially near the street, is a reminder of the road gravelling that took place nearly a century ago before cement paving became a matter of fact. This is particularly true of school yards.
Soil provides valuable ecosystem services including serving as a medium for plant growth, regulator of water supplies, habitat for organisms, modifier of the atmosphere, engineering medium and recycler of raw materials. There are four basic soil-forming processes that convert consolidated and unconsolidated geologic materials into soil:
- transformations occur when soil constituents are chemically or physically modified or destroyed and new materials are synthesized from the precursor materials.
- translocations involve the lateral and vertical movement of inorganic and organic materials through the soil, usually by water
- additions involve input of materials, such as organic matter, to the developing soil profile from outside sources; and
- losses from the soil occur by leaching of soluble constituents to groundwater by percolating water, erosion of surface materials and other forms1.
Urban areas are characterized by a variety of land uses, including commercial, industrial, residential and recreational, as well as high population densities. As a result, their soils are often disturbed. Examples of disturbances may include:
- cutting, filling and grading to level landscapes for homes and buildings
- fill of low-lying areas that are wet or which possess other undesirable soil characteristics
- filling of areas to dispose of dredged materials
- removal of topsoil and mixing of soil horizons
- addition of plant growth materials such as compost and municipal sludge
- atmospheric deposition of materials, including nutrients and metals
“Fill” is any material used to ‘fill in’ an area. It can be natural soil material (derived locally or not), waste materials (e.g., incinerator ash, construction debris, dredged spoils) or a mixture of both. Soils in urban areas often contain non-soil materials or human artifacts such as glass, brick, metal, wood, and various waste products.
Given the prevalence of disturbance, urban soils are characterized by a paucity of organic matter in areas with sparse vegetation, the presence of human-created artifacts such as from construction debris or municipal household waste, higher soil pH values compared to undisturbed natural soils due to addition of basic soil nutrients, such as sodium from road salts and calcium from concrete and plaster, greater compaction, and contamination by toxic substances, such as organic contaminants and heavy metals (see below).
So, what is beneath our feet as we tramp through the streets and neighborhoods of Chicago? Unless you are walking on concrete, gravel or asphalt, or beach sand along the shoreline, the answer is soil—both natural and disturbed. Depending on land use history, it is likely that these disturbed soils will contain various types of fill materials; however, like their undisturbed counterparts, even these disturbed soils are performing valuable ecosystem services and are experiencing the aforementioned soil-forming processes.
Indeed, the Natural Resources Conservation Service (USDA-NRCS) has mapped and catalogued urban soils in New York City, all of which have the typical types of soil horizons (e.g. A, E, B, C) associated with natural soils, and most of which contain considerable fill material. With testing and proper management, as described below, many of these disturbed urban soils have the potential to be quite agriculturally productive.
The Solution: Reconnect With the Urban Soil Resource
Urban soils have for too long received a bad rap, largely because many folks assume they have no economic or ecological value. However, the recent explosion of urban gardening and agriculture programs in inner cities, as well as an increasing emphasis on eating local has served to reconnect folks with the stuff under their feet.
Given the disturbed nature of many urban soils, anyone interested in using these soils for food production or even real estate development should consider the following:
- To Grow Vegetables in Urban Soil: Have your soil tested to determine whether it is safe to grow vegetables in it. Consider having your soil tested for pH and nutrient levels (e.g. nitrogen, phosphorous, potassium), as well as heavy metals (lead, mercury, cadmium, arsenic, nickel, copper, etc.) and organic pollutants. The cost of doing such testing will vary depending on the suite of soil tests you want performed. Most home soil-testing kits test only for PH, potassium, nitrogen, and phosphorus. In order to be sure to test for heavy metals and pollutants, contact a certified soil-testing laboratory, or consult with U.S. Environmental Protection Agency (EPA), Illinois office. Few detailed soil surveys exist for urban areas, so if you need a detailed analysis of the soil resource on your site, hire a soil scientist.If your soil contains high levels of contaminants, or if it is not practical for you to have your soil tested, then it is recommend that you create a raised-bed gardening with clean soil.
- To Develop on Urban Soil: If you are a real estate developer, you will probably be required to perform due diligence on the site, including having the soil tested for heavy metals and organic contaminants such as PCBs.
Written by Jim Montgomery, Environmental Science Program, DePaul University, and John Tandarich, Geography Program, Social Science Department, Triton College