Usefulness of Nitrogen-Sulfur Ratio for Determining Sulfur Status in Corn.

Sulfur availability indicators are necessary for rational use of S fertilizers. Few studies however, have evaluated the N:S ratio in corn plants as an indicator of crop S status. This research studied the sensitivity of the N:S ratio in the whole corn plant to external changes in S availability at different crop stages. Five field experiments with different rates of S were conducted in two contrasting areas in the Argentinean Pampas Region. Grain yield, N, and S concentration at four crop stages were measured. Nutrient concentration in corn plants tended to decrease with crop development as a result of a well-known dilution process. Sulfur application significantly increased corn grain yield in three of five experiments (P<0.1). The N:S ratio tended to decrease with crop development, likely because N uptake decreases considerably after silking (and N remobilization to grain begins) but S accumulation continues beyond this stage. For crop stages as early as V6 and 10d pre-silking, the N:S ratio seemed to be a poor indicator of crop S status since, in general, no significant decrease in the N:S ratio was found in situations where S application significantly increased grain yield. For advanced crop stages (especially at physiological maturity) however, the N:S ratio performed much better as an indicator of crop S status since significant decreases in this index (P<0.1) were generally found in situations where S application significantly increased (P<0.1) grain yield, or no decrease in the ratio was observed where S application did not increase grain yield. Based on these results, the N:S ratio seems to be a promising tool for monitoring the S status of corn. This index could be useful for determining if the crop grew under S deficiency. However, it may not be a precisely accurate diagnostic/predictive tool, although it can be useful in planning for long term S fertilization strategies. Further research is needed to confirm these results, especially under wider ranges of soil S levels.