Field tests for ferrous iron and ferric-organic complexes (on exchange sites or in water-soluble forms) in soils

A 1 M solution of ammonium acetate containing á, á¿-dipyridyl indicator is recommended for use in field tests for exchangeable and water-soluble ferrous iron in soils. In using the test, a soil sample may be added to a vial containing the solution, or the solution can be sprayed on to a freshly exposed soil face. A field test for ferric-organic complexes in soils, using the same vials, is proposed. This is based on the photochemical nature of the reduction of ferric to ferrous iron in the presence of oxidizable organic ligands. Two subsamples of soil are added to separate vials, and light is excluded from one. After 5-15 min, a positive test for ferrous iron in the vial exposed to light, and a negative test in the other, indicates the presence of ferric-organic complexes.