SULFUR REQUIREMENTS DURING EARLY GROWTH OF TRIFOLIUM-BALANSAE, TRIFOLIUM-SUBTERRANEUM, MEDICAGO-MUREX, AND PHALARIS-AQUATICA

Sulfur (S) requirements for early growth were determined for 3 legumes (Trifolium balansae, T. subterraneum, Medicago murex) and 1 grass (Ph alaris aquatica). Plants were grown in sand culture in a glasshouse an d supplied with nutrient solutions containing 6 rates of S (1-32 mu g/ mL). Legumes were supplied with nitrogen (N) at 168 mu g/mL, and phala ris with 28 or 168 mu g N/mL. Plants were sampled twice, the second sa mpling coinciding with flowering of the legumes. Diagnostic indices [t otal S (S-t), HI-reducible S (sulfate-S), oxidised S (S6+)], and ratio s (sulfate-S/S-t, S6+/S-t) were derived for whole shoots (WS), younges t open leaves (YOL) of legumes, and youngest expanded blades (YEB) of the grass. The largest responses to S by the legumes were made by bala nsa clover and murex medic, which both outyielded subterranean clover, but subterranean clover was the most efficient user of S and had the lowest external S requirement. Concentrations of S6+ were always highe r than concentrations of sulfate-S, but the 2 were highly correlated a nd much lower than S-t. No S fraction or plant part sampled was consis tently superior as an indicator of S status. The larger relative incre ases and sharper breaks in gradient of the relationships between yield and sulfate-S or S6+ compared with S-t were offset by their greater r elative variability. All indices were subject to Piper-Steenbjerg effe cts, although these did not seriously affect the critical values. Crit ical values of S-t and S6+ in YOL, S-t in WS of balansa clover, and S- t and S6+ in WS of subterranean clover changed little up to 60 days af ter sowing, but it was necessary to know plant age when assessing the S status of murex medic or phalaris. Critical concentrations of S-t in phalaris were little affected by N supply, but critical concentration s of sulfate-S and S6+ were higher with 28 than 168 mu g N/mL. Critica l values for subterranean clover agreed well with previously published data.