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 deficiency were derived for tissues of balansa clover (Trifolium balansae) cv. 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 each season. Symptoms of K deficiency appeared on leaves of plants at both sites, where <50 kg K/ha had been applied. Diagnostic indices were obtained for K in the dry matter (DM) of blades and petioles of youngest open leaves (YOL) and of whole shoots, and for K in petiolar sap of the YOL. In all species, critical K concentrations in dry matter were initially high but decreased steadily as the season progressed. Critical values were highest for YOL petioles, and simllar for YOL blades and for whole shoots, with good discrimination between deficient and adequately supplied plants for K concentrations in these tissues. Critical concentrations (%) of K in DM of YOL laminae of 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 from 2.48 in July to 0.68 in November, while critical K concentration (¦g/mL) in the YOL petiole sap declined from 1000 in May to 384 in November. 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 petioles 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 (¦g/mL) in murex medic petiole sap fell from 1000 in May to 471 in September. The decline in critical K concentrations in DM was related to date and was unaffected by rainfall before sampling, temperature, or sodium concentrations. Critical concentrations in DM of subterranean clover were similar to those in the literature for other cultivars. Critical concentrations in balansa clover agreed with those derived previously from glasshouse experiments. The test for K in petiolar sap was less satisfactory: the decline in critical concentration in sap was less consistent than that in DM, and the critical values for subterranean clover did not agree with published results.