the reducible material of a soil mainly composed of iron and manganese oxides, in particular know to fix the metals in trace amounts. such as clays, metal oxyhydroxides are secondary minerals resulting from weathering processes, slightly soluble in the pH rang of soils. tthey are divided into three size classes; in the form of "coatings" , "bridges" between the various components of the soil or "concretions" as isolated particles