Wildfire ash is composed of organic matter, so it acts as a soil fertilizer wherever wind and gravity disperse the ashes and they settle to the ground. However, ashes from burned houses and human-made materials present a different scenario. The Oregon Department of Environmental Quality cautions that ashes from house fires may be contaminated with multiple toxins such as asbestos, mercury, lead, cadmium and chromium. These chemicals can leech into the soil wherever ashes fall.
In most cases, the level of toxins mixing with the soil is minimal, but the OSU Extension Service warns that the closer your garden is to a house fire, the larger the chance the soil may be contaminated. In such cases, they recommend collecting several soil samples from the garden and sending them for testing. Their guide, “Analytical Laboratories Serving Oregon” (available at https://catalog.extension.oregonstate.edu) lists all of the soil analysis services provided by labs in our state. OSU’s Central Analytical Laboratory in Corvallis offers a full complement of soil testing, including analysis for heavy metals, chemical contaminants, soil health and nutrient content. Instructions for collecting soil samples are provided at https://cropandsoil.oregonstate.edu/cal.
If your garden has accumulated a thick layer of ash, it might be a good idea to amend the soil with compost or fresh soil, OSU says.
The Extension Service also provides useful guidelines for cleaning up ash from gardens and handling vegetables and fruit on which ash has accumulated. They recommend wearing a face mask when cleaning ash residue (N95 respirators are best if you can find them), and wearing gloves, eyewear and protective clothing.
Prevent dispersing ash into the air by using a gentle stream of water from the garden hose to rinse off plant foliage and produce. Direct the rinse water to a low-traffic area of the yard, rather than down the storm drain. Spritz large amounts of ground ash with water, and then gently sweep up the ash, seal in a bag and discard.
After rinsing ash from fruits and vegetables outside, they can be soaked in white vinegar (1 part vinegar to 9 parts water) and then rinsed again before eating or storing. As additional safety precautions, the skin of tomatoes, apples and root crops can be peeled off and the outer leaves of leafy greens can be removed.