Internal phosphorus requirements of six legumes and two grasses

Abstract

Critical phosphorus (P) values, both total (P,) and inorganic Pi, for the diagnosis of P deficiency were derived for a number of pasture species 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, Ornithopus compressus, and Lotus pedunculatus, and the grasses Phalaris aquatica and Danthonia richardsonii, were grown in sand culture in glasshouse experiments and provided with nutrient solutions containing 6 rates 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 for whole shoots. Critical Pt concentration in the YOL of T. balansae did not decline until full flowering and was the most stable indicator. The range of critical concentrations was 0.45-0.50% for both diagnosis of deficiency and prediction of seasonal yield. A Pi concentration of 150 mg/kg was critical for T. balansae during vegetative growth only. Critical concentrations in M. murex declined from an early stage, but a Pt 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 necessary to know plant age when interpreting them.