What happens to gypsum or limestone when stubbles are burnt? Gypsum and limestone are soil amendments – sometimes spread onto paddocks before burning of crop stubble.
Stubble or grass fires can affect limestone and gypsum in two ways:
chemical changes due to high temperatures, and
convective losses as ash rises.
Stubble fires are often reported to reach 186 to 300°C but with the right conditions- hot weather, strong winds and big fuel loads (the amount of stubble) the temperature at the soil surface can reach 500-600°C.
Header trail and windrow fires tend to reach higher temperatures (around 500°C) than would be expected in a whole-paddock burn – because there is more fuel concentrated into smaller areas.
Lime remains chemically stable during stubble burning as it does not decompose until temperatures reach 900°C.
Gypsum decomposes to its’ hemihydrate form at 128°C and then dehydrates completely at 163°C – forming something like plaster. Cooked gypsum is much less soluble than in its applied state, so potential benefits to soil structure will be delayed.
Limestone or gypsum that is resting on windrows or header trails when the stubble is burnt may be at risk of loss by convection as the hot air rises and carries away particles of ash. Particulate losses of ash can be 1 to 11 % of burnt material in a forest fire situation but we would expect less convective loss from a stubble fire.
Loss of limestone will be minimal after burning of stubble, and only occur through small convective losses along header trails and windrows. There will be no change to limestones’ chemical properties.
Gypsum might lose some solubility, as well as suffering some small convective losses, but only along header trails and windrows.
Mark Conyers, Nigel Wilhelm, Sean Mason.
Photo courtesy of the GRDC