A COMPARISON OF THE POTASSIUM REQUIREMENTS DURING EARLY GROWTH OF LOTUS-PEDUNCULATUS, MEDICAGO-MUREX, M-POLYMORPHA, MEDICAGO-TRUNCATULA, ORNITHOPUS-COMPRESSUS, TRIFOLIUM-BALANSAE, T-RESUPINATUM, PENNISETUM-CLANDESTINUM, AND PHALARIS-AQUATICA

A comparison was made of 9 pasture species, some of which are coming i nto increasing use in Australia, for potassium (K) requirements and ea rly growth. Plants were grown in sand culture in glasshouse experiment s and were provided with nutrient solutions containing 7 rates of K (0 -5 mmol/L), and nitrogen (12 mmol/L). Plants were sampled twice, the s econd sampling coinciding with flowering of most species. Symptoms ind icating K deficiency were recorded, and diagnostic indices were derive d for blades and petioles of the youngest open leaves (YOL), or for yo ungest expanded blades, and for whole shoots. Medicago murex and M. tr uncatula had the highest yield at the first sampling. Pennisetum cland estinum, Phalaris aquatica, and M. polymorpha had the highest yield at the second sampling and had a lower internal requirement for K than t he remaining species, while Ornithopus compressus had the lowest yield . In all species except M. murex, critical K concentrations in whole s hoots declined with plant age. Only in L. pedunculatus did the use of the YOL yield similar K concentrations at the 2 samplings. Ornithopus compressus was shown to have a low requirement for K and a high K effi ciency, but it also showed poor scavenging ability. All plant parts sa mpled could be used to discriminate between K-deficient and K-sufficie nt plants. Critical K concentrations were higher in petioles than in b lades of the YOL and, generally, were higher in whole shoots than in b lades. The critical K concentrations derived for some species were sim ilar to critical concentrations previously reported for other members of the same genera.