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Manganese is an important micronutrient, and its availability in soil is affected by a wide variety of factors. In properly aerated and drained soils, and during growth, it temporarily switches to a less available form, at the very moment when plants need it most. Changes in soil moisture can convert available forms into unavailable forms and vice versa. Rapid changes can either increase or decrease its availability, causing deficiency or even excesses to toxic levels. It is therefore important to consider environmental influences, in order to assess the deficiency risk and to interfere with foliar fertilization.
Manganese is involved in the reduction of nitrates and the synthesis of amino acids for building up proteins. Deficiency always leads to growth disruption, and therefore to a loss of yield. It plays an essential role in enzymes and in the synthesis of chlorophyll, which explains why chlorosis can develop between the veins of young leaves when a deficiency occurs. During full vegetative development manganese, together with nitrogen and magnesium, can be a determining factor in the success of most crops.
Changes in soil moisture can either increase or decrease the availability of Mn. Therefore even in properly drained and aerated soils, manganese can become the first limiting factor. Manganese is present as Mn+2, Mn+3 and Mn+4, but Mn+2 is the primary form in which Mn is absorbed by plants.
INTERACTIONS AND SPECIAL FEATURES
Soil application is less effective since its availability is mostly influenced by the soil properties and environmental factors. It is therefore necessary to intervene with foliar application, which is ideally split into several applications.
AVAILABILITY OF MANGANESE
The availability of manganese is influenced by pH and deficiencies are most likely to occur in calcareous soils. Mn is most available between pH 5 and 6.5. At very low pH (< 5), Mn toxicity is possible. Mn+2 is readily chelated by organic molecules which is favoured by a high organic matter content and also reduces Mn availability. Other nutrients such as Cu, Fe, Ni and Zn may inhibit Mn uptake.
Soils with a high granite and sandstone content are naturally poorer than volcanic or sedimentary soils. High organic matter content negatively affects the solubility of manganese. Liming has the same effect. Finally, dry weather is not conducive to the presence of absorbable manganese. In damp soils, such as waterlogged soils during winter months, manganese is identified by bluish-grey stains.
MANGANESE CAN BE FOUND IN THE SOIL IN DIFFERENT FORMS:
The oxidised, trivalent or tetravalent forms, which are very hard to assimilate, are the most common forms. These are Mn3+ and Mn4+.
The divalent Mn2+ form, which can be assimilated by crops, is absorbed in clay minerals and in organic matter, and is also contained in the soil solution.
Sensibility table & Symptomes
Due to the high mobility of Mn in plants the symptoms are first visible in the young leaves and can be recognized by interveinal chlorosis. It can be confused with Mg or Fe deficiency, but Mn deficiency occurs at the older leaves first and Fe deficiency is marked by a sharper contrast between the green veins and areas affected by chlorosis.
Excess & Needs
Excess manganese in the soil can negatively affect crops growing in damp soils, or under rainy conditions. The same applies under anaerobic conditions.
Effective Mn-compounds for foliar application are sulphates, oxides, nitrates and carbonates. Generally, manganese penetrates the leaves in a few days, which means that nutrient complementation can be repeated several times during the vegetative growth phase. The choice of formulations aims to sustain the nutrient supply of foliar applications over a longer period, in order to limit the number of applications.
Although most of the manganese present in soils is in the form of oxides, the manganese analysis through an EDTA or DTPA extractive is a good indicator of absorbable manganese.
ORGANIC MATTER CONTENT
High organic matter content causes a blockage of the manganese by the formation of a manganese/organic matter complex.
Light sandy textures are subject to constant aeration, which causes manganese oxidation.
Climate has a strong influence on the availability of Mn2+. Deficiency is most common under cold and damp conditions. But also, in favourable growth conditions, manganese can be oxidised and is no longer absorbable.
The pH value has a significant effect on the availability of manganese. At pH<6 the risk of manganese deficiency is low. At pH 7 and above, trivalent forms are in the majority and the risk of deficiency is heightened. However, in very acid and manganese-rich soils, the element can become toxic. Frequent liming causes a blockage of manganese, due to the rapid pH increase.