As part of the North Coast Soil Hub‘s Fall/Winter 2020 newsletter, our CivicSpark Fellow Cailin Notch wrote a piece on wildfire’s effect on soil health. Read the story here:
California wildfires set new records this year, with a total of 1,492,128 acres burned as of Nov 1, 2020. The LNU Lightening Complex Fire and the Glass Fire heavily impacted our North Coast community and left many of us wondering about the lingering impacts in food production and the nuanced relationship between wildfire and soil health in our region.
The impact of fire on soil can lead to both positive and negative outcomes depending on factors such as fire intensity and soil type. Generally, low intensity fires can be beneficial and improve the productivity of the soil. Inversely, high intensity fires can have significant negative effects on soil health, killing microorganisms and sterilizing the soil. In all instances, soil will regenerate, over time, although the regeneration process may take longer after high intensity fires.
Within each teaspoon of soil exists a microscopic world in and of itself. In fact, there are more microorganisms in one teaspoon of soil than there are people on the planet. Soils are composed of mineral particles, air, water and organic matter, the last of which acts like a glue binding all other components together. Soil also has different layers (or “horizons”): litter at the top (fresh leaves and needles, etc.), duff (decomposing litter), and then humus (thoroughly decomposed litter) at the bottom. In a fire, the top layers of the soil burn (the litter and duff); however, fires do not reach the deeper layers as soil is not a good conductor of heat.
Fire can impact the soil either directly or indirectly. Soil impacts include physical, chemical or biological changes. Direct impacts include the combustion of organic matter. Indirect impacts result from changes in the ecosystem such as vegetation cover or changes in flora. Impacts from fire can be short-term, long-term or permanent depending on fire severity. For example, soil fertility can increase after low intensity fires, which increases site productivity. Fires do this by restoring soil nutrients through the release of elements such as nitrogen, sulfur, phosphorus and carbon.
However, high intensity fires can change the physical characteristics and make them more susceptible to nutrient loss due to erosion. If fires are hot enough, they can kill microorganisms and partially sterilize the soil. Severe fires can increase soil density, reduce porosity and permanently alter soil texture. Due to decreased water infiltration rates and water storage capacity, damaged soils thereby accelerate run off and erosion. This can lead to increased sedimentation, which can negatively impact water supply and reduce reservoir storage.
Post-fire, soils can become water-repellent, which adds to increased runoff rates. To assess fire severity and water repellency, residents can do an easy DIY water drop penetration test (page 10 of this publication).
For winegrowers, a study showed that burned soils leached more dissolved organic carbon (DOC) concentrations, but less sulfur. The results are available in this abstract and suggest that winegrowers may need to amend soils with organic carbon (such as cover crops) when replanting burned vineyards.
House fires can contaminate soils. Houses provide plenty of fuels, causing fires that burn with high intensity. Building materials, furniture and consumer electronics produce toxic substances when burned. If you are worried that your soil may be contaminated from air pollution post-fire, a soil test is encouraged. If your soils are contaminated, it’s best to grow food crops in raised beds with clean soil and mulch the surrounding area to keep dust down.
Smoke and Ash
Many residents in the North Bay are not only concerned about fire itself, but also the effects of smoke and ash. A UC Davis study found chemical contamination in soils from smoke to be of low concern, as regulated by Prop 68. While those in the study encouraged for more testing to be completed, the study found low metal soil concentration and no detectable polychlorinated biphenyls (PCB)s from smoke alone. While chemical contamination in soil from smoke may not be of great concern, smoke still challenges plants as it reduces their ability to photosynthesize. Ash can also be detrimental to plant growth by clogging the stomatal pores. On the other hand, ash is composed of many of the essential nutrients that plants require, including calcium, magnesium and potassium.
As we continue to navigate our relationship with fire in California, we will undoubtedly learn more about this topic. We look forward to learning in partnership with you.
Extra Resources: Post-fire soil resources
- UCCE publication on Soils in Urban Agriculture with soil testing & sampling information
- Tips for Interpreting Soil Analysis
- UC Master Gardener of Sonoma County 2018 workshop video “Effects of fire on soil”
Post-fire landowner resources
Vineyards and Agriculture
- Making farms more climate resilient might protect California from wildfire damage
- Fire in California – Vineyard resources
- Fire, Water, and Wine: Effects of the North Bay Wildfires on Napa Valley Vineyard Soil Carbon and Sulfur Losses
- Napa, Sonoma wineries damaged in Glass Fire plan path forward
- Ancient Oak Cellars- process of vineyard re-plant following the October, 2017 wildfires
Post-fire food safety
- Best Practices for Produce Safety After Fire
- Understanding Risk: A community guide for assessing the potential health impacts of locally grown produce exposed to urban wildfire smoke
Firewise and sustainable home landscaping design in the defensible space zone
- UC Master Gardener Program of Sonoma County firewise landscaping
- Firewise & sustainable design and maintenance video by the Resilient Landscapes Coalition
Impact of smoke & ash on plants
- UC Berkeley Botanical Garden blog post on “The Effects of smoke and ash on plants“
- After the Napa Fires, a Disaster-in-Waiting: Toxic Ash