Soil nitrogen availability indices as predictors of sugarcane nitrogen requirements

Nitrogen recommendation systems for sugarcane (Saccharum spp.) generally does not consider the N supply from soil. Identifying a reliable soil test for estimating N availability is crucial to avoid yield losses or environmental pollution. Therefore, the objective of this study was to correlate and calibrate N availability indices with field based measures of soil N supply. Between 2006 and 2013, 15 trials for rate-response to N fertilizer by sugarcane ratoons were performed in sao Paulo, the main sugarcane-producing state in Brazil. The following indices were tested: KMnO4 oxidizable C, hot KCl extractable N, phosphate borate buffer distillable N, NaOH distillable N, Illinois Soil N Test, organic C, total N, mineral N, anaerobic incubation, soil respiration, substrate induced respiration, microbial biomass C, metabolic quotient, microbial quotient, and gross N mineralization. The indices were then correlated with sugarcane yield (Y-0N) and N content of the crop (N-0N) in N unfertilized plots, relative yield (RY), and the N rate predicted to achieve 90% of the RY (NR 90%RY). Although weak correlations were found between Y-0N with anaerobic incubation, total N, and soil respiration, as well as between NON and anaerobic incubation, no index correlated with RY or NR 90%RY. Grouping sites based on soil texture or byproduct management did not improve prediction of RY. Therefore, we concluded that none of the fifteen laboratory indices is a reliable predictor of soil N supply, and hence could not be used to adjust N fertilization rate for sugarcane. (AU)