Fires in Calgary, Edmonton, Montreal, Langley, and Timberville all have one thing in common – they were all caused by potting soils. But is that the entire story or can potting soil pose a legitimate threat to your garden, home, or your lives?
Generally, under ordinary circumstances, potting soil is not a fire hazard. With most organic matter, they can burn under the right conditions, and even so those conditions would be extreme to cause a fire.
Potting soil is often used as an alternative to regular soil in growing houseplants due to its comparatively lighter weight. As such, stories of potting soils becoming the main cause of fires are usually confined to the domestic space. But the soil itself is not the main cause per se as the composition, main causes, and other external factors should be taken into consideration as well.
The Chemical and Physical Composition of Potting Soil
Potting soil is a mixed medium for growing plants. Despite being referred to as soil, little to no soil is incorporated in the medium itself. The potting soil you find in the shops and gardening nursery is almost completely soilless!
It is often composed of:
- Peat moss or coconut coir fibers.
- Sand, perlite, grit, vermiculite to improve the drainage and water retention and porosity of the medium.
- Fertilizer such as pine bark and compost to provide the essential nutrients for the plant.
The composition of the soil produces a medium with good aeration, water retention efficacy, and drainage. The volcanic material in the medium aids in water retention and reduces weight which makes the soil apt for use in indoor environments or in instances where potted plants are moved from one place to another.
Can It Burn?
To resolve the main issue – YES, it can burn. Due to the organic composition of potting soil, it is combustible if exposed to temperatures above its normal working temperatures. However, they do not spontaneously combust under ordinary conditions. Like any dry organic matter, it will catch fire if exposed to the right conditions. More often than not, negligence is the root cause of why potting soil burns.
Main Cause of Potting Soil Fires
Unsurprisingly, the main cause of potting soil-related fires is human negligence. Cigarettes are often disposed of in pots, the individual unaware that the content is not regular soil but a mixture of compost and organic matter present in the medium. In conjunction with negligence in regularly watering potted plants create suitable conditions for starting a fire. The soil works to insulate the lit cigarette to an ignition temperature. The cigarette may continue to smoulder for several hours, exacerbated by the fertilizer or compost which act as oxidizers. The soil will break out into flames given enough time.
Though extremely rare, there have been instances where sealed potting soil bags have caught on fire due to moisture trapped in the bags and produced an anaerobic digestion process whereby heat and methane are produced. Given the right conditions (i.e. stored in a moist basement), these bags can produce fire. Similar to compost piles, excessive temperatures can cause a spontaneous combustion. However, even overheated compost piles are not prone to spontaneous combustion so the heat produced must be so extreme to catch on fire. The same applies to potting soil.
Provided with enough heat and if the proper conditions are met, it can occur. These occurrences can happen when indoor potting soil is used for outdoor purposes. However, like with any dry material exposed under soaring temperatures and direct sunlight, there is a possibility for it to become a fire hazard.
Pollutants Produced by Potting Soil Fires
As with a regular fire, pollutants are produced. The emissions of primary pollutants include carbon dioxide (CO2), sulfur (SO2), carbon monoxide (CO), nitric oxide (N2O), nitrogen dioxide (NOx). Additionally, it also produces hydrocarbons (HCs) and volatile organic compounds (VOCs)
In Comparison to Other Soil
In light of the facts mentioned, potting soil still stacks up fairly well against its contemporaries. However, it is much more expensive than gardening soil and topsoil due to its unique composition of organic matter enriched with compost and spaced out by volcanic rocks. Hence, the use of potting soil may become prohibitive to the budget-conscious gardener.
This table will provide a concise yet detailed comparison.
|Qualities||Potting Soil||Gardening Soil||Topsoil|
|Physical appearance||Light and grainy||Heavy texture and loamy||Dependent on the origin of the soil|
|Water retention efficacy (WRE)||Good||High (25%-100% WRE; brand dependent)||Average|
|Special properties||Expensive. Excellent drainage, aeration, and WRE.Almost soilless; often without sand or clay||Cheap.Greater water retention. Level Degree of drainage is dependent on the brand.||Cheap. Available anywhere|
|Composition||Peat moss, coconut coir, compost,||Topsoil enriched with compost and other organic matter||Sand, silt, clay, and humus|
|Usage scenario||Potted plants||Garden beds||To fill in a hole or to mix with gardening soil|
More often than not, potting soil is not the main cause of fire per se. It is human negligence as evidenced by the news sources reporting on various fires accidents in America and Canada. Apart from preventing unwise individuals from using potted plants as cigarette butt dispensers or ashtrays, we can prevent fire incidents through other means.
The Colorado Springs Fire Department promulgated a “Potting Soil Fire Safety” advisory which informs both newbie and veteran gardeners alike of what ought to be done. In summary, they are as follows.
- Warning: Do not use Pots as ashtrays. They are the main proximate cause of potting soil fires.
- Regularly water potted plants.
- Know your product. Read the manufacturer’s instructions on the label of the product.
- Do not use indoor potting soil for outdoor purposes as the indoor variety can dry out faster due a difference in composition.
- Use clay pots. Clay pots are non-flammable and will contain the burning soil compared to other types of pots if ever a fire does occur.
- Keep potting soil away from other combustible materials.
- Unused/leftover potting soil should be spread on garden beds
Follow these steps and the use of potting soils will be a safe and worry-free affair. No need to throw away your potting soil or repot your plants unless necessary.
In summary, potting soils are a wonderful and convenient alternative to regular soil, especially for growing indoor plants. However, its convenient use must be in conjunction with due diligence by the gardener to prevent any of the aforementioned fire incidents.
Instances of potting soil catching fire are rare. Potting soil is naturally safe. It is negligence, not the innate physical and chemical composition of potting soil, that causes fires.
As long as they’re kept in a safe place and no cigarettes get extinguished inside the pot, then there’s no impediment in relying on potting soil.
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“Effect of Growing Media on Water and Nutrient Management” by N/A in Center for Agriculture Food, and the Environment, University of Massachusetts Amherst
“Fires in waste composting activities – Risk factors and prevention measures: lessons learnt from feedback” by Ministry of Ecological and Solidarity Transition
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