Effects and interactions of phosphorus and sulphur on a mown white clover ryegrass sward .3. Indices of nutrient adequacy

necessary to determine whether the current fertiliser policy was succeeding in maintaining adequate and balanced nutrition throughout the year. Relationships were examined between relative yields of pasture with respect to phosphorus (P) and sulphur (S) applications, and clover herbage P and S concentrations and P/nitrogen (N) and S/N ratios in a P x S factorial fertiliser trial on a mown clover/ryegrass sward. The relationships between the yield of a treatment relative to the maximum yield obtainable with high rates of P alone (RY[P]) and both % P and P/N were affected by the application rate of S. S deficiency generally lowered the % P required for any given RY[P] but raised the P/N required. This latter effect was because S deficiency markedly lowered clover herbage N concentration. P deficiency had a similar but smaller effect on % S and S/N required for any RY[S]. S/N was much less sensitive to changes in clover S status than P/N was to changes in clover P status. This resulted from the strong stimulatory effect of S on clover % N. Relationships were also established between clover P and S adequacy indices and RY[PS] which is the herbage yield with P and S in physiological balance relative to the maximum yield with P and S in physiological balance. These relationships differed from those involving RY[P] and RY[S], requiring a higher value of the adequacy index for any specified RY. Nutrient concentrations (particularly of S and N) and ratios changed during a year, indicating that more than one sampling per year would be necessary to determine whether the current fertiliser policy was succeeding in maintaining adequate and balanced nutrition throughout the year.