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Manganese

Manganese is a transition metal occurring in nature in the oxide form e.g. pyrolusite (MnO2). Because the electron expansion occurs in a shell other than the outermost, there is considerable similarity of the transition elements within a horizontal sequence. So much so, that in some cases the horizontal similarity through the period is greater than the vertical resemblance down through the sub-group. This is not at all surprising, since the outermost shell contains so few electrons. Also, the atomic volumes are extremely small. Another characteristic property of the transition elements is that in forming compounds they exhibit many oxidation states, presumably because some or all of the 3d electrons can be used with 4= electrons in chemical binding.In the case of the element manganese, for example, it has an oxidation number of +2, +3, +4, +6 and +7, corresponding to the use of none, one, two, four and five of the 3d electrons respectively.When manganese takes part in a chemical reaction, it can do so by losing the two 4= electrons from the Mn ++ ion; in addition, one or more of the five 3d electrons can be used for the chemical bonding.

An interesting adjunct in the function of manganese in nutrition is its function as MnO2 in the oxidation of vitamin A1 to retinene, a component (vision purple), of the photosensitive substance "rhodopsin" which is found in the retina of the eye.As mentioned, enzymes are complex substances in biological systems which act as catalysts for biological processes. Occasionally we encounter a special type of catalysis, which we term as an autocatalysis, or self-catalysis as the name implies, this is a catalysis in which one of the products of the reaction is catalyst for a reaction. An example of just such is in the reaction of permanganate and oxalic acid.2MnO4_ + SH2C204 + 6H+-> 2Mn ++ + lOCO2 +8H20In this reaction, the product Mn ++ catalyses the reaction.In the event that a soil is fertilized in excess with phosphates, there is a negative reaction on the oxidation-reduction or "Redox" potential of hydrogen peroxides formed by cell decomposition. In a normal observation, hydrogen peroxide decomposes to water (H2O) and oxygen (O). The reaction is rapid enough that solutions of hydrogen peroxides are difficult to keep without decomposition. When certain substances such as phosphates are added, the rate of reaction slows down. Here, the application of phosphates act in the reverse of that of the catalysis, and, in fact, substances which slow down the rate of reaction have been called negative catalysts. This term is misleading, since the function of the phosphate in hydrogen peroxide appears to destroy the action of the catalysts already present in the hydrogen peroxide. An example of this can be found by experimentation, in that the decomposition of hydrogen peroxide is catalyzed by traces of Fe+3 from functioning as a catalyst.Manganese in its elemental form decomposes cold water and dilute acids to give hydrogen, and reacts with oxygen and nitrogen when heated. In a soil environment the formation of non-soluble and non-exchangeable manganese oxides is dependant on the pH - soil biological status - other cation and anion ratios - soil moisture and soil temperature. Environmental conditions that are conducive to the formation of manganese+4 result in a reduction of Hydrogen peroxides due to the oxidizing potential of this form of manganese.Manganese deficiency is likely to be induced by over liming, or applying other cation elements in amounts that will create a displacement in exchangeable manganese. Mulching also can induce deficiencies on crops when grown on soils low in manganese. It has been suggested; S.M. Bromfield, Pl. Soil, 1958 9 325; 1959, l0 147.; that crop roots probably play an active part in bringing about the reduction of manganese in insoluble oxide precipitates through their root excretions. These excretions can be washed out of the soil, but they are readily decomposable by soil organisms. This may be the case, however the function of manganese at soil level introduces other factors such as its oxidation-reduction potential especially in when soil conditions are such that encourage the formation of manganese+4.There has been little recognition that the fertility of the animal is indeed a function of the fertility of the soil, and in fact, the health of the animal is a reflection of the fertility of the soil.

One of the tendencies in terms of soil fertility recommendations, especially in New Zealand and Australia, is to assume that the trace element levels will be adequate for pasture growth and animal nutrition. It is due to this ill founded assumption that there has been little if any emphasis placed on soil requirements, with exception of the elements cobalt and to a lesser degree, copper. This is especially the case for elements such as manganese and iron.It is well accepted and equally as well documented that this element has a tendency to display excessive soluble levels on acid soils, and while that may be the commonly accepted theory, it is a menacing generalization. It is primary for this reason that little emphasis has been placed on this element in New Zealand and Australia, and especially the former, due to the predominance of the acidic tendency of our agricultural soils.The New Zealand Department of Scientific and industrial Research, give a more in depth view of this element and its reaction in New Zealand soils than is usually available through other sources. They published the following in Soil Bureau Bulletin 26 (2) 1968.
Essentiality
Manganese is an essential element for both plants and animals. It is involved in photosynthesis reactions of algae, but this effect has not been proved for plants. The element is thought to activate many enzyme systems, e.g. dehydrogenases and carboxylases. In oxidation-reduction reactions manganese is associated with iron compounds. It also increases the resistance of roots to attack by bacterial grey-speck disease.Manganese is essential in the diet of rabbit and chickens; its absence result in abnormal bone formation and slipped tendon in chickens.
Toxicity 

An excess of manganese may induce a deficiency of iron by converting it to the less soluble ferric form. Plants with manganese toxicity usually contain above 3,000 p.p.m. Mn. This toxicity occurs in overseas countries on acid soils with high amounts of manganese. Animals have a high tolerance to salts of divalent Manganese. Hens can tolerate 10,000 p.p.m. Mn in their diet, but 4,800 p.p.m. has been found to be toxic to chickens. 

Deficiency Symptoms The general symptoms of manganese deficiency in plants is mottled chlorosis with interveinal areas yellow, while the veins remain green. Small animals fed on a diet low in manganese show impaired growth, defective mineralisation of bones, and depressed reproduction. Poultry with a diet low in manganese develop perosis; this is characterized by enlargement and malformation of the tibio metatarsal joint, shortening of the leg bones, and slipping of the Achilles tendon from its condyles. This makes movement difficult and the birds squat frequently and walk on their hocks.

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