CRITICAL POTASSIUM CONCENTRATIONS IN TRIFOLIUM-BALANSAE, MEDICAGO-MUREX AND TRIFOLIUM-SUBTERRANEUM AS AFFECTED BY SAMPLING DATE

Critical concentrations of potassium (K) for the diagnosis of K defici ency were derived for tissues of balansa clover (Trifolium balansae) c v. Paradana, murex medic (Medicago murex) cv. Zodiac, and subterranean clover (T. subterraneum) cv. Karridale. The legumes were grown for 2 seasons at 2 sites to which 7 rates of K fertiliser were applied in ea ch season. Symptoms of K deficiency appeared on leaves of plants at bo th sites, where less than or equal to 50 kg K/ha had been applied. Dia gnostic indices were obtained for K in the dry matter (DM) of blades a nd petioles of youngest open leaves (YOL) and of whole shoots, and for K in petiolar sap of the YOL. In all species, critical K concentratio ns in dry matter were initially high but decreased steadily as the sea son progressed. Critical values were highest for YOL petioles, and sim ilar for YOL blades and for whole shoots, with good discrimination bet ween deficient and adequately supplied plants for K concentrations in these tissues. Critical concentrations (%) of K in DM of YOL laminae o f balansa clover declined from 4.00 in May to 0.70 in November, of YOL petioles From 5.80 in May to 1.40 in October, and of whole shoots fro m 2.48 in July to 0.68 in November, while critical K concentration (mu g/mL) in the YOL petiole sap declined from 1000 in May to 384 in Nove mber. The decline in critical K concentration (%) in DM of YOL laminae of murex medic was from 2.75 in May to 1.12 in September, of YOL peti oles from 5.58 in May to 1.57 in September, and of whole shoots from 1 .57 in July to 0.70 in November. Critical K concentrations (mu g/mL) i n murex medic petiole sap fell from 1000 in May to 471 in September. T he decline in critical K concentrations in DM was related to date and was unaffected by rainfall before sampling, temperature, or sodium con centrations. Critical concentrations in DM of subterranean clover were similar to those in the literature for other cultivars. Critical conc entrations in balansa clover agreed with those derived previously from glasshouse experiments. The test for K in petiolar sap was less satis factory: the decline in critical concentration in sap was less consist ent than that in DM, and the critical values for subterranean clover d id not agree with published results.