Iron-phosphorus interactions in the nutrition of seedling macadamia in organic potting media

Abstract

Macadamia integrifolia cv. Keauhou seedlings were grown in an organic potting medium based on ground pine bark and peat and of pH 5.9. Interactive effects of iron (Fe) and phosphorus (P) on shoot growth and quality were studied by amending the medium with single superphosphate to provide 0-240 mg P/L, and either FeSO4.7H2O at 0-3 g/L or FeEDDHA at 0.1-0.4 g/L. Shoots of all seedlings, except those at the highest levels of ferrous sulfate with low P addition, were chlorotic, with symptoms typical of Fe deficiency. Shoot quality, as judged by degree of chlorosis, was correlated with Fe concentrations in unbuffered DTPA (1:1.5 by volume) and double acid (DA) (1:5 by volume) extracts of the media (r2 = 0.683 and 0.688), poorly correlated with P concentrations in these extracts (r2 = 0.284 and 0.289), and highly correlated with concentrations of both Fe and P in the regression equation (r2 = 0.845 and 0.865). Shoot quality was not as well correlated with Fe and P in ammonium acetate (NH4OAc) extracts. Optimum shoot quality was achieved in media (pH 5.9) whose DTPA, DA and NH4OAc extracts contained 2-9, 1.54 and 2-4 mg P/L and at least 60, 15 and 0.43 mg Fe/L, respectively. Leaf chlorosis did not occur if the Fe : P ratios of the extracts were >4.5, >1.7 and >0.02, respectively.