Manganese: an essential element
Manganese is an essential trace element in the metabolism of all living organisms. Nevertheless, excessive doses are toxic, and the resultant disease may arise in the pulmonary system or the central nervous system (CNS). Available literature concludes that the problem presents itself if exposure is via inhalation and over long periods of time. The risk varies with the manganese species involved and their particle sizes and shapes.
The manganese industry has adopted specific measures to protect its workers. These include: reducing exposure levels and time of exposure, the use of exhaust ventilation, having workers in isolated control rooms or in air-conditioned or filtered air cabins, issuing better protective equipment.
Many countries have issued restrictions concerning the permissible amount of airborne manganese in dust and fumes (threshold limit value -TLV) and these will likely become tighter in the future.
Manganese, Plants and Soil
All plants require manganese for growth and reproduction. Manganese is the element chosen by nature to catalyse oxygen evolution in photosynthesis. When there is a lack of manganese, the structure of the chloroplasts is markedly impaired. Manganese also plays an important role in suppressing both leaf and root diseases. Some plants are particularly susceptible to manganese deficiency.
Manganese content in the soil can vary widely. It is only 50 ppm in some localities, but can reach 10,000 ppm in unleached alkali soils. Only the divalent cation (Mn+2) which is soluble and mobile in the soil, is easily available for absorption by plant roots. The tetravalent cation (Mn+4) is virtually insoluble. Both leaf-manganese and soil-manganese analyses are used to determine manganese deficiencies or toxicity.
Manganese deficiency in most crops is indicated by an overall chlorosis of the leaf, which leaves only the main veins and midrib outlined in dark green healthy tissue. The degree to which plants can absorb manganese is more important than its simple chemical presence in the soil.
Manganese sulphate is considered the most efficient source of manganese for fertilizer production, although manganese monoxide is also used. These are added in regions deficient in manganese. Manganese sulphate and other compounds can also be used in solution to spray on foliage.
It has been demonstrated that a fungal infection of wheat can develop as a result of manganese deficiency. To correct it efficiently, manganese must be brought to the roots of the plant. The hypothesis is that manganese has a direct negative effect on the fungus, and strengthens the host plant by modifying its resistance and susceptibility.
It should also be noted that an excess of absorbable manganese, which sometimes occurs in acid soil, can be toxic to some species. The problem is generally solved by adding lime.
