Fire intensity matters
The intensity of a fire in a paddock, how hot the fire is and for how long, will influence the effect on following crops.
Cool–moderate burns: most dead plant material is burnt with some seed and most subterranean clover surviving. There will usually be a small residue of unburnt pasture or stubble. There is an increased risk of wind erosion, but there is little effect on a following crop. Soil organic carbon is not reduced, and soil organisms are largely unaffected.
Hot burns: all dead plant material and many seeds are burnt. The topsoil usually appears charred and bare. Weed control in a following crop is improved; soil pH under burnt windrows will be increased. Soil organic carbon might be reduced in the top few millimetres, and soil organisms will be reduced or changed in that zone.
Very hot burns: the surface layer of soil is virtually sterilised. All surface plant material and seed is destroyed, and some sub-clover seed is burnt or sterilised. Soil organic carbon might be reduced in the top 10–15mm, and soil organisms will be killed or reduced in that zone.
Temperatures at the soil surface reach about 600°C in a very hot burn, about 100–250°C in a hot burn, and 50–150°C in a cool–moderate burn. The soil 15mm below the surface is usually not heated by more than 10°C and returns to its original temperature within five minutes (CSIRO). Plants that bury their seed or that have growing points below the surface have better survival after fire.
Destroying 100% of ryegrass weed seed requires exposure at 400°C for 10 seconds and for wild radish 400°C for 30 seconds (Australian Herbicide Resistance Initiative). Other grass weeds are likely to be within this range.
Burning narrow windrows typically destroys 100% of surface weed seeds as there is often 20 to 30 t/ha of dry matter in the windrow, which results in a suitably hot fire. Burning whole paddock residue of a 3 t/ha harvested crop can result in 400°C but only for a few seconds. This could kill 80% of ryegrass seeds and 10% control of wild radish seeds on the soil surface.
Rotation choices after fire
Wind erosion and sandblasting of crops is possible: Cereals have a lower risk from sandblasting, with growing points below ground at germination and are more resistant to sand blasting than canola, peas or lupins.
Sow into moisture as soon as possible to establish a crop and reduce erosion risk. Match variety to sowing time.
Lupins sown into bare burnt paddocks will have a higher-than-normal risk of brown leaf spot, due to increased soil splash of this disease with the absence of stubbles.
Pulse crops sown into burnt paddocks should be inoculated with the appropriate strain of rhizobial inoculum, as a precaution against the heat of the fire having sterilised the surface soil. This recommendation also applies to pasture legume seed sown into burnt soils.
Canola is not recommended on paddocks with severe wind erosion risk. Nitrogen management may be more difficult for canola in soils where surface residues have been destroyed and soil organic matter may be depleted by fire and/or subsequent erosion. Canola yields and quality will be more sensitive to any shortfall in N supply than cereals.
Fire, soil fertility and testing
Generally, fire reduces the amount of nitrogen available because organic matter near the surface is burnt, releasing nitrogen to the atmosphere.
Nutrients present in standing plant material are returned to the soil as the ash and at risk of loss through wind erosion events which can lead to a loss of nutrients.
Soil testing for nutrient analysis (N, P, K, organic C) is recommended, especially on paddocks that suffered significant topsoil loss after the fire. Nutrient levels at depth are not expected to be affected by fire. Testing should be done in autumn, so that test results are available to guide fertiliser decisions for sowing in consultation with your consultant. Regular monitoring of crops on burnt paddocks for nutrient deficiency symptoms, particularly N and trace elements, is also advised.
Stubble fires will not affect lime. However after burning, there is less protection against wind erosion, which could result in the loss of recently spread lime on the soil surface.” [excerpt from: Chris Gazey, Gaus Azam, Jenni Clausen & Zed Rengel. 'Soil Quality: 4 Soil Acidity.' SoilsWest, Perth, 2019]
Fire and weeds
Fire will reduce the weed seed load on the paddock surface but weed seed banks at depth will likely remain viable.
Soils laid bare by fire are at risk of weed invasion from nearby non-burnt paddocks and roadsides. Reduced competition from regenerating pastures and lack of grazing can enable weeds to gain a foothold in burnt paddocks.
Grasses with more of their seed at or near the soil surface, such as annual ryegrass, brome grass and barley grass, are reduced by hot to very hot burns. Other weed species including wild oats and wild radish might show a flush of seedling numbers following burning. Fire and smoke can break seed dormancy in many circumstances.
Reduced stubble and organic matter can affect soil active herbicides.
Extra care should be taken in using soil active herbicides such as simazine and atrazine on lighter soils with reduced surface residue and soil organic matter as a result of intense burning. This risk may restrict crop choice.
Fire and soil microbes and disease
Only very hot fires might reduce microbial populations more than a few millimeters below the surface.
About 50% of the soil’s microbial populations are found within the top five centimetres of the soil profile. If heat from a fire is severe enough to reach this depth, then it will reduce microbial populations and weaken the soils naturual microbioal buffer against some diseases.
Stubble-borne pathogens such as fusarium and take-all as well as other leaf diseases will be reduced in burnt paddocks.
The impact of a hot fire on soil microbes is variable:
- wildfires can cause significant, immediate but short-term reductions in microbial biomass and activity.
- fungi usually decline more than bacteria following a fire.
- recovery times vary but are apparently longer for fungi than bacteria, and depend on several factors, such as soil pH and available nutrients.
Fire and insects
Some reduction in insect pests
- Depending on the intensity of the fire at the soil surface, some reduction in insect pests is likely. Farmers on Lower Eyre Peninsula reported that redlegged earthmite, millipedes, lucerne flea, aphids, slugs and snails were all less prevalent in crops and pastures following the January 2005 fire in that region.
Diversity of beneficials
- Research following the 2019–20 fires on Kangaroo Island in South Australia found a surprising diversity of largely beneficial insects such as parasitoid wasps, honey bees, hover flies, spiders, moths, carabid and ladybird beetles in unburnt stubbles.
- These beneficial invertebrates are important predators and parasites of significant pests such as mites, aphids, caterpillars and earwigs.