Three methods have been used to extract iron (Fe) and aluminium (Al) from a wide range of soils. The three extractants were pyrophosphate reagent (p), acid-oxalate reagent (o), and dithionite-citrate reagent (d), and each reagent is thought to extract different forms of Fe and Al. The forms of Fe in the soils were studied before and after extraction using Moessbauer spectroscopy. Pyrophosphate-extractable Fe (Fep) does not specifically relate to any particular form of Fe in soils and it should not be used to estimate Fe in humus complexes. In some soils (e.g., Spodosol, Ochrept) Fep arises from Fe in ferrihydrite. In other soils (Ultisol, Hydrandept) Fep arises from goethite and ferrihydrite which is dispersed by pyrophosphate. Alp corresponds to Al in humus complexes and, in most soils, can be used to estimate Al in such complexes. Al and Si in allophane and imogolite were extracted in acid-oxalate reagent, and could be estimated from Alo-Alp and Sio, provided large amounts of ferrihydrite were not present. Ferrihydrite was also extracted in acid-oxalate reagent and, assuming complete extraction, could be estimated by multiplying Feo by the factor 1.7. Dithionite-citrate reagent dissolved goethite and hematite in addition to ferrihydrite. In some soils repeated extractions were required to dissoive most of the goethite and hematite. Quantitative estimates of Fe in goethite and hematite can be made from Fed -Feo. Other techniques, however, such as X-ray diffraction and Moessbauer spectroscopy, were necessary to identify iron oxides positively in most soils.

A 1 M solution of ammonium acetate containing �, �-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.

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The Mossbauer spectra of a size range of synthetic akaganeite (β-FeOOH) samples containing chloride, and one sample containing fluoride, in the structural tunnels, have been obtained over 4-295 K. At 295 K the spectra consist of overlapping paramagnetic doublets which indicate the presence of at least two sites for iron. Typical parameters for a spectrum of chloride-containing β-FeOOH fitted to two doublets are: (i) δ (relative to iron metal) 0.372 mm s-1, Δ 0.55 mm s-1, Г 0.27mm s-1; (ii) δ 0.375 mm s-1, Δ 0.94mm s-1, Г 0.33 mm s-1, with peak area ratios (i) : (II) of 60 : 40 for lath-shaped particles and 53 : 47 for spindle-shaped particles. At 77 K the spectra consist of overlapping magnetic hyperfine components which indicate the presence of at least three sites for iron, a three-component model giving internal magnetic fields of about 470 kOe, 460 kOe, and 435 kOe. The spectra of fluoride-containing β-FeOOH are very similar to those for chloride-containing β- FeOOH. The results are explained in terms of two types of octahedra within the lattice, FeO3(OH)3 and FeO2(OH)4, the extra proton in the second type balancing the charge on halide ions within the tunnels. Secondary effects may arise from the interaction of the halide ions with adjacent octahedra. The effects of modifying the surfaces of β-FeOOH particles (heating, phosphate sorption) are consistent with modification of the outer surfaces of the particles only, and it is not necessary to invoke the presence of large internal pores, of the order of 30Ǻ by 30Ǻ, as has been suggested by some earlier workers.