INTERNAL PHOSPHORUS REQUIREMENTS OF 6 LEGUMES AND 2 GRASSES

Critical phosphorus (P) values, both total (Pt) and inorganic P(i), fo r the diagnosis of P deficiency were derived for a number of pasture s pecies that are increasingly used in Australia. Trifolium balansae and Medicago murex, with T. subterraneum for comparison, were grown for 2 seasons in field experiments to which 7 rates of phosphate fertiliser were applied. The legumes T. balansae, M. murex, M. polymorpha, Ornit hopus compressus, and Lotus peduncillatus, and the grasses Phalaris aq uatica and Danthonia richardsonii, were grown in sand culture in glass house experiments and provided with nutrient solutions containing 6 ra tes of P (0.05-1.6 mmol/L). Diagnostic indices were derived for blades of the youngest open leaves (YOL) or youngest expanded blades, and fo r whole shoots. Critical P(t) concentration in the YOL of T balansae d id not decline until full flowering and was the most stable indicator. The range of critical concentrations was 0.45-0.50% for both diagnosi s of deficiency and prediction of seasonal yield. A P(i) concentration of 150 mg/kg was critical for T. balansae during vegetative growth on ly. Critical concentrations in M. murex declined from an early stage, but a P(t) concentration in YOL of 0.40% was the most useful indicator for diagnosis until flowering. The critical values for T subterraneum agreed well with previously published data. Critical P concentrations in O. compressus were similar at 2 sampling times. For the remaining species, critical concentrations declined with time and it was necessa ry to know plant age when interpreting them.