Application of synthetic siderite (FeCO₃) to the soil is capable of alleviating iron chlorosis in olive trees

Iron (Fe) deficiency chlorosis is commonly observed in olive trees cultivated in calcareous soils of southern Spain. The purpose of this study was to assess the efficacy of synthetic siderite (FeCO₃) in preventing Fe chlorosis in olive. The underlying hypothesis was that siderite could be easily oxidized and incongruently dissolved in soil to give poorly crystalline Fe oxides that can act as slow-release Fe fertilizers. Three-year (2008–2011) experiments were carried out in three orchards with highly calcareous soils located in the provinces of Jaén, Córdoba and Seville, and cropped with 25-year-old ‘Picual’, 11-year-old ‘Picudo’ and 20-year-old ‘Lechín de Sevilla’ olive trees, respectively. The experiments involved a control (no Fe) treatment, and treatments consisting of a single injection into the soil of suspensions of (i) pure siderite and (ii) siderite prepared in the presence of phosphate (“P-siderite”) in the spring of 2008. In the Córdoba and Seville orchards, suspensions of vivianite (an effective slow-release Fe fertilizer) and vivianite plus humic acids, in addition to a solution of Fe chelate (FeEDDHA) were also applied. Leaf chlorophyll concentration as estimated via SPAD was significantly higher in the Fe-fertilized trees than in the control trees at nearly all times, the strongest response to Fe fertilization being that for the Córdoba and the weakest that for the Seville orchard. Leaf weight increased significantly with Fe fertilization at nearly all times except in the Seville orchard. The effect of Fe fertilization on yield was generally positive, but significant only for siderite in the Jaén orchard. The effectiveness of the two siderite and two vivianite treatments was similar and persisted over three years; by contrast, the residual effect of FeEDDHA decreased markedly after the second year. Siderite suspensions are not only effective slow-release Fe fertilizers, but also easy to prepare in the field and nontoxic; therefore, they constitute a good alternative to other Fe fertilizers.