Andrew Curtis, of Central Otago, asks :-

What are the pros and cons of farmers burning off before planting?

Carol Smith, a soil scientist, and Roddy Hale, an agroecologist, at Lincoln University, responded.

In New Zealand, fire is used to manage stubble and crop residues in preparation for crop planting, and for control of weeds and pests. However, in residential zones close to agricultural areas, the impact of smoke on air quality is an issue.

PROS: Burning off is cheap, easy to do, and clears the land prior to sowing, especially if a lack of rainfall means ploughing in is too difficult. The remaining ash does contain nutrients such as phosphorus, potassium, calcium and magnesium which can be recycled to the soil. Controlled burning is more cost effective and results in less use of fossil fuels, pesticides and herbicides; compared to ploughing in residues and spraying. Burning off may also be useful against herbicide resistant weeds for clearing a new paddock.

CONS: Burning crop residues releases carbon dioxide, nitrogen, sulphur and particulate matter to the atmosphere, and reduces the recycling of carbon to the soil. Up to about 80% loss of nitrogen occurs in wheat during burning compared to turning in residues. Soil erosion can also occur on unprotected paddocks.

Is a systems approach to pest, weed and disease control the answer? Consider a wheat production system with no burning. Incorporation (mechanical and biological) will be easier in a moist soil. How is stubble cut and chopped up before ploughing back in? There’s some pretty neat machinery out there now that can do this. For instance you can set up a rotary mulcher and harrow combination behind your tractor that really chops up the stubble very finely (it actually, as the machine says, ‘mulches’ it). This mulch is then more easily incorporated into the soil by the harrow, providing a more even spread of organic matter and (because the residue is cut so small it can even help in pest control by destroying any pests that may be ‘hiding’ in the stalks etc). Of course there is a strong argument for no-till practices here where the mulch is left on the surface of the soil and future seed is direct-drilled, but that is a whole other discussion perhaps!

Will ploughing in affect pests and disease? Aphids will die in the soil environment, but what about fungal diseases like rust? To prevent soil erosion do you leave the stubble above ground for a month or plough it in immediately? There is a strong argument that stubble retention outweighs burning if a systems approach is consequently taken to pest, weed and disease control.

Managing the soil in a sustainable way is vital for agriculture in NZ; and we must consider the agronomic, plant health, economic viability and soil quality benefits of any management practice. Perhaps a new approach to crop rotation and residue management is needed; smoke from burning off is only one component of the sustainability equation.