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

There are several diagnostic approaches for determining sulphur 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 sulphur concentration and yield response to sulphur. Unfortunately, the critical values of certain potential indicators such as total sulphur, sulphate and glutathione change over the growth season, making them impractical for use. The nitrogen:sulphur (N:S) ratio is more stable but problems with standardisation of measuring total sulphur among commercial laboratories mean that results depend strongly on the laboratory used. Furthermore, when sulphur deficiency is moderate, about 20% of samples are diagnosed as being sulphur-sufficient when they are in fact deficient, resulting in yield loss and subsequent economic damage to farmers. The malate:sulphate ratio in leaves has been shown to be a reliable and practical indicator of sulphur 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.