Abstract
Chemical reactions in soils largely determine the availability of Fe to plants. The solubility of Fe in soils can best be expressed as: Fe(OH)3(soil-Fe) + 3H+ ⇌ Fe3+ + 3H2O having a log K° value of 2.70. This solubility constant is 0.84 log units less than that of freshly precipitated amorphous Fe(OH)3, and approximately 2.7 log units more soluble than goethite and hematite. Hydrolysis species of Fe3+ raise total soluble Fe to approximately 10-10.4 M in calcareous soils, but plants growing in soils require approximately 10-8 M soluble Fe. Without some modifying mechanism, most plants would show Fe deficiencies when growing in media above pH 5.0.
Some plants are capable of lowering redox next to respiring roots to the pe + pH range of 4 to 7. This highly reducing environment is capable of supplying available Fe2+ through the dissolution and reduction of Fe(III) oxides.
Chelating agents in the rhizosphere are beneficial because they raise total Fe in solution, increase diffusion gradients, and eliminate Fe depletion zones next to absorbing roots. Strong chelating agents such as FeEDDHA generally correct Fe deficiency, but in hydroponic solutions, they sometimes cause Fe deficiency. As plants absorb Fe, chelating agent is left behind and depresses Fe3+ activity making it less available to plants. Adding extra Fe to a nutrient media, keeps the chelating agent saturated with Fe and available to plants.
Both synthetic and natural chelating agents act as carriers to transport Fe to plant roots. The enriched electron environment of respiring roots reduces Fe3+ to Fe2+ thus decreasing Fe3+ and causing ferric chelate (FeL-) to dissociate. The newly formed Fe2+ is available for absorption by roots while the released chelating agent diffuses away from the root and becomes resaturated with Fe3+. Chelation and reduction are important mechanisms that take place near roots which affect the availability of Fe to plants.
Chemical speciation models such as MINTEQA2 and GEOCHEM-PC are useful for predicting and interpreting complex equilibrium relationships involving chelation, Fe solubility, redox, and nutrient uptake. Computer models are useful for examining equilibrium relationships that simulate root environments.
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Lindsay, W. (1995). Chemical reactions in soils that affect iron availability to plants. A quantative approach. In: Abadía, J. (eds) Iron Nutrition in Soils and Plants. Developments in Plant and Soil Sciences, vol 59. Springer, Dordrecht. https://doi.org/10.1007/978-94-011-0503-3_2
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DOI: https://doi.org/10.1007/978-94-011-0503-3_2
Publisher Name: Springer, Dordrecht
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