figure 11.1 Solubility of inorganic Fe in equilibrium with Fe oxides in a well-aerated soil. The shaded zone represents the concentration range required by plants for adequate Fe nutrition. (Redrawn from Romheld, V., Marschner, H., in Advances in Plant Nutrition, Vol. 2, Praeger, New York, 1986, pp. 155-204 and Lindsay, W.L., Schwab, A.P., J. Plant Nutr., 5:821-840, 1982.)
With an increase in soil pH from 4 to 8, the concentration of Fe3+ ions declines from 10"8 to 10"20 M. As can be seen from Figure 11.1, the minimum solubility of total inorganic iron occurs between pH 7.4 and 8.5 (14).
The various Fe(III) oxides are major components of a mineral soil, and they occur either as a gel coating soil particles or as fine amorphous particles in the clay fraction. Similar to the clay colloids, these oxides have colloidal properties, but no cation-exchange capacity. They can, however, bind some anions, such as phosphate, particularly at low pH, through anion adsorption. For this reason, the presence of these oxides interferes with phosphorus acquisition by plants, and in soils of pH above 6, more than 50% of the organically bound forms of phosphate may be present as humic-Fe(Al)-P complexes (15).
Although Fe(III) oxides are relatively insoluble in water, they can become mobile in the presence of various organic compounds. As water leaches through decomposing organic matter, it moves the Fe(III) oxide downwards, particularly at acidic pH, so that under such conditions podzols form. The iron is essentially leached from the top layers of soil as iron-fulvic acid complexes and forms an iron pan after precipitation lower down at higher pH. The upper layers are characteristically light in color, as it is the gel coating of Fe(III) oxide that, in conjunction with humus, gives soils their characteristic color. However, in soils in general, the intensity of the color is not an indication of iron content.
These organic complexes tend to make iron more available than the thermodynamic equilibrium would indicate (16), and in addition to iron-forming complexes with fulvic acid, it forms complexes with microbial siderophores (13), including siderophores released by ectomycorrhizal fungi (17). A water-soluble humic fraction extracted from peat has been shown to be able to form mobile complexes with iron, increasing its availability to plants (18).
In soils with a high organic matter content the concentration of iron chelates can reach 10"4 to 10" 3M (17,18). However, in well-aerated soils low in organic matter, the iron concentration in the soil solution is in the range of 10"8 to 10" 7M, lower than is required for adequate growth of most plants (13).
Under anaerobic conditions, ferric oxide is reduced to the Fe(II) (ferrous) state. If there are abundant sulfates in the soil, these also become oxygen sources for soil bacteria, and black Fe(II)
sulfide is formed. Such reactions occur when a soil becomes waterlogged, but on subsequent drainage the Fe(II) iron is oxidized back to Fe(III) compounds. Alternate bouts of reduction and oxidation as the water table changes in depth give rise to rust-colored patches of soil characteristic of gleys. Ferrous iron, Fe2+, and its hydrolysis species contribute toward total soluble iron in a soil only if the sum of the negative log of ion activity and pH together fall below 12 (equivalent to Eh of +260mV and +320mV at pH 7.5 and 6.5, respectively) (13,14). It is likely that the presence of microorganisms around growing roots causes the redox potential in the rhizosphere to drop because of the microbial oxygen demand, and this would serve to increase concentrations of Fe2+ ions for plant uptake (21).
Because the solubility of Fe3+ and Fe2+ ions decreases with increase in pH, growing plants on calcareous soils, and on soils that have been overlimed, gives rise to lime-induced chlorosis. The equilibrium concentration of Fe3+ in calcareous soil solution at pH 8.3 is 10"19mM (22), which gives noticeable iron deficiency in plants not adapted to these conditions. It has been estimated that up to 30% of the world's arable land is too calcareous for optimum crop production (23,24).
Iron deficiency can also arise from excess of manganese and copper. Most elements can serve as oxidizing agents that convert Fe2+ ions into the less soluble Fe3+ ions (25), and excess manganese in acid soils can give rise to deficiencies of iron although it would otherwise be present in adequate amounts (26).
Corn (Zea mays L.) and sugarcane (Saccharum officinarum L.) may show iron deficiency symptoms when deficient in potassium. It seems that under these circumstances iron is immobilized in the stem nodes, a process that is accentuated by good phosphorus supply (27). Iron can bind a significant proportion of phosphate in well-weathered soil (as the mineral strengite), and as this substance is poorly soluble at pH values below 5, iron contributes to the poor availability of phosphorus in acid soils (25).
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