Soil Testing for Iron

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.

The micronutrients in the extract are measured by atomic absorption spectrometry, inductively coupled plasma spectrometry, or neutron activation analysis.