Cobalt (Co) and manganese (Mn) are closely associated in soils because they have similar chemical properties. The main forms of Mn in soil are the water-soluble and exchangeable forms of Mn(II) and the insoluble Mn oxides, mainly Mn(IV) and to a lesser and more uncertain extent as Mn(III). The concentration of water-soluble plus exchangeable Mn(II) (WS+Exch Mn) is determined by the relative rates of the chemically independent and physically separate reactions, the microbial oxidation of Mn(II) and the chemical reduction of the Mn oxides (by organic matter). The solubility and availability of Co to plants is influenced greatly by the activity of the Mn oxides and the reactions which affect Mn. The Mn oxides also participate in sorption and oxidation reactions which impact on soil health in that the former affects the availability of trace metals and the latter oxidises organic moieties, of which some are phytotoxic.