Manganese is a plant micronutrient. It fulfils a number of roles and is used in photosynthesis (manganese is important for a number of aspects of photosynthesis), synthesis of chlorophyll and nitrogen absorption as well as the synthesis of riboflavin, ascorbic acid and carotene.
A wide variety of factors can affect manganese availability including:
- pH ‑ high pH reduces availability and low pH can increase availability to the point of toxicity if there is an excess in the soil
- Organic matter ‑ the more organic matter in the soil the lower the availability of manganese
- Moisture ‑ changes in soil moisture can convert available forms of manganese to unavailable forms and vice versa and rapid changes in soil moisture can cause deficiencies and toxicities
- Iron ‑ excess iron reduces manganese uptake by plants
- Silicon ‑ addition of silicon can reduce the likelihood of symptoms of toxicity in plants that uptake excess levels of manganese
- Nitrogen ‑ low nitrogen levels can reduce manganese uptake by plants
- Anions (negatively charged ions such as nitrate, phosphate, sulfate) ‑ excess anions can increase the uptake of manganese
Ideally, for healthy and productive soil the concentration of manganese should be 10‑50 mg/kg. it would be worth conducting further investigations if your soil contains more than 3800 mg of manganese for every kg of soil.
Manganese deficiency is most common on alkaline and poorly drained soils as well as those high in available iron.
Symptoms of Manganese Deficiency
Treating Manganese Deficiency
In the event that you detect a manganese deficiency in your plants, a number of steps can be taken. Firstly, check that the soil pH is close to neutral. Manganese is most readily available to plants when the soil pH is between 5 and 7 though most plants will be able to uptake sufficient manganese if the soil pH is between 4.5 and 7.5 provided that there aren't any other problems with the soil. If the pH is too high, use an acidifying agent, such as sulfur, to lower the pH but just ensure that the pH doesn't drop so low as to result in other nutrient imbalances.
If you have doubts about the amount of nitrogen your plants are getting, adding nitrogen can also boost the uptake of manganese. You can do this by growing legumes near your plants or if a soil test reveals a nitrogen deficiency, you can add a nitrogen containing fertiliser (a general fertiliser will also increase the concentration of anions in the soil and further improve uptake). If there is an excess of available iron in the soil, check that irrigation water or run off from a nearby property isn't the cause.
If a soil test reveals that the concentration of manganese in the soil is below 10 mg/kg, a manganese fertiliser may be applied sparingly. Manganese chelate can be an organic‑friendly option (check the label) or you may chose to use a synthetic fertiliser such as manganese sulfate or manganese oxide.
Manganese toxicity is more common on very acidic soil. It can be toxic in it's own right but excess manganese can also cause iron deficiency.
Symptoms of Manganese Toxicity
Manganese toxicity may present as distorted leaves with dark specks. In severe cases, leaves will start to die from the outer edges in.
Treating Manganese Toxicity
Adding organic matter to the soil may help prevent symptoms of toxicity as will maintaining a neutral soil pH. If the pH of your soil is below 5, lime can be added to neutralise it. Any supplemental fertiliser applied to the soil should be free of manganese and low in nitrogen. Soil moisture should be kept as constant as possible to prevent fluctuations in manganese levels.