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Iron nutrition

Author : Doctor Liu Date : 8/31/2011 6:50:59 AM


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 humicFe(Al)-P complexes 
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 , and in addition to iron-forming complexes with fulvic acid, it forms complexes with microbial siderophores , including siderophores released by ectomycorrhizal fungi . 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 .
10 soil solution is in the range of 10 plants .
In soils with a high organic matter content the concentration of iron chelates can reach 10 4  to 3 M


The typical symptoms of iron deficiency in plants are chlorotic leaves. Often the veins remain green whereas the laminae are yellow, and a fine reticulate pattern develops with the darker green veins contrasting markedly with a lighter green or yellow background . In cereals, this shows up as alternate yellow and green stripes . Iron deficiency causes marked changes in the ultrastructure of chloroplasts, with thylakoid grana being absent under extreme deficiency and the chloroplasts being smaller . As iron in older leaves, mainly located in chloroplasts, is not easily retranslocated as long as the leaves are not senescent, the younger leaves tend to be more affected than the older leaves . In extreme cases the leaves may become almost white. Plant species that can modify the rhizosphere to make iron more available can be classified as iron-efficient and those that cannot as iron-inefficient. It is among the iron-inefficient species that chlorosis is most commonly observed.

Iron toxicity is not a common problem in the field, except in rice crops in Asia . It can also occur in pot experiments, and in cases of oversupply of iron salts to ornamental plants such as azaleas. The symptoms in rice, known as ‘Akagare I’ or ‘bronzing’ in Asia, include small reddish-brown spots on the leaves, which gradually extend to the older leaves. The whole leaf may turn brown, and the older leaves may die prematurely . In other species, leaves may become darker in color and roots may turn brown . In rice, iron toxicity seems to occur above 500 mg