A simple method for estimating the return of nutrients in sugarcane trash

Sugarcane yield and return of nutrients nitrogen (N), calcium (Ca), magnesium (Mg), phosphorus (P), potassium (K) and sulfur (S) and dry matter (DM) in residues before and after burning operations, were quantified in a series of seven field experiments. The quantity of DM in sugarcane residues (tops, trash and total residue) was strongly linearly related to mechanically harvested sugarcane yield (fresh weight, FW) (r2 =: 0.71--0.88). The quantities of N, Ca, Mg, P and S (but not K) in these total unburnt residues were also linearly related to the FW cane yield (r2 = 0.50--0.83). When the standing crop was burnt before mechanical harvest, the mass of Ca, Mg and P (but not S, K or N) in the unburnt post-harvest residues was linearly related to FW cane yield (r2 = 0.53-0.75). Most of these linear regressions had significant intercepts, so extrapolation beyond the range of cane yields measured (ca. 50--125 t/ha FW) was not realistic. When the cane was burnt before harvest and the post-harvest residues burnt, the quantities of nutrients in residues were not related to FW cane yield. The quantity of nutrients in unburnt residues that would have been lost by burning (the unburnt-burnt differential) tended to increase as crop yield increased. Thus, for a sugarcane crop yielding an extra 50 t cane FW/ha (total cane yield of 50--125 t FW/ha), this differential was ca. 40 kg Calha, 15 kg Mg/ha, 4 kg P/ha and 4 kg S/ha. The differential return of N and K could not be adequately quantified.