The effects of nitrogen fertilization and age of regrowth on a number of indices for assessing the sulphur status of the perennial tropical pasture grassPanicum maximumvar. 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.