Use of Compost and Mulch Products on Fire-Damaged Land

Caltrans Erosion Control Toolbox

Wildfire destroys hundreds of thousands of acres of CaliforniaPDF download forest and chaparral each year. These fires frequently occur on steep and remote lands where access is difficult. Occasionally wildfires enter the rural or urban interface zones, often with devastating consequences for homeowners living in the path of the flames.

One of the impacts of a changing climate is that California’s (and the West’s) fire season is starting earlier and lasting longer. Less rainfall in winter means brush dries out earlier. Trees are under stress and dying in some areas. Higher temperatures and seasonal winds can quickly whip a small fire into a catastrophic event. Fires are larger now: seven out of the 10 largest wildfires in CaliforniaPDF download history have occurred in the Twenty-first Century, according to CalFire statistics. In February, 2015, a rare winter wildfire in high-elevation Mono County destroyed 40 homes and prompted Governor Brown to declare a state of emergency. In Fall 2017, in response to several large wildfires, the Governor issued states of emergency for Sonoma, Napa, Yuba, Ventura, Santa Barbara, and Los Angeles Counties. These fires include the second largest in California history, the Thomas wildfire in Ventura and Santa Barbara Counties, which burned over 459,000 acres. In 2018, the Camp Fire (also known as the Paradise Fire) in Butte County, one of the deadliest and costliest fires in United States history, burned over 153,000 acres, destroyed 18,793 structures, and resulted in 85 casualties. The Insurance Information Institute estimates insured losses from the Camp Fire of between $8.5 billion and $10.5 billion dollars.

Wildfires eliminate the soil’s protective vegetative layer, exposing it to wind and rain. Heavy rainfall on burned lands washes sediments into creeks, rivers,  roads, and neighborhoods. Particularly hot or slow-moving fires volatilize the organic matter on the top layers of the soil. When the soil cools a waxy layer forms that repels water. If the hydrophobic layer forms under a few inches of burnt, mineralized soil, then there is potential for dangerous mudslides, such as the one that destroyed 13 homes in Camarillo, CA in December, 2014.

Benefits of Compost and Mulch Use

• Compost helps restore soils by improving its physical, chemical, and biological properties. Whether the compost is incorporated or applied as a protective layer, soils treated with compost are rich in slow-release nutrients, see improved water infiltration and retention, and benefit from an environment where plants thrive. Allowing plants to develop healthy, extensive, and deep roots further improves soil structure and prevents erosion.
• Compost can bind and adsorb contaminants already in soil, including heavy metals, fuels, and polycyclic aromatic hydrocarbons, making it an ideal treatment for soil remediation (also known as bioremediation) and for filtration of runoff from contaminated land.
• Compost amendments and mulch help protect fire-damaged lands by protecting the soil from the erosive impact of heavy rain.
• Organic matter in compost helps soils increase water infiltration and water holding capacity, providing seeds with more water to germinate.
• Some nutrients in compost, while tightly bound in organic complexes, are highly soluble in synthetic fertilizers and rapidly leach into soil and water; this means nutrients in compost will not easily wash away in the first rainstorm and will be there longer than synthetic fertilizers to sustain plant growth and health.
• Compost helps establish vegetation and treat stormwater runoff. When soil can re-grow its protective vegetative layer quickly, the result is less erosion, cleaner waterways, and less time and money to dig out critical infrastructure.
• Mulch suppresses weeds and moderates soil temperature.

The benefits discussed above accrue in applications of finished, mature compost and are not associated with applications of uncomposted green materials. Help ensure product quality by purchasing your compost from a facility that follows CalRecycle regulations for pathogen reduction and metals testing and is permitted by or registered with a Local Enforcement Agency that follows CalRecycle regulations for pathogen reduction and metals testing.

Compost and Mulch Treatments Used in Fire Remediation

• Biofiltration strips and swales
• Compost blankets
• Compost socks
• Engineered soil
• Hydroseed and hydromulch
• Soil amendment

Research

CalRecycle sponsored research into the impacts of compost application on nutrient runoff from fire scarred lands.

Resources

• When wildfires destroy neighborhoods, CalRecycle often leads the clean-up efforts.
• California wildfire statistics: Cal Fire Statistics and Events
• CalTrans Erosion Control Toolbox

For more information contact: Organic Materials, organics@calrecycle.ca.gov