Summary
Several authors are advocating the use of the SO4−S/total S ratio in the plant as the best index of S status. We have traced the arguments put forward in support of this index, and we show that they are based either on unfair comparisons with other indices, such as SO4−S or total S alone, or inappropriate statistical treatment.
The SO4−S/total S index has two fundamental disadvantages compared with SO4−S or total S alone: (1) the numerator (SO4−S) is the major variable in the denominator, so the ratio is likely to be less sensitive than either of the measurements alone; (2) its determination involves twice as much analytical work as either measurement alone.
Examination of some of the source references indicates that SO4−S by itself is the most satisfactory S index. Whenever whole plants are analysed, any index which includes organic S is subject to variation due to tissue age.
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Scaife, A., Burns, I.G. The sulphate-S/total S ratio in plants as an index of their sulphur status. Plant Soil 91, 61–71 (1986). https://doi.org/10.1007/BF02181819
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DOI: https://doi.org/10.1007/BF02181819