Iron comprises approximately 5% of the earth's crust and is the fourth most abundant element in the lithosphere (Tisdale et al., 1993), and as a result, plants are growing in a "sea of Fe". However, the bioavailability of Fe in alkaline soil is very low (Chen and Barak, 1982; Jolley and Brown, 1994; Lindsay and Schwab, 1982; Vose, 1982).
It is well documented that Fe deficiency in field crops primarily occurs in high pH, alkaline soils (calcareous conditions) (Hansen et al., 2003; Inskeep and Bloom, 1984). These soils have pH in the range of 7.5 to 8.5, with higher pH values in sodium accumulating soils. The solubility of Fe minerals decreases exponentially for each pH unit increase in the pH range common for soil (Lindsay, 1974; Lindsay and Schwab, 1982). Free soil carbonate exacerbates the unavailability of Fe due to the formation of poorly soluble Fe phosphates. As much as one-third of the world's surface soils are calcareous, primarily in arid and semi-arid regions (Brown, 1961; FAO, 2005) with additional land underlain with calcareous subsoils. Although Fe deficiency is relatively common, most field crops have adapted to grow normally in these environments.
Iron deficiency generally occurs when susceptible crop varieties are cultivated in soils that promote the problem. In areas where Fe deficiency is a concern, the deficiencies generally occur heterogeneously rather than uniformly across a field. Chlorotic patches often occur in fields spatially uniform with respect to high soil pH, indicating that alkalinity is not the only factor controlling the availability and uptake of Fe (Hansen et al., 2004). Other soil factors associated with the expression of Fe deficiency include: carbonates, salinity, Fe composition, moisture, bulk density, and concentration and form of interacting elements and compounds. In addition, environmental conditions interact to promote Fe deficiency problems. In some years severe Fe deficiency is observed, while in other years little or no Fe deficiency occurs in the same locations. Cool, wet conditions often aggravate Fe deficiency, but the effects of temperature and moisture are complex and often contradictory.
High levels of bicarbonates (HCO3-) in the soil solution will induce Fe deficiency stress (Coulombe et al., 1984, Inskeep and Bloom, 1984). The concentration of HCO3- in the soil solution is affected by the concentration and reactivity of soil carbonates, exchangeable bases, soil moisture content, and the concentration of carbon dioxide (CO2). Several studies have compared soil conditions where no or serious deficiency symptom is apparent in the same field. Some have documented that higher solid phase carbonate concentration measured as calcium carbonate equivalent (CCE) relates to chlorosis expression (Franzen and Richardson, 2000; Inskeep and Bloom, 1987). Others have found that soil CCE did not differ with different degrees of chlorosis, but that the reactivity of carbonates (Morris et al., 1990) or the clay-sized fraction of carbonates had a strong relationship with Fe chlorosis (Inskeep and Bloom, 1986).
Soil salinity has also been linked to the occurrence of Fe deficiency in some crops, including soybean and corn. Reason for this relationship is lacking, but it is believed that decreased root growth with increased soil salinity may be at least part of the cause. Inskeep and Bloom (1984) compared soil characteristics for soybean differing in Fe deficiency and found that low chlorophyll content was associated with higher soil Mg2+, Na+, and Cl- concentrations and also a higher Mg/Ca ratio. Soluble salt concentration (EC) is related to differences in Fe deficiency chlorosis in soybean with higher EC observed for the chlorotic areas than for the non-chlorotic areas of the same field (Hansen et al., 2004). Similar observations have been made elsewhere (Franzen and Richardson, 2000; Inskeep and Bloom, 1987; Loeppert et al., 1994; Morris et al., 1990).
Neither total nor extractable soil Fe concentrations are useful as predictors of the risk of Fe deficiency in field crops. Total Fe levels may be quite high at the same time that available Fe concentration is limiting due to low solubility (Lindsay and Schwab, 1982). More important than total soil Fe are the concentration, mineralogy, and crystalinity of soil Fe oxide (Vempati and Loeppert, 1988). The DTPA extractable Fe test has been used as an indicator of available Fe (Olson and Ellis, 1982). However, the identification of soils prone to Fe deficiency using soil DTPA-Fe has been mixed. Hansen et al. (2003) found that soil DTPA-Fe levels were lower in Minnesota production areas where soybeans showed Fe deficiency symptoms than in areas without symptoms, while Inskeep and Bloom (1987) concluded that there was no difference in soil DTPA-Fe levels when comparing chlorotic and non-chlorotic areas of individual fields in Minnesota. In North Dakota, Franzen and Richardson (2000) found differences in DTPA-Fe between chlorotic and non-chlorotic areas for some sites but not for others. McKeague and Day (1966) and Morris et al. (1990) found a positive correlation between leaf chlorophyll concentration and the quantity of amorphous Fe oxide in calcareous soils.
Soil moisture content relates to the expression of Fe deficiency with deficiency most likely to occur in wet, but unsaturated soils. In greenhouse trials, chlorosis expression is exacerbated for soybean and oat with increasing soil moisture in calcareous soils (Inskeep and Bloom, 1986; Ocumpaugh et al., 1992). In field production of soybean, soil moisture was greater in areas where soybeans were Fe deficient than in areas where soybeans grew normally (Hansen et al., 2003). A combination of the increase in soil bicarbonate concentration due to high soil-water content and associated poor soil aeration leading to oxygen deficit and the resulting root growth reduction is the possible cause of chlorosis. The differences in Fe chlorosis versus chlorosis from hypoxia are distinct enough to diagnose the problem. High soil moisture content also causes accumulations of volatile compounds, such as ethylene, in the root environment, which may affect root growth and induce chlorosis expression (Inskeep and Bloom, 1986).
Field observations indicate that chlorosis is affected by soil compaction and the associated change in soil bulk density. In some instances, chlorosis in tractor-wheel tracks is worse than chlorosis outside of wheel tracks. This may be related to the fact that as air-filled porosity of soil decreases due to compaction, there is an inhibition of diffusion of plant and microbial-produced CO2 away from the rhizosphere. This localized accumulation of CO2 results in an increase in concentration of HCO3- and a decrease of Fe availability (Geiger and Loeppert, 1988). In other instances, chlorosis in wheel tracks is less severe than in surrounding areas (Hansen et al., 2004). This phenomenon has not been explained, but is likely related to impacts on soil temperature or water movement.
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