Interactions Between Iron and Phosphorus in the Nutrition of Banksia ericifolia L.f var ericifolia (Proteaceae) in Soil-Less Potting Media

Abstract

Interactive effects of iron and phosphorus on the growth and appearance of Banksia ericifolia L.f. var. ericifolia seedlings were investigated. The seedlings were grown in a pinebark-based soil-less potting medium amended with 0-40 mg L^-1 P added as single superphosphate and with Fe supplied either as FeSO₄.7H₂O (at 0-3 g L^-1) or FeEDDHA (at 0-430 mg L^-1).

Fe deficiency was the dominant visual effect on shoots; its severity increased as P supply increased. Classic symptoms of P toxicity (death of oldest leaves) were produced in plants at the highest levels of P supply and low Fe supply.

At each level of added P, Fe tended to lower %P in the shoots, mainly by allowing increased production of dry matter as Fe deficiency was overcome. The highest rate of FeSO₄.7H₂O addition reduced accumulation of P in the leaves and increased it in the stems. High P additions reduced accumulation of Fe into leaves. At non-toxic additions of P, young leaves had higher concentrations of P than did old leaves, but the reverse applied as P supply became excessive. The leaves of high quality plants contained less than about 0-14% P and more than 30 mg kg^-1 Fe. P:Fe ratios for the leaves of such plants were in the range 14-53 (mean 23). Numbers of proteoid root clusters were greatest on plants in media of high Fe supply and low P and declined rapidly as P supply increased above the adequate range. No evidence was found for Fe stress leading to greater development of proteoid roots.

Quality scores of at least 85% of maximum were mainly confined to media giving < 3 mg L^-1 P in 2 mM DTPA (1:1.5 volume) extracts and amended with at least 1.5 g L^-1 FeSO₄.7H₂O or 215 mg L^-1 FeEDDHA. Response in ferrous sulfate treatments was better correlated with extractable P and Fe than with totals added, but there was no difference with FeEDDHA treatments, because additions of FeEDDHA did not decrease extractable P and ferrous sulfate did.