ABSTRACT
Manganese (Mn) is an essential micronutrient for the growth and development of crop plants. Natural background concentrations of Mn in soil range from 0.5 to 5000 mg kg−1, and the oxidation state ranges from 0 to +7, with +2, +3, and +4 the most common oxidation states in natural environments (Hernandez-Soriano et al. 2012). In soils, Mn solubility is determined by two major variables, pH and redox potential. In well-aerated alkaline soils, Mn may not be sufficiently available for healthy plant growth (Graham, Davies, and Ascher 1985; Curtin, Martin, and Scott 2008). Mn3+ and Mn4+ hydroxides precipitate at pH above 5.5 and under aerobic conditions (Hernandez-Soriano et al. 2012). The Mn2+ form is highly soluble and is the thermodynamically most stable form in soils at low pH (Porter et al. 2004) or under reducing conditions such as may happen in flooded rice (Foy 1984; Weil, Foy, and Coradetti 1997).
