East Sussex Fire & Rescue Service logo
East Sussex Fire & Rescue Service logo

Fire caused by 'spontaneous combustion of compost'

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Planter with compost


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Damaged solar light

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Burnt Plant

 Burnt Plant

How this fire happened

A fire station was contacted by a householder informing Kent Fire and Rescue Service (KFRS) that she had a small solar light in a plant pot which had apparently ‘spontaneously’ caught light.

She had extinguished the fire herself using water from her garden hose.

A member of the KFRS Fire Investigation and Research Team (FIRT) visited the occupier and examined the plant pot; the tree that was planted in the plant pot, the remaining compost in the plant pot and the remains of the solar garden light that the occupier stated was located in the plant pot before the fire.

The pot was of ceramic construction and showed no signs of fire heat or smoke damage. There was however evidence of the original level of the compost and the burnt remains of compost that had partially smouldered to ash.

The bottom of the apple tree had smoldered to charcoal.

There was an ants’ nest in the compost with live ants below the pot when it was lifted.

The solar light was the self contained type. It was designed to use a single solar cell to recharge a battery during daylight and then automatically turn on the light after dark. The light was designed to be inserted into soil using a plastic spike under the light housing. The solar light showed signs it had burnt from outside to in, from below, with no apparent damage to the circuitry.

It is believed this fire had not been caused by the solar light at all, but had in fact been produced by spontaneous combustion of the compost within the ceramic pot, made worse by the presence of an ants’ nest within the compost.

Ants in soil or compost aerate the soil, thereby increasing ventilation. They also change the PH (acidity) of the soil and introduce microorganisms (bacteria). 

These factors can lead to an increase in temperature of the soil, or in this case compost. It is believed this allowed the compost to reach smoulder temperature and thereby cause this fire.

The effect it had

The occupier felt that it had been useful to share this information with KFRS as; had she not been in, or the pot closer to the wooden cladding on her house and patio, the consequences could have been worse.

Safety message

Compost can spontaneously ignite given the correct circumstances.

If smoke is seen rising from planters, ensure copious water is applied immediately.

Should this not deal with the problem call out the fire service.

Further information

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This case study was contributed by:
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Latest Update :
02 October 2020
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