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The use of s" as an index of the sulfur status of plants

A. Pinkerton, P.J. Randall and P.A. Wallace

CSIRO Division of Plant Industry, P 0 Box 1600, Canberra ACT 2601

Total sulfur (S), sulfate (SO4) and the ratio SO4: total S have each been used as indices of S status of plants (1). The fastest and most reliable method of analysis for total S is X-ray fluorescence spectrometry (XRS). Recently it has been shown that the two principal forms of S present in plants, oxidised S (S") and carbon-bonded S, can be determined by XRS at the same time as total S, and that S' is highly correlated with SO4 (2). Here, we illustrate the use of S' as an indicator of S deficiency.

Methods

A number of species were grown in sand culture with nutrient solutions supplying a range of S concentrations. Plant parts were sampled and analysed for total S and 56+ by XRS, and for HI- reducible S (essentially SO4), and the concentrations related to dry weight at the time of sampling.

Results and discussion

For the parts of some species, for example, sunflower stems and petioles, S6+ and SO4 are apparently the same fraction (data not shown), but for most species, although correlation is high and the gradient is close to unity, there is an intercept for S6+ that some oxidised S is not reduced by HI. Typical relationships between tissue S fractions and yield are shown in Figure 1.

Figure 1. Relationships between concentrations of forms of S in young leaves of three species and shoot dry weight. • = total S; • = S'; = HI-reducible S.

It is clear that S6+ has the same attributes as SO4. It shows larger relative changes than total S, and is more sensitive as an index. Coefficients of variation for S6+ and for SO4 are comparable. Methods of analysis for "sulfate" do not necessarily determine only inorganic SO4, nor is it certain that all inorganic SO4 is included. The use of S' is advocated because it can be determined simply and rapidly.

References

Fre1. ney, J.R., Randall, P.J. and Spencer, K. 1982. In: Sulphur '82. (Ed. A.I. More) (British Sulphur Corp. Ltd: London). pp. 439-444.

Pin2. kerton, A., Randall, P.J. and Norrish, K. 1989. Commun. Soil Sci. Plant Anal. 20, 1557-1574.

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