Predicting Iron Chlorosis of Lupin in Calcareous Spanish Soils from Iron Extracts

Abstract

Iron deficiency chlorosis is a major nutritional problem affecting sensitive cultivated plants in calcareous soils. Our main objective was to study the suitability of various Fe extracts for predicting the incidence of Fe deficiency chlorosis in calcareous soils from southern Spain. Six single Fe extractions were used: unbuffered hydroxylamine (room temperature), DTPA (diethylenetriaminepentaacetic acid) at 2 and 17 h, “rapid” ammonium oxalate, ammonium oxalate, and pyrophosphate. The Fe forms that account for most of the Fe supplied to plants were studied by using a sequential Fe fractionation process involving four extractants: citrate–bicarbonate, citrate at pH 6, citrate–ascorbate (a mild reductant), and citrate–bicarbonate–dithionite (a strong reductant). Suitability of Fe extracts for predicting Fe chlorosis was checked by establishing correlations between the amount of Fe extracted from soil and the chlorophyll content in white lupin (Lupinus albus L.). The amounts of Fe extracted by hydroxylamine accounted for a minimal portion of Fe associated with oxides, thus indicating that this extractant is not efficient at reducing Fe oxides. Hydroxylamine Fe, however, accounted for 63 and 72% of the variance in chlorophyll meter readings at 2 and 3 wk of growth, respectively, in contrast to “rapid” oxalate (32 and 11%) and DTPA at 2 h (29 and 15%). At 3 wk of growth, the variance in meter readings explained by hydroxylamine Fe was much higher than that explained by the active CaCO3 equivalent (56%). This ability of hydroxylamine Fe to predict chlorophyll meter readings is due to the relationship of the Fe extract to poorly crystalline Fe fractions, which showed the strongest relationship with the active CaCO3 content (R2 = 0.84, P < 0.001).