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Beware the dangers of adding ash to your soil: It’s risky and could be harming you and the soil

Updated: Apr 1, 2021

Among the many things we have learned about ourselves during the pandemic and ensuing lockdown and restrictions, one stands out.

We are a nation of fire starters. Whether it was barbecues in the spring and summer or chimineas and fire pits as the nights drew in and the temperatures started to drop in the autumn, it is evident we like burning things. A lot. Add in Bonfire Night and traditional burning of garden waste and we’re a hot smoky mess of a nation.

There are numbers that bear this out.

According to The Telegraph, John Lewis has seen sales of outdoor heating equipment over the past two months soar by a whopping 77 per cent. Sales of barbecues and fire pits rose by 29 percent and 35 percent, respectively.

“Our customers have been making the most of their outside space this year, buying outdoor sofas, bistro sets or garden games,” said Lucy Rees, an assistant outdoor buyer at John Lewis. “But now the colder months are ahead of us, customers are looking to make changes that mean they can be in this space for longer.

“Adding a heater or a fire pit is one of the quickest ways to make your garden more inviting as the temperature drops and an open flame can make a lovely focal point."

That it can.

Other retailers report similar spikes in sales of the things that allow us to burn stuff.

(If you're looking to get in on the act, check out Hello Gardening's guide to incinerator bins.)

There was a time when this would have led to a byproduct that was considered fabulously useful: Ash. Gardeners used to swear by it. As did farmers. Indeed, we can see the evidence in the aftermath of forest and brush fires as lush green growth starts to spring up and replace the charred landscape in just a few weeks.

Ash had been used as a fertiliser for years and is commonly used on acidic soils to raise their pH or make them more alkaline. Indeed, the word potash originates from the way it was made before it was mined - using pots to soak ashes before the liquid was boiled off. 

Note that potash is a common ingredient in fertilisers and is a generic name for any salt containing water-soluble potassium. It is also where the word potassium comes from.

Granted, potassium is vital for the advancement of nutrients through plants and consequently the development of healthy (and abundant) fruit and flowers. It is also an essential element for human life. According to the British Nutrition Foundation, potassium “is essential for water and electrolyte balance and the normal functioning of cells, including nerves.”

It adds that: “Low blood potassium levels (hypokalaemia) can result from severe diarrhoea. Symptoms include weakness, mental confusion and, if extreme, heart failure.”

Though, as ever, too much of a good thing turns into a bad thing. 

“High supplementary doses of potassium can be harmful especially if the kidneys are not functioning properly.”

So, back to that ash in the soil.

First, remember that wood ash raises the pH of soil, making it more alkaline, which can be hugely damaging to plants that prefer acidic soil such as many fruits including blueberries and raspberries as well as rhododendrons and roses.

And finally, there’s a potentially toxic aspect to all this, which is dependent on what it is that we’re burning.

Traditional bonfires tend to be assembled using wooden pallets. The wood in these has often been treated. Likewise, old furniture is often made from particle board rather than solid wood - this contains adhesives that produce toxic fumes when burned in addition to byproducts in the resulting ashes. Painted wood such as disused doors and window frames also produce toxic fumes and, in a worst-case scenario if the wood is old, extremely toxic fumes because older paint often contained lead. The resulting ashes will also contain the lead and other potentially toxic elements that, when added to soil, can contaminate your growing space. And make you sick.

And then there’s coal ash, which is even worse. It contains contaminants like mercury, cadmium, chromium and arsenic as well as polycyclic aromatic hydrocarbons (PAHs), a group of chemicals that are produced during incomplete combustion of coal, oil and other organic products. Some PAHs are classified as being carcinogenic and others are deemed to “likely” be so.  Short-term exposure to these can result in irritation of the nose and throat, dizziness, vomiting and shortness of breath. Long-term exposure can result in liver and kidney damage, cardiac arrhythmia and multiple cancers. 

Nevertheless, some swear by the stuff and what it can do for the garden. 

Catherine Mansley of Gardeners’ World offers one potential approach for the die-hard fans of coal ash and suggests that it is ok to use coal ash with ornamentals but handle the stuff with care and beware the risks should you choose to grow things for consumption down the road.

That’s because some of the dangerous elements can remain in the soil indefinitely. Indeed, anyone suspecting that unsafe ash may have been used on their growing space, would be well-advised to consider running a standard contamination test on their soil to make sure all is well.

In the end, the best way to dispose of ash left from all those bonfires, barbecues and fire pits is to put it in council rubbish collection. Because, as we like to ask at Safe Soil UK, why chance it?

If you think your soil may have been contaminated by wood or coal ash, Safe Soil UK offers testing suites that can help put your mind at ease. See all the available options here.

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