Illuvial horizon

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The illuvial horizon is the washing-in or enrichment horizon of a soil layer in geology and soil science .

The layer is clearly delimited from the overlying eluvial horizon and, depending on the dominant displacement products, is colored black ( aluminum ) to rust-red ( iron ).

Low-molecular compounds such as polysaccharides , carboxylic acids and water-soluble low-molecular humic substances (e.g. fulvic acids) washed into the upper soil layer form metal-organic complexes ( chelates ) together with aluminum and iron ions . If further ions are bound due to the low pH value in the soil, the complexes can flocculate. If, on the other hand, there is an increased calcium content (higher pH value ) in the subsoil , this can lead to the decay of the complexes, to flocculation or to polymerisation of the low molecular weight acids.

The enriched substances have a stabilizing or solidifying effect in the deposition area, especially with repeated drying out. The resulting B-horizon can become humus-rich (Bh-horizon), iron-rich (Bs-horizon) or clay-rich (Bt-horizon) through the storage.