A Glasshouse Evaluation of 'critical' N and P Concentrations and N:P Ratios in Various Plant Parts of Six Eucalypt Species

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Abstract
The nutritional response of Eucalyptus citriodora, E. cloeziana, E. crebra, E. orgadophila, E. papuana and E. tessellaris to N (0-200 kg N ha-1 as NH4NOs) and P (0-120 kg P ha-1 as Ca(H2PO4.H20) was investigated using a sandy clay loam as the growth medium. Sensitivity indices were employed as a means of identifying the ability of plant parts to reflect the greatest changes in N and P concentrations per unit increase in relative yield. The most sensitive plant parts for N were the three youngest mature leaf blades and undeveloped leaf blades, whereas stems plus petioles had the best ability to reflect changes in P concentration. Old leaf blades and stems plus petioles were the most sensitive parts for N : P ratio. For the three youngest mature leaf blades, the ranges for 'critical' (90% relative yield at the highest applied rates of N or P) N concentration, P concentration and N : P ratio were > 1.3-> 1.8%, 0.07-> 0.21% and 4-10 respectively. The large capacity of the eucalypts studied to respond to applied N and P , especially together, was a major limitation in deriving definitive critical N and P concentrations and N : P ratios.