Manganese in the soil
Manganese occurs mainly in oxide form but also occurs in silicates. Mn2+ ions are released into the soil solution during weathering of silicates. As well as the clay content of the soil, the pH and the redox potential of the soil are also important factors determining the potential of a soil to hold such readily exchangeable manganese.
With a decreasing pH value and decreasing redox potential comes an increase in the concentration of plant available Mn ions. A low redox potential occurs during low oxygen concentrations in the soil i.e. compaction, flooding, standing water). In contrast, a high pH value and adequate soil aeration decreases the concentration of Mn-ions.
Manganese deficiency mainly occurs in organic and carbonate rich soils, high pH soils and very light sandly soils which are often well aerated because of manganese fixation. Humus rich and podsol sandy soils are rather Mn poor as the manganese is less fixed.
Manganese in the plant
Manganese is taken up by the plant only as Mn2+-ions. This process can be inhibited by high concentrations of Mg2+-, Ca2+-, Cu2+- and iron ions. Manganese either stimulates, or is a component of many enzymes and, therefore, greatly affects the metabolism of the plant.
Functions of manganese in the plant:
- directly affects photosynthesis by assisting the synthesis of chloroplasts.
- Important role in synthesis of fatty acids.
- Affects energy budget by regulating carbohydrate metabolism.
- Reduction of nitrates in plants is only possible if sufficient manganese is present.
- Increases growth of secondary roots.
- Stimulates growth due to effect on elongation of cells.
- Similar to copper, manganese is important for immobilisation of free oxygen radicals.
- Manganese and magnesium increase the concentration of valuable ingredients such as citric acid and vitamin C. They increase the quality of frozen vegetables and the resistance of potatoes against discoloration during processing to mash and potato powder.
- youngest and mid leaves show chlorotic spots between the veins because the development of chloroplasts is negatively affected.
- Gramineae show chlorotic and necrotic strips.
- especially characteristic are the deficiency symptoms in oats which are called: grey speck or early blight; here the plant exhibits dirty grey strips or spots on the base of the leaves. The entire water balance is affected.
- Manganese deficient plants have a lower cell volume. Cell elongation and growth of secondary roots are negatively affected.
- occurs on acidic soils because these soil solutions mainly contain Mn2+ ions which are easily taken up.
- older leaves, leaf bases and stems show black-brown spots as a consequence of MnO2 deposits. Later on they show chlorotic margins.
- Induced deficiency of iron, magnesium and/or calcium can occur and add to the symptoms exhibited by the plant.