Influence of iron-containing pigments on the color of soils on alluvium of the Middle Kama plain

The study of the color in the CIE-L * a * b * system proved that, among the soils developed on the recent and ancient alluvium in the Perm Cis-Urals region, there are no soils composed of only brown horizons: the low-redness topsoils occur even in the automorphic soils. The color of the bulk of the soils does not depend on the total content of Fe-(hydr)oxide particles. The soils developed on the two-layered deposits are an exception, since the color of the lower layer of the heavy loam is affected by hematite-containing clay particles. In the upper horizons of other automorphic soils, red pigment is produced rather by Fe-containing hydroxides (feroxyhyte δFeOOH and Fe-vernadite Fe-δMnO2) than by hematite αFe2O3. The gleyed horizons are rich in free iron compounds (up to 3.2% (Fe2O3)d) that exert a weak effect on the redness. An identification procedure of the horizons showing hydromorphic features is developed, including the color control both in the untreated samples and in the samples in which the organic substance has been oxidized with H2O2. The soil becoming green after the organic matter oxidation appears to be a distinctive feature of the hydromorphic horizons, while reddening is a property of the automorphic horizons.