Garden Recovery After A Bushfire

In 2016 and 2017 Sophie Thomson was asked by the Department of Water and Natural Resources to provide several workshops for people whose gardens had been burned in the Pinery fires in South Australia. While the information is developed for dry, ‘Mediterranean’ climate areas of South Australia, the principles are the same wherever bushfires have burned in Australia and we would like to thank Sophie for sharing this excerpt of her article with our members.

Rebuilding after the fire

Remediating and caring for the soil; Starting from the ground up

While many people think of soil as an inanimate object, the reality is that soil is a living breathing organism and all fires, regardless of their intensity, have an effect on the soil.

The effect of fire on soil includes:

1. The moisture content of the soil is reduced.
2. The microbiology within the soil can be destroyed.
3. Ash impacts soil and moisture absorbing efficiencies.
4. Soil can be prone to erosion after loss of vegetation.
5. Retardants used to fight fires can leave residues in the soil.

This effect, even if only minor and temporary, must be understood when rebuilding a garden. If there has been significant burning in your garden knowing how to remediate your soil will help you to rebuild your garden successfully.

Effect of ash on the soil

PH Levels

Across all of Australia soils vary from acidic to alkaline while some people are very lucky and have naturally neutral soil PH. Ash is highly alkaline and used as a liming agent and in areas where soils are very acidic such as parts of the Adelaide Hills, parts of Victoria, and parts of NSW small amounts of ash are recommended as being a beneficial additive for the garden. However, a lot of Australia has alkaline soils and so adding ash to the garden is not recommended.

Where the fire has burnt areas with small amounts of crop stubble and very little vegetation, the effect of the ash will be minimal. However, in many of the 2019/2020 fires there has been much more to burn leading to a lot more ash deposited or a deeper “ash bed”. These areas are likely to have the pH level increased and the soil made more alkaline.

Hydrophobic soils

Ash is hydrophobic by nature which means it repels water, so a layer of ash left on top of the soil, will stop or at least alter water from rain or irrigation soaking into the soil. Ash is a small sized particle and very light, so when the strong winds affected the fire affected region shortly after the fire, much of the ash and topsoil moved. Heavy rain can also wash the ash into water way or dams where it causes pollution of the water.

Contamination

Soil contamination in fires can occur from burned structures and from some fire retardants.

If the fire has burned structures, there is the chance that the soil is contaminated. CCA (Copper, Chromium and Arsenic) treated timber known as Permapine is frequently used in garden settings as log garden edging, retaining walls, fence posts, or play equipment. Any structures containing asbestos which burnt will also contaminate the soil.

In addition, fire retardants are widely used in Australia to fight bushfires that are otherwise inaccessible. There are short- and longer-term soil effects that can impact your new garden. In South Australia, the fire retardant used is Phos-Chek which is an ammonium polyphosphate. Most of the effects of this retardant return to normal within 12 months, including increased soil salinity and reduced seed germination and viability, however the effects of reducing soil acidity and increasing soil phosphorus levels are evident after this time. Different firefighting services across Australia may use different retardants and so it is worth checking what is used in your jurisdiction to see what the soil affect is.

Soil Testing

If your garden has had a fire retardant applied to it, or there is possibly other soil contamination, consider soil testing to provide an insight into what remedial action you should take. You can utilize the VegeSafe service soil testing from Macquarie Uni – great value at just $20. https://research.science.mq.edu.au/vegesafe/ Knowing what and where your soil is contaminated means you can plan to grow your productive plants such as vegies and fruit trees in uncontaminated, or remediated soil, while your ornamental gardens can be grown on soil with low level contaminants.

Fire effects on Organisms and Biological Processes

Microbial communities include fungi and bacteria that are critical for the health of our soils. Microbes are killed by extreme temperatures and also rely on moisture in the soil to thrive. Extremely hot fires like we have experienced this fire season are lethal for the biota and destroy their habitat in the topsoil. It’s not all bad news though. Microbial communities are some of the most resilient of all and while the response to and recovery from fires is not well understood, normally the biota will recover with time.

Read the whole article here.