Because of the major impact of soil pH and bicarbonate content on the availability of iron to plants,
it is not common to test a soil for iron extractability. Tests of soil pH and lime content are much
more valuable in assessing where lime chlorosis is likely to occur.
Where testing of iron content is desired, early methods were based on determining the
exchangeable iron by extraction with ammonium acetate (104)
. Nowadays, soil iron is extracted by
the use of a chelating agent, in some cases EDDHA but more commonly DTPA (diethylenetriaminepentacetic
acid). This method, first proposed in 1967, is used for the analysis of zinc, iron,
manganese, and copper in soils together, and involves adding DPTA to a soil solution buffered at
pH 7.3 (105)
. The mixture contains CaCl2
so that any CaCO3
in the soil is not dissolved, with corresponding
release of otherwise unavailable micronutrients.