After fires: How to start over
Losing a garden to bushfire is devastating, but with the right soil remediation and planning, it’s possible for gardens to be rebuilt and thrive, writes SOPHIE THOMSON
In 2016 and 2017, I was asked by the Department of Water and Natural Resources to provide several workshops for people whose gardens had been burnt in the Pinery fire in South Australia. These talks, together with the work I did on the Sampson Flat fire recovery and my more recent learnings about the impact of fire on soils, provide the basis for this story. While the information has been developed for the summer-dry, Mediterranean-climate areas of South Australia, the principles are the same wherever bushfires have burned in Australia.
the soil
Soil is a living, breathing organism, and all fires, regardless of intensity, have an effect on it. There may be lower moisture content because of the temperature of the fire; loss of microbial life; contamination from burnt structures and fire retardants; and a change in pH levels from the layer of ash. These effects, even if only minor and temporary, must be understood when rebuilding a garden.
pH levels
Across Australia, soils vary from acidic to alkaline. Ash is highly alkaline – gardeners with acidic soil use it as a liming agent to raise the pH for crops such as peas or broad beans. In alkaline soils, adding ash is not recommended, as it can lock up certain nutrients, making them unavailable.
Where fire has burned areas with small amounts of crop stubble and very little vegetation, the ash and its effects will be minimal. However, in many of the 2019– 2020 fires, there has been much more to burn, leading to a deeper ‘ash bed’. It’s likely in these areas that the ash will raise the pH level, making the soil more alkaline.
Ash causes other problems because it is hydrophobic, meaning it repels water
– it stops, or at least reduces, the amount of water soaking into the soil from rain or irrigation. Ash is easily blown around in winds, exposing the burnt topsoil, which also blows away. And ash can be washed by heavy rain into waterways or dams, where it pollutes the water.
contamination
When a fire burns structures, there’s a chance of soil contamination. Copper, chromium and arsenic enter the soil via ash from burnt timber treated with CCA (copper chrome arsenate). Also known as permapine, this timber is frequently used in garden settings in log edging, retaining walls, fence posts and play equipment. Any asbestos-containing structures that are burned will also contaminate the soil.
In addition, fire retardants are widely used in Australia in aerial firefighting, and these can have short- or long-term effects on the soil. In South Australia, the fire retardant used is Phos-Chek, which is ammonium polyphosphate. Most of the effects of this retardant, including increased soil salinity and reduced seed germination and viability, return to normal within 12 months. However, longer-term effects on the soil – decreased acidity and increased phosphorus levels – will become evident over time.
Firefighting services across Australia may use different retardants, so it’s worth checking what is used in your jurisdiction to determine its effects on the soil.
If your garden has had fire retardant applied to it, or there is possibly other soil contamination, soil testing will help you establish what remedial activity you need to do, and also how you might plan your new garden. For example, if you know that there’s contamination in a particular spot, you can avoid growing vegies or fruit trees there. The soil-testing service provided by Macquarie University’s VegeSafe program tests for metal and metalloid contaminants in soil samples, and is great value at just $20 (for more information, visit research. science.mq.edu.au/vegesafe).
soil organisms
Microbial communities include fungi and bacteria that are critical for the health of our soils. The extremely hot fires we’ve experienced in the last fire season are lethal for soil microbes. Fire also destroys their habitat in the topsoil, removing the moisture they need to thrive. It’s not all bad news, though. Microbial communities are some of the most resilient of all, and while the response cycle is not well understood, soil biota does normally recover in time.
Fire retardants are widely used in Australia in aerial firefighting, and these can have short- or long-term effects on the soil.
how to remediate soil
Thin, dry, fire-damaged soils are going to need some help. If your soil is acidic, it may benefit from the sweetening effect of a layer of fire ash 10–20cm deep, but if it’s alkaline, scrape away the ash in areas where you wish to establish a garden, and move it to a spot where it won’t be a problem. Then, unless you are planting local indigenous species, work on improving the soil. That goes for acidic and alkaline soils.
All soils – especially those in vegie gardens – benefit from the addition of compost or aged animal manures. This improves the soil’s structure and dramatically increases its water-holding capacity, turning it into a sponge.
A 10cm layer of compost incorporated into the top 15cm of soil increases the soil’s water-holding capacity by more than three buckets of water per square metre. Organic matter feeds and attracts earthworms, too, along with all the soil microbes that are unseen but essential workers in the garden.
Heavy clay soils benefit from some gypsum, while sandy soils are improved with the addition of clay via a clay slurry.