Other mineral and organic iron sources

Ferrous sulfate, applied to the soil or to the tree (leaf treatments, trunk injection), has been the major therapy against Fe chlorosis from the first description of this nutritional disorder until the introduction of Fe synthetic chelates, and is still widely used by fruit growers in some developing countries, due to its low cost. If supplied alone, Fe(II)sulfate is of little or no agronomic value in calcareous soils, where the Fe2+ is subject to rapid oxidation. For example, Fe sulfate was not effective for curing Fe chlorosis in Actinidia deliciosa in a soil with a high CaCO3 content (32%), while a quite complete recovery was achieved by Fe-EDDHA (Loupassaki et al., 1997). The effectiveness of soil applied Fe sulfate may be improved by the addition of organic substrates able to complex the Fe (e.g. animal manures, sewage sludge, compost, peat, etc.). Plant extracts or plant residues (e.g. from Amaranthus retroflexus) enriched with Fe salts may represent a promising way to improve soil Fe availability in both field crops (Matocha, 1984; Matocha and Pennington, 1982) and in fruit trees (Rombolá, unpublished).

Soil injection of a synthetic Fe(II)-phosphate (Fe3(PO4)2.8H2O), analogous to the mineral vivianite, achieved a long-term prevention of chlorosis in pear (Iglesias et al., 2000), olive (Rosado et al., 2002), kiwifruit (Rombolá et al., 2003b) and peach (Rombolá et al., 2003c). Synthetic vivianite is relatively inexpensive and can be directly prepared by growers simply dissolving ferrous sulfate heptahydrate and mono-ammonium (or di-ammonium) phosphate in water (Rosado et al., 2002). Vivianite particles range between 2-10 ^.m in length (Iglesias et al., 2000) and, unlike Fe-chelates, are hardly mobile through the soil profile and remain at the depth of application. According to Rosado et al. (2002) the long-term effectiveness of vivianite depends on the poorly crystalline status of Fe oxides (ferrihydrite and lepidocrocite) resulting from the oxidation and incongruent dissolution of vivianite. The formation of these oxides mainly depends on the continuous removal of phosphate from vivianite (Roldán et al., 2002). The vivianite suspension, besides being an effective Fe fertilizer, also contains significant amounts of N, which should be considered in the orchard fertilization program.

Blood meal is an organic Fe source containing 20-30 g of ferrous Fe kg-1, chelated by the heme group of the hemoglobin. Blood meal is a by-product of industrial slaughterhouses and examples of its effectiveness have been reported (Kalbasi and Shariatmadari, 1993). According to Mori (1999), the incorporation of Fe from hemoglobin into the root cells may follow a similar mechanism as the uptake of Fe in animal cells (endocytosis). Under field conditions, the application of blood meal (70 g tree-1) alleviated Fe chlorosis symptoms of pear plants (Tagliavini et al., 2000). It should be considered that blood meal is one of the main N fertilizers allowed in organic farming.

Injection of Fe salts (mainly Fe2+ sulfate and Fe ammonium citrate) in liquid form into xylem vessels has been reported to alleviate Fe chlorosis in apple, pear, peach, kiwifruit and olive (Wallace and Wallace, 1986; Wallace, 1991; Fernández Escobar et al., 1993). In spite of the prompt re-greening and long-lasting effect (2-3 years), this technique is seen as an emergency procedure for curing severely chlorotic trees (Wallace, 1991) and may be only feasible for low density planting systems. Main difficulties are related to the risk of causing phytotoxicity on leaves when Fe concentration and time injection are not properly chosen. Solid Fe implants formulations may not induce leaf burning and have been successfully applied to control Fe chlorosis of Spain (Larbi et al., 2003).

The increase of soil organic matter content greatly reduces the risk of Fe chlorosis. Animal manure, particularly from cow, has been traditionally used to enhance soil organic matter content and fertility in orchards. The beneficial effect of organic matter on Fe chlorosis prevention depends on the direct Fe chelating ability of the humic and fulvic substances and on the stimulation exerted by organic components on soil microbial activities and root growth. In addition, organic matter improves soil aeration and may prevent the re-crystallization of ferrihydrite to more crystalline oxides in high pH soils (Schwertmann, 1966). Manure and compost are excellent substrates for bacteria (e.g. Citrobacter diversus) producing powerful Fe siderophores (Chen et al., 2000), that have an important role on Fe acquisition by roots (Crowley et al., 1991, 1992). Masalha et al. (2000) have shown that destroying soil microflora by sterilization impairs of Fe nutrition of Strategy I and II plants. Several siderophores have been isolated from manure (Chen et al., 1998; Crowley, 2001; Yehuda et al., 2000).

Water extractable humic substances (WEHS) may complex Fe from Fe-hydroxides and make it available for root uptake (Cesco et al., 2000; Pinton et al., 1999; Varanini and Pinton, 2005). The beneficial effect of WEHS of Fe nutrition also depends on their stimulation of proton ATPase and root growth (Pinton et al., 1999). Very low concentrations of Fe-WEHS complexes in nutrient solution induced a rapid re-greening of Fe chlorotic grapevines (Baldi et al., 2004). The efficiency of soil applied organic matter is often improved by incubation with Fe salts before application. For example, Fe deficiency of orange trees was prevented by applying a combination of peat and ferrous sulfate to a small portion of the root zone, whereas the application of peat alone was less effective (Horesh et al., 1986).

Foliar application of Fe salts alone or mixed with organic acids has been tested in several studies. Canopy-applied ferrous sulfate represents a valuable, inexpensive alternative to Fe-DTPA or EDTA (Abadía et al., 2002a; Álvarez-Fernández et al., 2004a; Pestana et al., 2002; Rombola et al., 2002c). Since the existence of inactivated Fe pools in chlorotic leaves has been proposed (Mengel, 1994), the possibility of inducing leaf re-greening under field conditions by spraying acidic solutions (e.g. citric, sulfuric, ascorbic acids) has been studied (Aly and Soliman, 1998; Tagliavini et al., 1995b). Kosegarten et al. (2001) have reported that spraying citric and sulfuric acid resulted in a decrease of apoplastic pH followed by leaf re-greening. Under field conditions, however, this approach causes only a partial recovery from chlorosis (Tagliavini et al., 2000). Effective re-greening of severely chlorotic kiwifruit leaves has been achieved by foliar application of Fe(III)-sulfate in combination with citric acid, malic acid and sorbitol (Rombola et al., 2002c).

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