Plant and soil diagnostic tests for assessing the phosphorus status of seedling Macadamia integrifolia

Abstract

Seedling macadamia (Macadamia integrifolia cv. Hinde) were grown in pots in two glasshouse experiments for 23 weeks. Experiment 1 comprised ten soils at two P levels (nil and a rate calculated to be non-limiting to growth) with six replications. Experiment 2 consisted of another two soils with eight rates of added P (0-2560 mg P per 4 L pot) and six replications. Whole plant tops were harvested, dried and weighed, and leaves analysed for P. In addition, leaves from Experiment 2 were analysed for Cu, Zn, Mn and Fe. Roots were recovered from the soils, separated into proteoid and non-proteoid root material, dried and weighed. Control (nil added P) soils were analysed for soil solution P and Colwell, Olsen, Bray 1 and 0.005 M CaCl2 extractable P. At 90% of maximum whole plant top growth, P concentration in the leaf was 0.08%. When the leaf Fe/P ratio < 0.07 in Experiment 2, there was a significant yield depression associated with symptoms of severe iron chlorosis. Critical soil P levels at 90% of maximum whole plant top growth were 50, 23 and 29 mg kg-1 for Colwell, Olsen and Bray 1 extractable P, respectively. It was not possible to define a critical CaCl2 extractable P or soil solution P concentration because of the large increase in relative growth with a small increase in these parameters. Proteoid root growth (as a percentage of total root weight) decreased with increasing level of soil phosphorus, and there were very few proteoid roots at >100 mg kg-1 Colwell extractable P. Applying P to maintain high soil test levels (>100 mg kg-1 Colwell extractable P) would have detrimental effects on proteoid root development.