The use of N:S ratios as a means of diagnosing S deficiency in a number of pasture species.

The subject was introduced by a review of N and S uptake and plant fractions, and concepts for diagnosing nutrient requirements. The relationship between total and alcohol insoluble plant N:S ratios ((N:S)t and (N:S)p) and yield was initially studied in pot and field experiments with oats, and subsequently extended to pot experiments with perennial ryegrass, foggrass, rape and white and subterranean clover, and to pasture trials. In all species severe N and S imbalances substantially affected the respective (N:S)p 's, but with the exception of rape, (N:S) p's were markedly less affected than corresponding (N:S)t's, and if extreme N and S imbalances are excluded from consideration, then the respective (N:S)p's were either constant, or their variability was such that they were arbitrarily referred to as tending towards constancy. These \constant\" values varied between species (about 15 in subterranean and white clover and about 11 to 13 in oats foggrass and ryegrass) and with physiological development the effects of physiological development appearing to be more pronounced in relatively young than in old plants. This sensitivity of (N:S)p was considered to be due to (a) analytical methods of determining Np and Sp (b) the composition of the protein fraction (c) the severity of N and S imbalance and (d) differences in physiological age. In both the gramineous and clover species severe S stress was always accompanied by (N:S) t 's substantially exceeding the respective constant values to which their (N:S)p 's tended whilst in corresponding S sufficient plants (N:S) t's were usually equal to or less than corresponding \"constant\" (N:S)p ranges. This latter group frequently included low N-low S treated plants suggesting that in these plants the internal S status was in balance with the internal N status. A progressive increase in (N:S) t's above their corresponding \"constant\" (N:S)p 's tended to be associated with increasing S stress but it was difficult to establish when this increase became physiologically critical. The investigations with gramineous plants were restricted to total foliage analysis. In clovers the S status was best reflected in laminae-only samples but for diagnostic purposes a precise separation of laminae from petioles was not necessary. Unlike pot experiments where the S status of grass-clover swards could be diagnosed by (N:S) t 's in any of the species in pasture situations only (N:S) t changes in clover components were sufficiently substantial for meaningful diagnosis. This was demonstrated in a field experiment in which management and S treatments were varied. Results of 19 simple grazing trials emphasise that the diagnosis assesses the S status (in relation to N status) as at the time of sampling and is not a reliable guide to future requirements and that the time of year is critical for meaningful sampling."