The Importance of Manganese in Plant Nutrition

Manganese (Mn) is an essential micronutrient critical for plant growth and development. Despite being required in relatively small amounts, its functions in plant physiology are indispensable, affecting processes ranging from photosynthesis to enzyme activation and disease resistance.

Manganese in Plant Physiology

Manganese is a central component of the oxygen-evolving complex in photosystem II, which is crucial for the photolysis (splitting) of water molecules during photosynthesis. This process releases oxygen and drives the light-dependent reactions that convert light energy into chemical energy. Adequate manganese is essential for efficient photosynthesis, and its deficiency can significantly impair this process, leading to reduced plant growth and vigor.

Similarly to magnesium, manganese acts as a cofactor for a variety of enzymes involved in critical metabolic pathways. These enzymes are essential for nutrient utilization, energy production, and the structural integrity of plant cells. It activates enzymes that regulate nitrogen assimilation, carbohydrate metabolism, and the synthesis of lignin and phytoalexins.

Lignin is a complex polymer that strengthens cell walls and provides structural support to plants. Lignin also acts as a barrier against pathogenic infections, particularly soil-borne fungi. If manganese is deficient in plants, the lignin synthesis can be impaired, making plants more susceptible to diseases and physical damage.

Manganese also plays a role in scavenging reactive oxygen species (ROS), which are harmful by-products of metabolic processes. Manganese is a cofactor of a superoxide dismutase (Mn-SOD) which is an enzyme that help neutralize ROS. Through this mechanism, manganese helps plants cope with environmental stresses such as drought, high temperatures, and pathogen attacks. This function is crucial for maintaining plant health and resilience under adverse conditions.

Manganese Deficiency Symptoms

Manganese deficiency is a widespread problem, particularly in sandy soils, peat or organic soils, and in a pH above 6-6.5. The deficiency is often exacerbated by cool and wet conditions. Also in warm and dry conditions with less water, the manganese uptake is reduced. Manganese has a relatively low phloem mobility in plants, so the typical leaf symptoms of Mn deficiency first develop in younger leaves. These symptoms vary among plant species but generally include:

• Interveinal Chlorosis: One of the most common symptoms is interveinal chlorosis, where the areas between the veins of young leaves turn yellow while the veins remain green. This symptom is particularly noticeable in dicotyledonous plants and in young leaves because the nutrient is almost immobile in plants.
• Reduced Growth and Yield: Manganese deficiency can lead to stunted growth, reduced dry matter production, and lower crop yields. The deficiency also weakens the plant’s structural resistance to pathogens and reduces its tolerance to environmental stresses.
• Gray Speck: In cereal crops like wheat and barley, manganese deficiency can cause a condition known as gray speck, characterized by pale green or yellow patches and necrotic spots on older leaves.

Manganese deficiency in tomato crop (Photo courtesy by ICL Italy)

Manganese deficiency in tomato crop (Photo courtesy by ICL India)

Manganese toxicity can occur in poorly drained and acidic soils. It can be recognized by interveinal chlorosis with leaf-cupping and darkening of leaf veins of older leaves. Necrosis may occur if toxicity is severe.

Different plant species have different tolerance to Mn. For example, beans, lettuce and potatoes are considered sensitive to higher Mn levels, while corn, rice, sugarcane and tomatoes are more tolerant.

Toxic levels of manganese can also impair the uptake of calcium, magnesium and iron by plants.

Managing Manganese in Agriculture

Effective management of manganese in agricultural soils involves several strategies to ensure adequate availability and uptake by plants:

1. Soil Testing and Monitoring: Regular soil testing is essential to determine manganese levels and identify potential deficiencies. Soil tests help farmers make informed decisions about manganese fertilization and other management practices.
2. Soil pH Management: Maintaining an optimal soil pH is crucial for manganese availability. Manganese is more available in slightly acidic soils (pH 5.5-6.5). Liming acidic soils to raise the pH can reduce manganese availability, so careful management is needed to balance pH and manganese levels.
3. Plant Tissue Testing: Plant tissue testing can diagnose manganese deficiency and should be used alongside soil tests to recommend Mn fertilizer use. Since Mn concentration varies with growth stages and plant parts, accurate interpretation requires testing specific plant parts at specific times during the growing season.
4. Manganese Fertilization: Applying manganese fertilizers can correct deficiencies and improve crop health. Manganese sulfate (MnSO₄) and manganese chelate are commonly used sources of manganese, providing a readily available form of the nutrient. Foliar applications of manganese are the most effective, especially in correcting deficiencies quickly.
5. Crop Rotation and Selection: Rotating crops that have different manganese requirements can help manage soil manganese levels. Selecting manganese-efficient crop varieties can also improve nutrient uptake and reduce the risk of deficiency.
6. Water Management: Proper irrigation practices can influence manganese availability. Over-irrigation can lead to waterlogged soils, reducing manganese uptake by roots. Conversely, drought conditions can limit manganese mobility in the soil. Balanced irrigation practices are essential for optimal manganese management.

ICL’s Solution for Manganese Management

Manganese is applied to plants either as MnSO₄ or in the chelated form (Mn-EDTA) which is more suitable for plant uptake. ICL offers a wide range of water-soluble and liquid products which contain chelated manganese within a standard package of trace elements. Apart from those, there are also special products designed to address directly manganese deficiencies. For example, water-soluble fertilizer for foliar applications, Agroleaf Special Mn, offers the highest 12% concentration of manganese in a chelated form. The Agroleaf Liquid range also features manganese-based products, either as a stand-alone solution (Agroleaf Liquid Manganese) or in mixture with zinc (Agroleaf Liquid Man Z+, Agroleaf Liquid Zinc M+). In fertigation systems, manganese can be added to soil with Agrolution Liquid Drip MZ-74.

Conclusion

Manganese is a vital micronutrient that significantly impacts plant growth, development, and resilience. Its roles in photosynthesis, enzyme activation, lignin synthesis, and oxidative stress resistance are crucial for maintaining healthy and productive crops. Managing manganese levels in agricultural soils through soil testing, fertilization, pH management, crop rotation, and proper irrigation practices is essential for optimizing crop health and yield. By understanding and addressing manganese requirements, farmers can enhance the sustainability and productivity of their agricultural systems.