Sulfur requirements during early growth of Trifolium balansae, Trifolium subterraneum, Medicago murex, and Phalaris aquatica

Abstract

Sulfur (S) requirements for early growth were determined for 3 legumes (Trifolium balansae, T. subterraneum, Medicago murex) and 1 grass (Phalaris aquatica). Plants were grown in sand culture in a glasshouse and supplied with nutrient solutions containing 6 rates of S (1-32 ¦g/mL). Legumes were supplied with nitrogen (N) at 168 ¦g/mL, and phalaris with 28 or 168 pg N/mL. Plants were sampled twice, the second sampling coinciding with flowering of the legumes. Diagnostic indices [total S (St), HI-reducible S (sulfate-S), oxidised S (S6+)], and ratios (sulfate-S/St, S6+/St) were derived for whole shoots (WS), youngest open leaves (YOL) of legumes, and youngest expanded blades (YEB) of the grass. The largest responses to S by the legumes were made by balansa 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 higher than concentrations of sulfate-S, but the 2 were highly correlated and much lower than St. No S fraction or plant part sampled was consistently superior as an indicator of S status. The larger relative increases and sharper breaks in gradient of the relationships between yield and sulfate-S or S6+ compared with St were offset by their greater relative variability. All indices were subject to Piper-Steenbjerg effects, although these did not seriously affect the critical values. Critical values of St and S6+ in YOL, St in WS of balansa clover, and S, and S6+ in WS of subterranean clover changed little up to 60 days after sowing, but it was necessary to know plant age when assessing the S status of murex medic or phalaris. Critical concentrations of S, in phalaris were little affected by N supply, but critical concentrations of sulfate-S and S6+ were higher with 28 than 168 ¦g N/mL. Critical values for subterranean clover agreed well with previously published data.