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Home Garden Soil Safety

After a wildfire, soils can be impacted in several ways and the cumulative effects of these changes can modify soil productivity and biological diversity, as well as pose health risks to humans and animals. Urban soils are at an increased risk for chemical and heavy metal contamination post-fire because urban environments tend to have features and materials that release these contaminants when burned. After a wildfire, assess, test, and remediate your home garden soils to reduce the likelihood of exposure to potentially harmful contaminants.  

 

Understanding Your Site

Step 1: Assess Your Site

When evaluating your soil post-fire, the first thing to do is understand the history of your site, both the natural conditions and modifications from human development. Understanding the history of your site can provide clues as to what contaminants may be present and where they might go. 

  1. Visually inspect the area for evidence of impacts.
  2. Determine past land use if possible.
  3. Obtain soil samples for chemical analyses if surrounding hazardous materials are identified as potential sources of contamination.

Step 2: Test Your Soil

An example of conducting testing on soil.

Testing is critical to understanding how wildfire may have affected your soil. Laboratory soil testing is recommended, especially if the soil is currently being used or is going to be used for urban agriculture. Residents directly impacted by fire can access post-fire soil test results from their clean-up contractor or seek out testing themselves. DIY soil testing kits are not recommended for this type of testing because they cannot test for many contaminants or heavy metals that are of concern post-fire. 

To learn more about soil testing options, visit UC ANR’s Healthy Soils for a Healthy California website or contact your local UCCE Soil Advisor or UC Master Gardener Program. 

 

Promote Safe Soils

Remediation

After determining the history and state of your soil, the next steps will depend on what your soil testing results indicate. If your soil test results indicate that no contaminants have been found exceeding acceptable levels, no immediate action is required for contaminant remediation. If your soil testing indicates that contaminants have been found at or exceeding acceptable levels, consider the following remediation strategies:

Person shoveling compost into a wheelbarrow.
  • Work with your existing soil. This is most feasible for sites with no history of industrial use and whose soil test results indicate low to moderate levels of contaminants. Existing soil can be amended with quality soil and organic matter, and maintained at a neutral pH.
    • Contaminants are often concentrated in the top two inches of soil and can be immobilized with a layer of organic matter or mulch until removal and remediation.
    • Soil pH should be maintained at or close to neutral (6.5-7.0). Soil nutrients are most available to plants at this pH, while lead and some other heavy metals are less available.
    • Add organic matter, compost, or soil known to be free of contaminants to your existing soil and continue to do so with each planting that follows.
  • Immobilize contaminants to contain them within a certain area. Immobilization can isolate contaminants to prevent them from traveling into or out of your site.
    • When planning how to isolate contaminants, consider the water features and water drainage on your property as contaminants can settle in those areas.
    • Cover soils with mulch or compost, and control run-off to the best of your ability.
  • Manage contaminants by working with an advisor or remediation professional to plan a strategy for your site. If your property burned, you will likely work with your county Office of Emergency Services to coordinate highly contaminated soil removal. For low to moderate contamination, you may be able to amend the soil or utilize bioremediation strategies for particular contaminants such adding plants to take up toxins, or adding fungal inoculants to soils.
    • All bioremediation needs a lifecycle analysis and safe removal of hazardous waste byproducts (e.g. post-phytoremediation composts). 

Alternative Options

An image of raised beds at Treasure Island Job Corps Farm in San Francisco, taken by A. Baameur.
  • Use raised beds or other containers and fill them with clean soil. If you build your own frames, use sturdy materials that won’t release contaminants into the soil, like redwood or other non-treated lumber, brick, concrete, or rocks. If your raised bed does not have a bottom barrier, consider creating one using various materials (i.e., water-permeable fabrics, rocks, etc.) to separate potentially contaminated soil from imported, clean soil.
    • When importing soil, consider purchasing topsoil or planting mix.
    • Other types of containers besides raised beds can be used as well. Avoid using items as planting containers that could contaminate soil as they degrade over time as planting containers (e.g., tires).
  • Remove contaminated soil from your garden site and replacing it with soil that is certified safe. The contaminated soil should be disposed of according to your local regulations or relocated elsewhere in your yard away from your garden site. 

 

Best Management Practices for Near-Home Garden Soil

If you are concerned about inhaling or ingesting chemical contaminants from plants, produce, or soil, use the following best management practices for interacting with soil that has been potentially affected by wildfire:

Wear gloves, a long-sleeved shirt, and other protective gear when interacting with contaminated soil.
  • Wear gloves, boots, eye protection, a long-sleeved shirt and long pants when interacting with contaminated soil to prevent ingestion and direct contact.
  • Use raised garden beds and import clean soils to avoid inhalation, ingestion, and plant intake of contaminants over time.
  • Amend soil with clean, high-quality compost to improve soil health. Be mindful to use the appropriate amount of compost.
  • Use mulch to cover the soil to prevent airborne soil and dust up-splash.
  • Promote good drainage and use drip irrigation to prevent up-splash, particularly at the bottom of slopes that burned and after long wet periods. Contaminants can accumulate in these areas under wet conditions.
  • Be mindful not to track contaminated soil into your home.
  • Wash hands before and after harvest and rinse produce before it is consumed.

 

Additional Resources

Check out these post-wildfire home garden soil management-related resources from the UC ANR Fire Network and our colleagues!