Analytical Methods:Fe Fractionation

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Iron Fractionation
The form, distribution and total concentration of iron in soil is strongly affected by soil forming processes such as weathering, leaching, podzolisation and gleying (waterlogging). Identifying the effects of these processes are important in reconstructing past environmental conditions from buried soils. The effects of these processes can also have significant effects on the field appearance of deposits and on the distribution and concentration of a range of soil elements, not just iron. The possible occurence of podzolisation is also important if a deposit is to be radiocarbon dated as organic matter may be translocated down profile alongside iron.

The fractionation of iron from the iron rich spodic horizon, either using pyrophosphate is used as one of the tests to identify podzol soils in many soil classification systems. For example, in England and Wales (Avery, 1980) a spodic horizon should contain more than 0.3% iron extractable in potassium pyrophosphate (0.1M K4P2O7, Bascomb, 1974), and in the USDA and FAO systems aluminium + 1/2 iron concentration in an ammonium oxalate extract totalling 0.5% or more (Soil Survey Staff, 2006), or twice that in the overlying horizon (FAO, 1998).

Analysis of the extract can be made using a colorimeter or atomic absorption spectrophotometer, alongside appropriate standards and blank solutions.

Related Techniques

 * Field determination of free iron and reducing conditions
 * Elemental analysis

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