Fire and soil health

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Is fire bad for soils?

Fire can damage soils by:

• Exposing the surface to erosive forces: and
• Reducing fertility.

Damage is directly proportional to the intensity and duration of the fire..

Fire (as controlled burning) may however be used to assist land and soil management for :

• Weed control
• Pest control (e.g. slugs and snails)
• Surface fuel reduction (e.g. windrow or fuel reduction burning)
• Ecological burning

What types of fire are there and what are their characteristics?

The intensity and duration of a fire is the key factor in calculating its effect on soil. This in turn is determined by fuel type, flammability and quantity.

Windrow (log heap) burning has been recorded as having the biggest effect on soil fertility and damage to soil structure. Grassland fires are recorded as having a relatively small effect. Further information is available at the reference page, Fire and its influence on soil.

Of course, the condition of the soil in terms of moisture content, and soil type (texture and porosity) will modify the changes caused by a particular fire at the surface.

What happens?

Fire, when travelling across a soil, burns flammable materials and causes heating.

Part of the heat transfers to the soil (reported as about 5% of the total generated) and then transmits down the profile. The dryer the soil the more heat transferred and transmitted.

A soil will experience the following, listed in order of increasing intensity of burn:

• Reduction in quantity of surface cover
• Death of plants and other soil organisms
• Exposure of physical soil materials
• Progressive drying of soil profile
• Oxidation and volatilisation of organic matter
• Sterilisation
• Breakdown of clay minerals

Details of the temperatures at which these changes occur is available at the reference page Fire and its influence on soil.

What is the effect?

Where a wild fire has burned all the surface organic material, the following will generally result:

• Water repellence
• Increased runoff and erosion from rainfalls
• Loss of nutrients
• Depletion of organic carbon thus knocking-out accumulated energy and structural resources

The first two are surface effects and will occur for all wild fire occurrences. The extent of the latter two will depend on how far the heat wave has penetrated into the soil profile. At the higher temperatures, physical changes will be cause in soil minerals - see Fire and its influence on soil.

What management practices help recovery of a burnt area?

This depends on what intensity of fire has occurred at the soil surface and how much heat has been transmitted within the profile.

An intense wildfire sterilising to a depth of 200 mm and losing much of its organic matter to 100 mm will have severely reduced biological activity. The rate of recovery of soil health will depend on the speed of plant colonisation. Plant growth can then initiate the cycling of organic materials into the soil which in turn provide energy and nutrients for operation of the soil ecosystem. Care is needed in what primary species are encouraged. There are plenty of examples of good intentions leading to major weed problems.

Wild fire in grasslands and annual crops will not be as damaging nor sterilising. The main concern will be water erosion with storm activity before vegetative cover is restored

Control burns and fast moving grass fires may need little mitigative attention.


What can I do to prevent (or prepare for) fire?

Go to - CFA Publications (external link)