Quantifying the loss of nutrients from the immediate area when sugarcane residues are burnt

Retention of crop residues, instead of burning them, may allow greater organic matter and nutrient retention. This would have implications for fertiliser requirements and water use. The recovery of nutrients (nitrogen, N; calcium, Ca; magnesium, Mg; phosphorus, P; potassium, K; and Sulphur, S) and dry matter (OM) from burnt trash were quantified in a series of seven field experiments, and by ignition of small samples of trash in a furnace (at 600 °C and 900 0C). The nutrient and OM recoveries after the fires ranged from 5-77% of their initial inputs. The lowest recoveries were after fires in standing cane crops. Recoveries of K were consistently the greatest (23-77%), while those of Ca (11--41 %) and N (5-48%) were generally the lowest. Nutrient and OM recoveries after a standing crop was burnt in the field before harvest were: K (30%), OM (23%), P (23%), N (23%), S (18%), Mg (17%), Ca (11 %). The data presented in this paper therefore indicate substantial additional quantities of these nutrients may be retained when residues are not burnt. Recoveries of all nutrients (except K) in residues tended to be linearly related to recoveries of DM. These data of DM and nutrient recovery after fires in sugarcane, indicate their retention only in the immediate area of the fire. Redeposition within areas predominantly burning cane would effectively increase recoveries and reduce net nutrient export.