Management of Fe nutrition is unique compared to most essential nutrients for which deficiencies are easily corrected by adding relatively inexpensive fertilizer. Although soil chemistry and environmental conditions result in various inefficiencies with these other nutrients, these deficiencies are generally corrected with adequate fertilizer addition (Tisdale et al., 1993). Alternatively, Fe deficiencies are not easily corrected with fertilizers applied to the soil in typical fashion (Anderson, 1982; Chen and Barak, 1982; Cihacek, 1984; Clark, 1982; Lucena, 2003; Vose, 1982). This anomaly is due to the extreme nature of the equilibrium between the solid and solution phase forms of Fe in the soil. This relationship is so "one-sided" in alkaline soils, that essentially an infinitesimal concentration of soluble Fe resides in the soil solution from which roots draw nutrients (Lindsay, 1974; Lindsay and Schwab, 1982). Iron fertilizer materials dissolve and temporarily elevate solution Fe concentration, but this increased solubility is short-lived and the net gain in terms of Fe uptake for plants is essentially nil.
Iron deficiencies are often spatially variable and are related to soil characteristics and landscape position (Franzen and Richardson, 2000; Hansen et al., 2003; Vempati and Loeppert, 1986). Areas of chlorosis tend to occur within portions of fields rather than across entire fields. Thus, managers must decide whether to address Fe-deficiency problems over all areas of a field or to explore management focusing on zones where Fe chlorosis is most likely to occur. Variable rate management of the practices listed in this section may prove to be economical solutions if expense of treatment for Fe chlorosis is limited to affected areas (Hopkins et al., 2005; Franzen and Richardson, 2000). Management of Fe deficiency is also complicated by year to year variability associated with temperature and precipitation patterns. Thus, managers may be faced with decisions without even knowing if deficiencies will develop.
As evidenced by the diverse and vigorous flora growing in alkaline soils, plants have evolved mechanisms to facilitate Fe uptake into their roots despite the virtual non-existence of Fe in soil solution. As stated previously, field crop production often requires management practices designed to enhance Fe availability to plants susceptible to Fe chlorosis. These management options include various cultural or fertilizer practices and/or selection of species, varieties, cultivars, or hybrids that have the capability of mobilizing Fe for root uptake (Jolley and Brown, 1994). These "Fe-efficient" or chlorosis tolerant plants have either evolved naturally or have been bred to possess some of the mechanisms which enable Fe solubilization and absorption under less than ideal soil conditions. Screening approaches and techniques have evolved for different crop species.
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