Does soil really burn? What’s happened to it during a fire? These are questions that at least once crossed my mind and probably yours. After all, security is at stake. The good news is that despite this can happen, it is quite rare.
Studies have demonstrated that soil starts burning at around 190°C when wood material (lignin) and old leaves (hemicellulose) start to break down because of heat. At temperatures above 600°C flames become visible and above 1000°C the fire is auto sustained. The soil burns because of organic dry matter.
Fires in Calgary, Edmonton, Montreal, Langley, and Timberville all have one thing in common – they were all caused by soil burning. But is that the entire story or can soil pose a legitimate threat to your garden, home?
Table of Contents
Can Soil Burn?
The Chemical and Physical Composition of Potting Soil
Soil is often used as an alternative to regular soil in growing houseplants due to its comparatively lighter weight. As such, stories of 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.
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 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 soil burns.
Main Cause of Soil Fires
Unsurprisingly, the main cause of soil-related fires is human negligence. Cigarettes are often disposed of in pots by people 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 smolder 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 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.
High Temperature Driven By Direct Sunlight
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.
What Happens When Soil Burn? Effects
When soil burns, the organic matter will turn into ash. At the same time, 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)
Soil Fire Comparison by Type
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|
During a fire, the majority of soil nutrients in their natural form is lost. This is because nitrogen, phosphorus, and potassium (the main soil nutrients) tend to decay at temperatures above 700°C.
Some nutrients like nitrogen change form but still remain available to plants after a fire. Studies clearly demonstrated that nitrogen becomes ammonium (NH4-N) after a fire. University investigations have shown that ammonium can be used by plants for growth.
A soil fire also changes the ability to repel water. Studies have shown that soil after a fire is very likely to become more water repellant. This has a serious negative effect and water runoff and erosion make the soil more prone to get fire again.
The natural microorganism that benefits plant growth gets killed during a fire. This has a major negative effect on the plant that survived as it reduces drastically the number of nutrients that a plant can use. In forests, this has a long-lasting effect.
Prevent Soil Fires
More often than not, 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 soil fires.
- Regularly water potted plants.
- Know your product. Read the manufacturer’s instructions on the label of the product.
- Do not use indoor 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 soil away from other combustible materials.
- Unused/leftover soil should be spread on garden beds
Follow these steps and the use of soils will be a safe and worry-free affair. No need to throw away your soil or repot your plants unless necessary.
Can Burning Soil Kill Weeds?
Burning soil is not an efficient way to kill weeds. Not only the plans will regrow new stems from the roots, but the combustion alters the pH of the soil and it is a serious fire hazard and security threat. The only way to kill weeds by burning is to repeat the process at least 4-5 times.
I, as many gardens, have tried to burn the soil in the attempt of removing weeds. This technique unfortunately does not work. Indeed, the wounded/burned top of the weed will often develop new shoots. Many weeds indeed behave similarly to tubers. Until the roots are unaffected, they will keep growing.
The only way to kill weeds is to burn the soil at least 4-5 times. Indeed, the weed will exhaust its energy to regrow new shoots and eventually wil die off.
However, more effective systems and week killer are available in the market.
In summary, soils, dirt, and potting can all burn due to the presence of dry organic material. During the burning process, the organic material will turn into ash releasing pollutants like CO2 and other volatile compounds.
Potting soil, is 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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“Chemical and Physical Properties of Various Commercial Garden Soils and their Effects on Plant Growth” by F. Hancock, S. Howington, and C. Currie in University of Georgia College of Agricultural & Environmental Sciences
“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
“Here’s The Dirt on Potting Soil: There’s No Dirt In It” by Tom MacCubbin in Orlando Sentinel
“Potting Soil catches on fire, threatens house” by Victoria Wood in WHSV
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“Who Knew? Dry Potting Soil A Fire Hazard” by Laura Bates in OKOTOKS Online