Focus on nutritional elements: manganese 

29 may 2018

Manganese is a micro nutritional element. A plant only needs a small amount of it. It is an important building brick in several enzymes necessary for different plant processes. This month in ‘Focus on nutritional elements’, everything about manganese.

Manganese (Mn) is an important building brick in several enzymes in the plant that take care of plant processes such as photosynthesis, respiration, protein metabolism and multiplication of cells (mitosis). Manganese is taken up by the roots of the plant in the shape of manganese ions (Mn²⁺). The mobility of manganese in the plant is scarce, but not as scarce as calcium, boron, copper and iron. The manganese level in plants is approximately between 0,002 and 0,005 percent of the dry matter.

Manganese is added to most potting soil base fertilizers. The availability of manganese in the substrate moisture depends on the acidity of the substrate. In case of a lower pH, manganese, if present, becomes more soluble. Manganese can already be available in substrate components, for example in bark. Therefore application of such substrate components often leads to slightly higher manganese levels in the substrate moisture.

The effect of a manganese deficiency is visible in young leaves, but not in the youngest leaves. This is because of the level of mobility of manganese in the plant. A deficiency first shows as chlorosis (degradation of chlorophyll) between the nerves. Visibly it is similar to an iron deficiency. Deficiency usually occurs in growing media with a high or very high pH. Chlorosis can be followed by (red brown) necrosis. In rose culture and gerbera culture manganese deficiency is a well-known phenomenon. In roses a shortage of manganese can cause the leaves to fall off the plant. A manganese toxicity also happens regularly. A toxicity is often caused by a high degree of fertilizing with manganese. This is not a problem with a normal pH, but a damaging surplus may develop in case of a decreasing pH. Manganese toxicity develops faster in an acidic environment. This is expressed by purple red dots on especially the older leaves. These dots may spread along the edges of the leaves. This concerns precipitated manganese oxide (M_nO₂). Manganese toxicity may occur combined with an iron deficiency. The iron deficiency is usually caused by manganese forcing the iron out of the chelates, causing it to become unavailable to the plant.

Left: manganese deficiency in Gerbera. Right: damage to Geraniums because of, among others, manganese and zinc toxicity because of strongly decreased pH.

Manganese is added to most potting soil base fertilizers. Among other things, the manganese levels of RHP certified products are checked before application in a substrate. For substrates with the RHP Horticulture quality mark the substrate producer and the grower decide together what the manganese level should be, suitable to the culture.