Field Correlation and Calibration of Soil Nitrogen Tests in Sugarcane Production.

Currently, the recommendation of fertilizer N for sugarcane (Saccharum spp.) in Brazil is performed based on the concept of expected yield. The identification of a soil test that can predict N mineralization can help to improve N fertilizer use efficiency by this crop. The objectives of this study were to correlate and calibrate soil N tests with sugarcane N response parameters obtained under field conditions. Between 2006 and 2013, 21 sugarcane N-response trials were performed in the state of São Paulo, Brazil. Soil sampling was taken at 0-20 and 20-40 cm depths in control check plots (no N added). The following biological, chemical, microbiological and isotopic (¹⁵N) methods were tested: anaerobic incubation (ANI), permanganate oxidizable C (POXC), hot KCl extractable N (HKCl), phosphate-borate buffer at pH 11.2 extractable N (PBB), direct steam distillation (DSD), Illinois Soil N Test (ISNT), basal respiration (BR), induced basal respiration (BRi), soil microbial biomass C (SMB-C), induced metabolic quotient (qCO_2i), microbial quotient (qMic), and gross N mineralization (GNM). The methods used to predict soil N supply were then correlated with stalk yield and N accumulation in the control treatment, relative stalk yield (RSY) and the N rate required to achieve 90% of the RSY of sugarcane (NR 90% RSY). No method was able to predict the soil N mineralization in sugarcane fields. Weak correlations were verified between ANI and stalk yield, and HKCl and PBB with N accumulation by the crop. Indexes did not correlate with RSY or NR 90% RSY, even when sites were grouped based on soil texture, fertilizer and by-product management or soil organic matter content. Thus, it can be concluded that none of the methods studied is recommended for adjusting N fertilizer rates to sugarcane ratoons in the state of São Paulo, Brazil.