Derivation of diagnostic indices for assessing the sulphur status of Panicum maximum var. trichoglume

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Abstract
The effects of nitrogen fertilization and age of regrowth on a number of indices for assessing the sulphur status of the perennial tropical pasture grass Panicum maximum var. trichoglume (green panic) have been examined in pot experiments. A non‐rectangular hyperbola regression model has been used as an aid in deriving critical sulphur concentrations and evaluating their confidence limits. The merits and limitations of this model together with problems associated with other methods of deriving critical nutrient concentrations are discussed. Results indicate that critical total S concentrations in whole plant tops declined markedly with age of regrowth. Critical sulphate S concentrations were more stable with age of regrowth, concentrations in excess of 0.012% being indicative of adequate sulphur for maximum plant yield. It is also suggested that plants have adequate sulphur when more than 12% of their total S content is in the sulphate form. N:S ratios may provide a useful guide for assessing sulphur status but should be treated with caution when the nitrogen supply to the plants is high.