A METHOD OF DIAGNOSING THE SULPHUR NUTRITION STATUS OF HERBAGE

The present investigations were undertaken to estimate rates of utilisation of absorbed sulphate at varying internal nitrogen levels in growing herbage. Previously clipped plants of perennial ryegrass were maintained on solutions with increasing nitrate and equivalently decreasing sulphate supply and analysis made of the three-weeks-old herbage for total, organic, and protein nitrogen and sulphur. It was found that S and N are utilised in the building up of protein substance in an atomic ratio (S/N)p = 0.027. Since the major proportion of the organic form of these elements is converted into proteins the ratio of organic-S to organic-N in the total mass of herbage: (S/N)₀ approximately equals (S/N)p = 0.027. Any sulphate absorbed in excess of this proportion is stored up as inorganic sulphate. From these results the minimal sulphur requirements for normal growth would show an organic sulphur content which is given by 0.027 × organic-N, expressed as gram atoms of the elements. If nitrate is low relative to total-N, Kjeldahl-N becomes sufficiently identifiable with organic-N, and the sulphur requirement approximates 0.027 × Kjeldahl-N. When sulphur is present in excess of this requirement, the excess relates to inorganic sulphate in the herbage and its presence indicates that sulphate supply by uptake proceeds at an adequate rate for normal growth. It is only when total-sulphur assay in the herbage yields values lower than 0.027 × Kjeldahl-N that a true condition of S-deficiency occurs. Features of this condition are the absence of inorganic sulphate in the herbage or its presence in amounts lower than 0.01 gram atoms S per kilogram dry weight, chlorosis, and reduced utilisation of nitrogen in protein formation relative to total nitrogen assimilation. A check is made on the frequency of sulphur shortage in field-grown herbage by the use of information from several sources. No conclusive evidence is obtained that inadequate sulphur supply was of significant frequency in the pastures investigated.