Diagnosing sulfur deficiency in field-grown oilseed rape (Brassica napus L.) and wheat (Triticum aestivum L.)

The effects of increasing sulfur applications on field-grown oilseed rape (Brassica napus L.) and wheat (Triticum aestivum L.) were investigated in 1998 and 1999, and the critical values and efficiency of several diagnostic indicators for S deficiency were determined. Critical values for leaf concentrations of total S, sulfate and glutathione changed over time and were not suitable for diagnosing S deficiency early in the growth season. The N:S ratio was more reliable but involved two analytical measurements. A practical and reliable indicator for S deficiency was the malate:sulfate peak area ratio as measured by ion chromatography, which required only a single analysis and was independent of the time of sampling or calibration of the samples. A malate:sulfate ratio less than or equal to 1 indicated S sufficiency at the time of sampling, whereas a ratio > 1 suggested S deficiency at the time of sampling. The malate:sulfate ratio was reliable at growth stage 3.6-3.7 (flower stalks extending to first flowers yellow) for oilseed rape and at growth stage 22-25 (main stem and 2-5 tillers) for wheat, which was sufficiently early in the growth season to apply remedial sulfur fertilizer, if necessary.