Most of the nitrogen (N) and carbon (C) contained in crop stubbles is lost when they are burnt. Around 100 kg/ha of urea could be needed to replace the N lost from burning stubble from a high yielding wheat paddock. Substantial amounts of stubble sulfur (S), phosphorous (P) and potassium (K) are also lost.
The nutrients are lost via smoke, airborne ash, and later via losses of windblown and waterborne ash. How much nutrient is lost depends on the stubble load and extend of ash movement. The significance of the loss depends on the paddock’s nutrient status.
Some nutrients from retained stubbles can also be lost, especially P and K when there is rainfall. However, these losses will be smaller than losses from burning stubbles without significant rainfall. Nutrients washed from retained stubble will be in the topsoil.These nutrients may later become available to the next crop.
How much is lost?
Nutrient loss increases with higher stubble loads and hotter burn temperature. Higher yielding crops produce more stubble. This formula can estimate the stubble load:
Amount of stubble (t/ha) = grain yield (t/ha) / harvest index
The harvest index value is around 0.4 to 0.5, depending on the season. Reference values for the crop can give a general indication nutrient concentrations in stubbles. The best way to estimate nutrient concentrations is to tissue test the specific stubble. Cut and collect stubble from several small areas that reflect majority paddock conditions.
Table 1: Nutrients lost from burning a 5 t/ha wheat crop stubble. Source: Managing stubble (GRDC publication)
What happens to nutrient availability?
Burning converts most nutrients into more available forms. Nutrients remaining in ash are around 80% available to crops grown in the next season. Despite nutrient losses, burning stubble may lead to an increase in immediate nutrient availability to emerging crops. Burning prevents the tie-up of soil N in the decomposition of retained stubble.
The carbon in crop stubble provides a food resource for soil micro-organisms. Ash carbon is relatively inert, what carbon remains after burning bypasses the labile carbon pool.
Bad or good?
Repeated stubble burning can run down soil fertility. Occasional burning can have management benefits, but the nutrient loss should be factored into nutrient budgets.