Critical Sulfur Dilution Curve and Sulfur Nutrition Index in Maize

Core Ideas Critical S dilution curve was established for maize: S concentration = 2.13 × biomass −0.23 . The S nutrition index successfully diagnosed S status at early crop stages (∼V 6 ). Sulfur concentration and chlorophyll meter reading, both in the upper leaf, were good predictors of the S nutrition index.Sulfur (S) deficiency can severely limit maize ( Zea mays L.) yield. This deficiency could be predicted by quantifying the S concentration (S C ) in the shoot biomass (BM) and by calculating the S nutrition index (SNI = observed S C /critical S C ). However, as shoot S C determination is laborious, alternative S diagnostic methods should be developed in maize. Thus, the objectives of our study were to: (i) determine and validate a critical S dilution curve, (ii) quantify the critical SNI, and (iii) explore the use of S C and chlorophyll meter reading (CMR), both determined in the uppermost developed leaf at vegetative growth stages, to predict SNI and for diagnosing S status in maize. Six field studies evaluating fertilizer S rates were executed, collecting maize shoot and uppermost developed leaf samples and determining Sc, CMR, yield, and SNI parameters. The main outcomes from our study were: (i) a maize critical S dilution curve was fitted (S C = 2.13BM −0.23 ) and validated with an independent dataset; (ii) a critical SNI threshold of 0.79 adequately diagnosed S status at ∼V 6 stage; (iii) at V 6 , SNI was linearly related to S C ( R 2 = 0.65) and CMR ( R 2 = 0.85). As a result, a threshold value of 2.0 g S kg −1 for S C, 47.5 units for CMR, and 0.94 units for relative CMR can be used to successfully diagnose S deficiencies at early vegetative (∼V 6 ) maize stages. Future studies testing S diagnostic tools for maize should be performed in environments with severe S deficiency and nutrient co‐limitations.