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

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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.

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