Hydroxidogenesis is weakly pronounced in steppe landscapes: iron hydroxides are found only in 50% of fine-earth samples. Hydroxidogenesis is more pronounced in forest landscapes: iron hydroxides are found in 82% of fine-earth samples. In forest and steppe soils, different hydroxides form similar sequences with respect to their abundance in the fine earth: goethite > ferroxyhyte > ferrihydrite > (≈) lepidocrocite. In the fine earth of these soils, neosynthesis of unstable iron hydroxides (ferroxyhyte, lepidocrocite, and ferrihydrite) is in balance with their transformation to stable goethite. Hydroxides form the following succession by their occurrence in forest soil concretions: ferroxyhyte > goethite > lepidocrocite > ferrihydrite. As compared with the fine-earth fraction, concretions are only enriched in thermodynamically unstable iron hydroxides. The diversity of hydroxide types in concretions as a whole reaches a very high value (H_hydr = 0.92) due to a significant local variation in Eh. The investigation of mineral composition of concretion-forming (hydr)oxides permits passing from the existing chemical classification of concretions to a more sophisticated mineralogical classification. Many of the ferromanganese concretions in forest soils of the temperate zone belong to the ferroxyhyte-vernadite class, whereas some concretions in gley soils developed on varved clay belong to the goethite class.