Keywords: Malate:sulfate ratio, N:S ratio, Soil testing, Critical value, Yield.

There are several diagnostic approaches for determining sulfur deficiency, including modelling, soil testing and plant tissue analysis. Plant tissue analysis has been shown to be more useful than soil testing because there is a closer relation between plant sulfur concentration and yield response to sulfur. Unfortunately, the critical values of certain potential indicators such as total sulfur, sulfate and glutathione change over the growth season, making them impractical for use. The nitrogen:sulfur (N:S) ratio is more stable but problems with standardisation of measuring total sulfur among commercial laboratories mean that results depend strongly on the laboratory used. Furthermore, when sulfur deficiency is moderate, about 20% of samples are diagnosed as being sulfur-sufficient when they are in fact deficient, resulting in yield loss and subsequent economic damage to farmers. The malate:sulfate ratio in leaves has been shown to be a reliable and practical indicator of sulfur deficiency, with only 4% of samples incorrectly diagnosed as being sufficient when they are not.

Dr. Mechteld M A Blake-Kalff, Dr. Fangjie Zhao and Prof. Steve P McGrath, Division of Agriculture and the Environment, Rothamsted Research, Harpenden, Hertfordshire, AL5 2JQ, UK.

23 pages, 8 figures, 1 table, 64 references.