Predicting the incidence of iron deficiency chlorosis from hydroxylamine-extractable iron in soil

Management of crops sensitive to iron deficiency chlorosis (IDC) includes the use of effective methods to predict the incidence of this nutritional disorder. The objective of this work was to validate the use of hydroxylamine-extractable Fe ([Fe.sub.ha]) for predicting the incidence of IDC in olive (Olea europaea L.), grapevine (Vitis berlandieri Planch x Vitis rupestris Scheele), chickpea (Cicer arietinum L.), and sunflower (Helianthus annuus L.) typical of the Mediterranean climate of Spain. This test was compared with the 'active lime' test and oxalate, citrate-ascorbate, and diethylenetriaminepentaacetic acid extractions. Overall, [Fe.sub.ha] provided the best estimates of leaf chlorophyll concentration, accounting for 26 to 66% of the variance (linear-plateau regression) depending on the particular crop. The optimum extractant/soil ratio for accurately predicting the chlorophyll content was 100 for olive and 20 for grapevine, chickpea, and sunflower. The concentration of Fe in the hydroxylamine extracts was found to be related to that of poorly crystalline Fe oxides in the soil, which testifies to the ability of this test to accurately predict the incidence of IDC. Proposed critical levels of [Fe.sub.ha] below which IDC is likely to occur were 60 mg [kg.sup.-1] for olive, 11 mg [kg.sup.-1] for grapevine, and 10 mg [kg.sup.-1] for chickpea and sunflower. Abbreviations: ACCE, 'active' calcium carbonate equivalent; CCE, calcium carbonate equivalent; DTPA diethylenetriaminepentaacetic acid; IDC, iron deficiency chlorosis; [Fe.sub.ca], citrate-ascorbate-extractable iron; [Fe.sub.ha], hydroxylamine-extractable iron; [Fe.sub.ox], oxalate-extractable iron.