Iron chlorosis in macadamia as affected by phosphate‐iron interactions 1

Iron (Fe) chlorosis induced by heavy phosphate (P) fertilizations is a serious problem for macadamia (Macadamia integrifolia) in Hawaii. To address this problem, a study was conducted to quantify the effects of P‐Fe interaction on macadamia leaf composition and chlorosis. The soil used was a limed Oxisol (Tropeptic Eutrustox, Wahiawa Series), pH 5.5. Phosphate was added as treble superphosphate at 0, 150 and 500 mg P/kg. The 150 mg P/kg rate was designed to yield approximately 0.04 mg P/L in the soil solution, a level considered adequate for macadamia growth. The 500 mg P/kg rate was intended to produce approximately 0.2 mg P/L, a level required by many horticultural crops but considered excessive for macadamia. Iron was added as Fe‐DTPA at 0, 5 and 10 mg Fe/kg soil, and factorially imposed on the P treatments. Color Index, a numerical rating based on hue, value and chroma from a Munsell Color Chart for Plant Tissues, was correlated with leaf chlorophyll concentration and used as an indicator of chlorosis.

Phosphate concentrations in leaves increased with increasing P application rates as expected, but decreased remarkably with increasing Fe rates (at a constant P rate). Plant Fe unexpectedly remained unchanged with increasing Fe rates but decreased with increasing P rates. The results suggest that (1) soil‐solution Fe was not a limiting factor to macadamia growth as is often incorrectly assumed for high P‐fertilized soils, (2) Fe uptake was restricted not because soil‐solution Fe was low but because plant P was excessively high, and (3) Fe translocation from roots to leaves may have been hampered by high P in the plants. Consequently, Fe chlorosis was intensified primarily by P fertilization (actually, by high plant P concentrations) and secondarily by P‐Fe interactions. Chlorosis, as measured by Color Index, can be diagnosed by a leaf Fe/P ratio < 0.06, and predicted by a soil‐solution ³√Fe/P ratio < 15.