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The amount of Fe and Al extracted from soil by 0·1 M pyrophosphate (Fep, Alp) has long been used as a criterion in the designation of spodic horizons of podzols (Avery, 1973). Although Adams et al. (1980) showed that in a group of podzolic soils developed from Lower Palaeozoic sedimentary (LPS) rocks Fep was very closely correlated with the quantities of Fe eluviated and illuviated, such evidence validating the use of pyrophosphate is not generally available. Recently there has been renewed interest in the identification of forms of Fe and Al extracted by pyrophosphate in order that greater confidence might be gained in the use of Fep and Alp in soil classification (McKeague & Schuppli, 1982).Our interest in pyrophosphate extraction was stimulated when Nto (1981) discovered that Fep was an excellent predictor of P sorption in soils developed from LPS rocks. Indeed, the level of predictability was not improved by the inclusion, in multiple regressions, of either Fe or Al extracted by any other of the commonly used selective extractants.

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SUMMARY Lepidocrocite and goethite are pedogenetic crystalline Fe oxyhydroxides in the soils of mid‐Wales. Lepidocrocite is more abundant and widespread, occurring in brown earths (Denbigh series) as well as morphological gleys. It is commonly dominant in horizons intermediate in depth. Goethite occurs in subsoils and was found to be dominant in the subsoil of surface water gleys (Cegin series). Apart from the Cegin series, a large proportion of the total free Fe occurs in non‐crystalline forms. This amounted to 50–80% of the free Fe in organic and eluvial horizons of podzols. Despite lower proportions of non‐crystalline Fe in Bs horizons, amounts were similar to those in eluvial horizons. Data on the distribution of crystalline and non‐crystalline free Fe together with correlations observed between Fe and C extracted by pyrophosphate suggest that transformation and translocation of Fe in the podzols examined are governed by its association with organic fractions.

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