Distribution of iron forms in different types of black earths in the Chełmno Lake District (Northern Poland) as an indicator of soil-forming process

Abstract

An important part of soil science research is the classifi cation of soils. Numerous soil scientists have paid special attention to various dependencies related to the content and distribution of iron forms in the soil profi le. This allows the taxonomic unit to be determined according to soil classifi cations. The research objective was to evaluate the distribution of iron forms as an indicator of soil-forming processes in four profi les of arable black earths. The research subject was soils formed from glacial sediments in the Chełmno Lake District (northern Poland). They were determined by morphological description to be Phaeozems representing four subtypes: Haplic Phaeozem, Luvic Gleyic Phaeozem, Gleyic Phaeozem and Gleyic (Cambic) Phaeozem. Soil horizons were described according to FAO guidelines. The soil samples were analysed for the content of free iron oxides (Fe d ) according to Mehra and Jackson's method, as well as the content of amorphous iron oxides, (Fe o ) according to Schwertmann's method. Based on the content of Fe d and Fe o , the content of crystalline iron oxides (Fe c ) was calculated using the formula: Fe c = Fe d -Fe o and the iron oxides activity index was calculated from the Fe o /Fe d ratio. The content of Fe t was highest in the Bt horizon of profi le II. In individual profi les, Fe d content was highest in the Bt horizon of gleyic Phaeozem and the AB horizon of gleyic (cambic) Phaeozem. In Luvic Gleyic Phaeozem and Gleyic (Cambic) Phaeozem iron was removed from the surface horizon to be accumulated in the illuvial horizon. The uniformity of content of total iron and its free iron oxides in the parent material of the analysed soils indicates its genetic homogeneity. The release of iron in the Ap horizon is characteristic of chemical weathering. The relationship between the content of Fe o and Fe d determines the degree of crystallization of free iron oxides. Based on the statistical analysis, a signifi cantly positive correlation between the content of clay fraction and all the iron forms determined was found. All the profi les were characterized by similar degrees of weathering of the soil material, which was determined on the basis of the Fe d /Fe t ratio. The soils also demonstrate a low value of iron mobilization, as confi rmed by the values of the Fe d /Fe t ratio. Determining iron forms and interpreting analysis results help in the correct classifi cation of soils, because the distribution of iron forms in soil profi les depends mainly on pedogenesis.