Understanding Urban Soils
Jack Titchener | Oklahoma Conservation Commission
Urban soils have most likely been converted from agricultural use to nonagricultural uses as suburban sprawl encroached into previously rural areas. More often, cropland, grazing land, or wildlife habitat was the primary management use. Today, most urban soils are formed from different (i.e., human-altered and human-transported) parent materials than natural soils and need different considerations for use and management. This is often due to construction and demolition activity at the site or surrounding sites. The mixing and addition of off-site soils is relatively common in developed areas.
Examples of how urban soils are altered are:
- Soils are excavated and stirred, often replaced in the profile in a different order.
- The areas may be graded to level high spots and fill low areas.
- Fill is often brought in from off-site and frequently has characteristics different from the native soils.
- Compaction layers and non-soil materials are becoming part of the soil profile.
Over Time
Urban lands can have great variation in the characteristics of the soils. Although urban soils can be wildly variable and, in some areas, polluted, they have potential for urban agriculture if they are managed and used correctly.
Compaction is a vital soil health indicator. Urban soils may have been compacted from previous agricultural use. This is appropriate for many structural construction purposes, but it can create an unsuitable growing medium. Soil compaction is the process by which soil is compressed and pore space is reduced. Pore space is where water and air move into the soil and where roots can grow through the soil. Flattened roots can be an indicator of compaction, as well as pooling or puddling of water in low areas. Many urban soils are compacted from the construction and building of suitable housing or road structures.
During any site assessment phases, testing for contaminants should be considered to protect the producer, gardener, and the public from health risks. Testing should be conducted by trained environmental professionals using proper personal protection equipment. Common contaminants in urban soils include pesticides, petroleum products, radon, asbestos, lead, chromated copper arsenate, and creosote. In urban areas, soil contamination is largely caused by human activities. Some examples are manufacturing, industrial dumping, land development, local waste disposal, and excessive pesticide or fertilizer use.
Digging should only be done on a site after any assessments are completed. Utility, right of way, previous dangerous industrial uses, or chemical plants located upstream would be major concerns and are potential reasons for soil testing. At a minimum, the investigation should include a soil sample done with a spade slice. A soil probe may be used instead of a spade to examine soil structure. The investigator should look for such things as soil quality, compaction, and organics.
Contamination, compaction, and low organic matter are common problems with soil in urban areas and must be addressed before putting urban land into food production. One of the most significant issues to identify and resolve is the risk of exposing farmworkers and community members to soil that is contaminated with toxic compounds. Working with environmental experts to carefully assess the site and its land-use history, along with testing the soil, will help you evaluate the risks and determine if it’s feasible to use the site for urban food production. The strategies for improving degraded, contaminated soils include physical (such as soil removal), chemical (such as altering pH), and biological (such as adding composts) practices.
Remediation, or excavating large amounts of contaminated soil and replacing it with clean soil, can be expensive and is usually reserved for only the most contaminated sites. Burying contaminated soils with healthy soil material may be a more economical option. In-place mixing of organic materials and subsequent mulching and use of appropriate plantings are often good options for green spaces and gardens.