The Importance of Soil Testing in Urban Gardening

Communities are embracing urban agriculture and developing urban gardens on sites previously used for industrial and commercial purposes. In addition to the health benefits of gardening and fresh, local produce, a garden on formerly blighted land can become a new community asset with the potential to increase property values for adjacent properties, improve neighborhood aesthetics, and grow social capital and community pride.

However, these sites also have the potential to be contaminated. According to the U. S. Environmental Protection Agency (EPA), a good rule of thumb is that recreational or residential spaces are at lower risk for contamination, while commercial and industrial uses can be considered higher risk. But, because all sites are different, it’s important to have your soil tested for qualities such as pH and nutrient availability and also to determine the amount and types of contamination, as well as the risks associated with those contaminants.

Not all types of contamination will have the same effect on you as a gardener or on your crops. Research on soil metal chemistry and plant uptake conducted at the U.S. Department of Agriculture (USDA) has found that most metals are so insoluble or so strongly attached (i.e., adsorbed) to the actual soil particles or plant roots that they do not reach the edible portions of most plants in levels which would compromise human health when eating grown crops.

Maintaining a neutral soil pH can control much of the risk of exposure via plant uptake. For example, lead is known to be toxic to humans and can be found in extremely high concentrations in some urban soils where extensive lead-based paint was used or where historical lead industry activity occurred. The risk to the gardener inhaling dust or ingesting actual soil from dirty hands is much higher than the risk of the consumer eating the properly washed crops grown from this soil. Important exceptions to the strategy of keeping a neutral pH include soils with high concentrations of cadmium and cobalt, which can be toxic to humans, and sometimes molybdenum and selenium, which are more of a concern for livestock.

Other soil metals, such as copper, are phytotoxic and will kill the plant before the metal concentration in the soil would be harmful to a gardener. In these cases, accidental ingestion of the actual soil during initial preparation or as part of ongoing gardening activities would have the greatest negative health effect.

Consult with your state environmental agency, local health department, or county USDA Cooperative Extension office to determine what kinds of samples you should take to accurately represent the conditions at your site. These offices also can provide lists of facilities that offer soil-testing services.

If your soil tests show contamination, it doesn’t mean you can’t grow food on your site. There are ways to recondition your soil or segregate it completely from the food you grow. For more information on how to accomplish this, visit EPA’s Brownfields website at, which is chock full of useful tools and resources to help.

Reference: U.S. EPA. 2011. Brownfields and Urban Agriculture, Interim Guidelines for Safe Gardening Practices.

Posted on: March 12th, 2012