Manganese is an essential plant micronutrient that is necessary for optimal plant growth and development. While required in small amounts, manganese plays an important role in several plant processes including photosynthesis, enzyme activation, and carbohydrate production. Ensuring adequate manganese nutrition promotes overall plant health and maximizes crop yields. This article will examine the functions of manganese in plants, manganese deficiency symptoms, and different fertilizer sources that can supply manganese to meet crop requirements.

Role of Manganese in Plants

As an immobile nutrient, manganese is not easily translocated within the plant once a deficiency occurs. It is primarily involved in enzyme activation, specifically activating enzymes involved in chloroplast reactions that provide energy for photosynthesis. Manganese helps catalyze the breakdown of water molecules and release electrons during photosynthetic light reactions. It also activates enzymes involved in respiration, nitrogen metabolism, and biosynthesis of different organic compounds in plants.

Manganese deficiency can severely impact plant growth, development and yield potential. Visible symptoms include interveinal chlorosis of new leaves, necrotic spots, distorted leaf growth, and stunted plants. Because manganese is immobile, symptoms first appear on young leaves while older leaves often remain green. Prolonged deficiency leads to necrosis, browning of leaf tissue, premature leaf loss, and reduced crop quality.

Sources of Manganese Fertilizers

When soil manganese levels are insufficient, fertilizers can supply manganese and prevent deficiencies. Both organic and inorganic manganese fertilizers are available.

Inorganic Fertilizer Sources:

  • Manganese sulfate – Contains approximately 28% manganese in a readily available form. Manganese sulfate is approved for organic farming.
  • Manganese oxide – Typically contains 45-65% manganese and must be converted into soluble forms by soil microbes before uptake.
  • Manganese chelates – These contain manganese bound to organic molecules (chelates) like EDTA or EDDHA that protect manganese from soil interactions. Chelated forms are more stable in soils.
  • Foliar sprays – Contain manganese sulfate, oxides or chelates that can be sprayed directly on plant leaves for rapid absorption and correction of deficiencies.

Organic Fertilizer Sources:

  • Compost and manures – Contain variable manganese content based on source materials. Compost provides manganese in organic forms that must mineralize into plant-available forms.
  • Cover crops and green manures – Legumes like clover and soybeans take up manganese from subsoils and recycling this organic matter provides manganese.
  • Seaweed extracts – These concentrate manganese absorbed from sea water by marine algae. Extracts or seaweed meal can be applied as liquid sprays or soil amendments.
  • Manganese-rich rock dusts – Mineral powders like basalt and granite contain manganese oxides that can slowly dissolve and release manganese.

Manganese Fertilizer Application Methods

Proper application and placement of manganese fertilizers can optimize manganese availability and correct deficiencies. Manganese can be applied to soils or foliage through different methods.

Soil Application:

  • Broadcast spreading – Uniform surface application of manganese fertilizers which are incorporated into soils before planting. This provides a broad distribution of manganese.
  • Banding- Concentrated manganese fertilizer bands are placed in soil below and to the side of seeds during planting. This localizes manganese near root zones.
  • Side-dressing – Fertilizer bands applied alongside plant rows during the growing season. Effective for immobile nutrients like manganese.
  • Fertigation – Injection of soluble manganese fertilizers into irrigation water for delivery to root zones. Only suitable for pressurized irrigation systems.

Foliar Application:

  • Foliar sprays – Manganese fertilizer solutions sprayed directly on plant leaves and rapidly absorbed. Used to correct nutrient disorders quickly during the growing season.
  • Low biuret urea sprays – Urea fertilizer containing manganese can be absorbed through leaf tissues. Minimizes leaf burn risks.

Proper nutrient stewardship with soil and plant testing helps determine if manganese applications are warranted and which methods are most suitable. Always follow safety precautions when preparing and applying manganese fertilizers.

Using Manganese Fertilizers in Different Cropping Systems

The demand for manganese fertilization depends on the crop. Understanding manganese needs in different cropping systems and horticultural plants allows efficient use of manganese fertilizers.

Field Crops – Crops like corn, soybeans, sorghum, and cereals require manganese for photosynthesis and nitrogen metabolism. Deficiencies most likely in very high pH or sandy soils. Typical application rates are 2-4 lbs manganese/acre for row crops.

Vegetable Crops – High manganese demand for leaf and fruit development. Spinach, sweet potatoes, tomatoes, beans, and peas often respond to manganese fertilization. Can use 1-3 lbs/acre manganese, with foliar sprays as needed.

Fruit Crops – Important for growth of tree fruits, grapes, strawberries, and blueberries. Deficiency causes reduced yields and quality. Often applied as foliar sprays containing manganese sulfate or chelates.

Turfgrass – Necessary for turf color, density, and vigor. Greens and fairways may receive granular or foliar manganese at 0.5-1.5 lbs/acre. Avoid over-application on turf.

Greenhouse Crops – Added to soilless media or applied as foliar sprays in greenhouses. Monitor through tissue testing. Use chelated forms in nutrient solutions.

By considering specific crop needs and production practices, manganese fertilizers can be effectively leveraged to maintain proper plant nutrition. Always consult agronomic guidance to avoid providing excess manganese.

Potential Risks of Over-fertilization with Manganese

Over-fertilization of soils with manganese can lead to several risks for crops and the environment. While manganese is an essential plant micronutrient, excessive amounts can become toxic to plants. High manganese levels can interfere with iron uptake, leading to iron deficiency chlorosis in crops. This impairs photosynthesis and stunts plant growth.

Over-fertilizing with manganese also increases the risk of manganese toxicity, causing brown spots and necrotic lesions on plant leaves and stems. Excessive manganese in soils can remain for years, leading to accumulation over time. This has potentially harmful effects, as manganese can leach into groundwater or be taken up in edible plant parts destined for human or animal consumption. High manganese intake can cause neurological problems in mammals.

Additionally, manganese over-fertilization disrupts soil nutrient balances, impacts beneficial soil microorganisms, and may negatively affect future crop yields if not properly managed. Careful soil testing, appropriate fertilizer application rates, and avoiding over-application are important to minimize the risks of manganese over-fertilization.

Future Trends in Manganese Fertilization

The future of manganese fertilization will likely see more precise application rates and placement based on soil testing. Widespread soil mapping and routine testing will help identify deficient areas in fields requiring manganese applications. Variable rate technology will allow custom fertilizer prescriptions tailored to specific soils and zones within fields.

Controlled release manganese fertilizers are also being developed to better regulate availability and reduce toxicity risks. These enhance efficiency by synchronizing release with peak crop demand. Foliar manganese products applied directly to leaves may supplement soil manganese applications. However, foliar manganese mainly provides short-term benefits and does not correct long-term deficiencies.

As organic farming expands, there is growing research on plant-based and mineral manganese sources approved for organic systems. Overall, the future emphasis will be on building soil manganese levels through regular soil testing and judicious fertilization rather than over-applying manganese annually. These advances will help provide manganese to crops more precisely while minimizing environmental risks.