VENTURA, Calif. -

Spontaneous combustion appears to be to blame for a mulch fire in Ventura.

The pile containing hay and horse manure was used to grow mushrooms at the California Mushroom Farm that closed down last month.

The sign in front of the shuttered business on Olivas Park Drive has an auction sign posted next to it.

In March of 2001, when Pictsweet owned the property, a similar fire burned for three weeks and led to lawsuits and environmental fines.

Firefighters are trying to contain the one-acre mulch fire that is threatening to spread into the river bottom.

The  historic Olivas Adobe and golf course are also next door.

The fire is being monitored for hazardous materials. Residents are advised to stay out of the smoke if possible.

Ventura County Fire officials said the incident is currently in a unified command with the city of Ventura. More than 70 firefighters are working to contain it.

Authorities said the fire is deep-seated and creating hazardous material and environmental concerns.

Slideshow: Mulch Fire Burning at Old Mushroom Farm

POSTED: 05:08 PM PDT Oct 16, 2013    UPDATED: 05:18 PM PDT Oct 16, 2013 
One acre of manure on fire

Ventura County Fire Department is on scene at a large mulch fire burning at a mushroom farm on Olivas Park Drive.

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Credit: Ventura County Fire Dept.

4440 Olivas Park Dr.,  Ventura, CA 93001