Manganese (Mn) is a divalent cation and the tenth most common element on land. Mn is attracted to negatively charged matter, such as clay and other organic matter. This leads to a greater accumulation of Mn in soils with more carbon. The Mn in soil forms incredibly stable bonds with humic acids, preventing plants from accessing it.

Mn2+ is the most soluble species of Mn, whereas Mn3+ and Mn4+ and their related oxides are very insoluble. Oxidation processes make Mn less soluble but reducing and acid environments make Mn more soluble and bioavailable to plants. In acid soils at pH <5.5 and in reducing conditions, the insoluble species can be reduced to the mobile Mn2+. Microbially-mediated manganese reduction is a primary mobilizer of Mn under low oxygen, low nitrate conditions. In conditions where Mn is excessive, liming can be used to raise the pH and make it less soluble.

Crops grown on calcareous soils with an elevated pH, or porous sandy soils with low organic matter, are more likely to experience manganese deficiency, because these conditions favor the oxidation of available Mn to Mn oxides. Mn deficiency is extremely detrimental to the plant’s health. A lack of Mn causes reduced electron transport and increased oxidative stress in the photosynthetic apparatus. Given the role of Mn in a wide range of enzymes, a deficiency can affect everything from ATP synthesis to the biosynthesis of lignin, fatty acids, and lipids.

Manganese is an essential micronutrient for plants, acting as an enzyme cofactor and structural component of many proteins and enzymes. Its primary function is in photosystem II (PSII), which splits water molecules, generating electrons necessary for photosynthesis. Mn is also an enzyme cofactor for superoxide dismutase (SOD), which helps plants cope with oxidative stress. Proper Mn nutrition ensures that plants grow to their full potential and are able to cope with whatever stresses they encounter.

When too much Mn is present in a plant, other ions are outcompeted and are unable to perform their proper functions. Mn can degrade the photosynthetic apparatus and other enzymes, as well as affect the absorption and utilization of other mineral nutrients.

Furthermore, Mn phytotoxicity often presents as interveinal chlorosis in leaves and necrotic leaf spots and the excess Mn cannot properly function as a cofactor for superoxide dismutase (SOD). The SODs are unable to properly regulate the levels of reactive oxygen species (ROS), making the plant particularly vulnerable to sunlight, which generates ROS in plants. Elevated Mn concentrations are associated with shallow acidic groundwater, often having leached through porous rock or near mining spoils. Sewage and other anthropogenic inputs can also raise Mn levels. Overapplication of N fertilizers can create a surge of nitrification in groundwater that results in acidification and mobilization of Mn and other trace elements.