Manganese

Manganese is a micronutrient used by plants to make chlorophyll. Manganese can also be phytotoxic, which means it can be poisonous to plants.

Made by large stars just before they go supernova, manganese is the 12th most abundant element of the Earth’s crust, and early man used manganese as a pigment in cave paintings some 20,000 years ago. How we use it in the garden can help or harm our plants.

Cave paintings of  Lascaux (Public Domain)

Manganese cycle

You may have heard about the nitrogen cycle, the Calvin cycle, or the carbon cycle, but did you know there is a manganese cycle? I didn’t either. It ends up that manganese can take many different forms, depending on what it is attached to. [Mg2+ is the form most commonly used by plants.] 

Unlike many other elements, which can exist on their own in nature, manganese prefers being attached to other minerals, usually iron. This can cause a whole Domino Effect when it comes to feeding your plants. According to studies conducted by Cornell University, high levels of copper (Cu), iron (Fe), nickel (Ni), and zinc (Zn) can make it difficult for plants to absorb manganese. At the same time, plants low in calcium (Ca), iron (Fe), magnesium (Mg), phosphorus (K), or silicon (Si) are also more likely to be sensitive to high manganese levels.

How plants use manganese

Manganese is used by all living things as an antioxidant, to counteract the toxic effects of oxygen. In plants, it is an important component of chloroplasts. Chloroplasts are where chlorophyll is made. Manganese is also used during photosynthesis, in many enzyme reactions, and to make potassium and calcium more readily available. Crops such as oats, wheat, and barley use a lot of manganese, with corn using moderate amounts.

Once inside a plant, manganese stays where it was first used. As a highly immobile plant nutrient, this means that deficiencies are most often seen in new growth, while toxicities are seen in older growth.

Manganese toxicity has a big impact on plant health (Alandmanson) CC BY 4.0

Manganese toxicities

Plants can absorb too much manganese in acid soils, or under drought conditions. When acid-forming fertilizers, superphosphates (fertilizers made by treating phosphate rock with phosphoric or sulfuric acid) are used, or when nitrate (NO3-) is used as a nitrogen source, those acidic conditions can occur. Manganese is most available to plants when the soil pH is between 5.0 and 6.5. Soils with neutral or alkaline pH slow the solubility of manganese, so toxicities are less likely.
The most common symptoms of too much manganese look a lot like the symptoms of too much boron: 

• darkening of leaf veins, especially on older leaves
• leaf cupping
• older leaves exhibit pale, sunken, irregularly shaped patches of interveinal tissue, with patches closest to leaf margins turning black [these spots are more regularly spaced in boron toxicities]
• minor veins on the underside of leaves may appear black in older leaves
• soybean leaves crinkle and cup downward
• apple stems develop pustules in a condition called apple measles
• watermelon plants may wilt and drop leaves in only days, in a condition called sudden crash
• chlorosis occurs on younger leaves due to an induced iron deficiency

Too much manganese interferes with root growth and causes overall stunting, especially in alfalfa, small grains, and beans.

While magnesium is needed by all living things, too much magnesium can be very, very bad. At high doses, inhaled magnesium can lead to neurological damage called manganism, a condition similar to Parkinson’s disease. If you have to work with manganese, wear protective gear.

Manganese deficient roses (Malcolm Manners) CC BY 2.0

Manganese deficiencies

Being an immobile nutrient, manganese deficiencies are first seen in new growth. When manganese is in short supply, you will see interveinal chlorosis (yellowing between leaf veins). If there is a sharp distinction between veins and yellowing, it may be an iron deficiency, or a combination of insufficient manganese and iron.  If this symptom is seen in older leaves, it is more likely to be a magnesium (Mg) deficiency.

Manganese deficiencies are more common in mucky soil, which means providing good drainage can prevent this problem. Cold and wet conditions can also interfere with manganese uptake. Due to its immobility as a plant nutrient, foliar (leaf) sprays of manganese are recommended if deficiencies have been identified.

Looking at plant leaves can tell you a lot about what they have and what they need.