Manganese (Mn⁺⁺)

Manganese is essential for many plant functions. Some of them are:

• The assimilation of carbon dioxide in photosynthesis.
• It aids in the synthesis of chlorophyll and in nitrate assimilation.
• Manganese activates fat forming enzymes.
• It functions in the formation of riboflavin, ascorbic acid, and carotene.
• It functions in electron transport during photosynthesis.
• It is involved in the Hill Reaction where water is split during photosynthesis.

• Soil pH: High soil pH reduces Mn availability while low soil pH will increase availability, even to the point of toxicity.
• Organic Matter: Mn can be “tied up” by the organic matter such that high organic matter soils can be Mn deficient.
• Soil Moisture: Under short-term waterlogged conditions, plant available Mn⁺⁺ can be reduced to Mn⁺, which is unavailable to plants. However, under long-term reducing (e.g. waterlogged) conditions, available Mn can be increased. As soil dries, Mn availability undergoes changes. Some unavailable Mn⁺ is oxidized to available Mn⁺⁺, while some available Mn⁺⁺ can be oxidized to unavailable Mn⁺⁺⁺⁺. When the soil is in transition from flooded to normal moisture content, there can be a temporary “flush” of excess Mn⁺⁺ giving the possibility of a temporary toxicity, especially if other conditions are favorable to the presence of excess Mn⁺⁺.
• Mn:Fe Balance: Soils high in available Iron (Fe), or high Fe applications can reduce Mn uptake.
• Mn:P Balance: There is conflicting research that high soil P can either increase, or decrease Mn uptake by various plants species. Until more definite evidence is available, we probably should not include the soil P level in our consideration of Mn availability.
• Mn:Zn Balance: There is conflicting research high soil Zn can either increase, or decrease Mn uptake by various plant species. Until more definite evidence is available, we probably should not include the soil Zn level in our consideration of Mn availability.
• Mn:Mo Balance: One researcher observed that Mn concentrations were reduced in half by molybdenum (Mo) fertilization. This limited evidence should not be used to make Mo recommendations due to the possible toxic reactions of high Mo contents that could occur in animals grazing or eating the crops grown on high Mo soils.
• Mn:Si Balance: Research has shown that silicon (Si) applications can alter the Mn distribution in leaf tissue in such a way as to reduce the possibility of Mn toxicity from excess Mn uptake.
• N STRESS: Low N availability decreases the vigor of plants to an extent that it may fail to take up adequate amounts of many other nutrients. Manganese uptake can be affected in this way.
• Mn:S Balance: The Sulfur interaction is primarily one-way, as the Sulfur content of the plant is diminished so also is the Manganese content.
• Mn:Anion Balance: Heavy fertilization with materials containing Cl^-, NO₃^-, SO₄^–, can also enhance Mn uptake (termed the anion effect).

While this is an essential element for all plants, these crops have been found to be especially responsive: alfalfa, beets, cauliflower, citrus, cotton, large-seeded legumes, lettuce, onions, potatoes, small grains, sorghum, soybeans, spinach, sweet corn, and tobacco.

Because Mn is not translocated in the plant, deficiency symptoms appear first on younger leaves. The most common symptoms on most plants are interveinal chlorosis. Sometimes a series of brownish-black specks appear in the affected areas. In small grains, grayish areas appear near the base of younger leaves. Manganese deficiencies occur most often on soils with a high pH and/or naturally low Mn content. Conifers will exhibit a general yellowing of the current season's needles.

Manganese toxicity is a relatively common problem compared to other micronutrient toxicity. It normally is associated with soils of pH 5.5 or lower, but can occur whenever the soil pH is below 6.0. Symptoms include chlorosis and necrotic lesions on old leaves, dark-brown or red necrotic spots, accumulation of small particles of MnO₂ in epidermal cells of leaves or stems, often referred to as “measles”, drying leaf tips, and stunted roots. Sometimes the interveinal tissue will show “puckering” or raised areas in the leaves. Toxic symptoms can sometimes be alleviated by using Iron chelates applied to either the soil or preferably the foliage. Some acid-loving plants such as blueberries, cranberries, Christmas trees, azaleas, etc. may accumulate very high levels of Mn in their tissue due to the required low soil pH. However, these plants normally will tolerate much higher tissue Mn than other species.

Broadcast applications are not recommended because Mn that is not concentrated in a band or similar method is quickly converted to unavailable forms when it comes into contact with the soil. Liming soils to the proper pH for the crop is the most practical way to avoid the majority of problems with Mn. On high pH soils, the use of acid-forming fertilizers in the row can increase the uptake of Mn, and other micronutrients. One example of an acid-forming fertilizer blend would be one based on monammonium phosphate and ammonium sulfate. Where foliar Mn is used, multiple applications throughout the season are often needed to compensate for soil deficiencies.

NOTE: It has been reported that if a Mn-chelate (EDTA) is added to the soil to correct an apparent deficiency problem, the most common result is increased Mn deficiency. This occurs because the affinity of chelates for iron is greater than their affinity for manganese and substitution occurs. The Fe-chelate is rapidly taken up by the plant and the ensuing interaction increases the Mn deficiency.