Manganese (Mn) is an essential micronutrient that plays a critical role in the growth and development of vegetable crops. Although required in small quantities, its functions in plant physiology are indispensable, affecting processes ranging from photosynthesis to enzyme activation and disease resistance. For vegetable growers in North Carolina, understanding manganese's role, its soil dynamics, and effective management practices is key to optimizing crop health and productivity in the state’s diverse agricultural systems.

In plants, manganese primarily serves as a cofactor for a range of enzymes involved in critical metabolic pathways. One of its most notable roles is in the oxygen-evolving complex of photosystem II, where it facilitates the splitting of water molecules during photosynthesis, releasing oxygen and driving the light-dependent reactions. Manganese is also involved in lignin synthesis, contributing to cell wall structure and plant rigidity, and plays a role in detoxifying reactive oxygen species, helping plants mitigate stress from environmental factors such as drought and temperature extremes. Additionally, manganese activates enzymes that regulate nitrogen assimilation and carbohydrate metabolism, processes essential for plant growth and fruit development.

In practical terms, this means that manganese is crucial for the basic functions that keep plants healthy and productive. Here’s how it applies to everyday farming:

1. Photosynthesis Efficiency: Manganese is essential for photosynthesis, the process where plants use sunlight to produce energy. Without enough manganese, the plant struggles to convert water into oxygen and energy, leading to slower growth and reduced vigor.
2. Strong Plant Structures: Manganese helps in the production of lignin, a compound that strengthens cell walls. This means plants will have firmer stems and leaves, making them more resilient to physical damage and environmental stresses.
3. Stress Resistance: Manganese helps plants cope with environmental stresses like drought or high temperatures by neutralizing harmful by-products (reactive oxygen species). A manganese-deficient plant is more susceptible to damage under harsh conditions.
4. Nutrient Use and Growth: Manganese activates enzymes needed for nitrogen and carbohydrate metabolism. This ensures that the plant efficiently uses nutrients for growth and energy production, supporting healthy development and optimal fruiting.

Manganese exists in soils in several forms, including exchangeable Mn, manganese oxides, and Mn bound to organic matter. The availability of manganese to plants depends heavily on soil pH, redox conditions, and organic matter content. In well-drained, aerobic soils, manganese predominantly exists as manganese oxides, which are not readily available to plants. In contrast, poorly drained or waterlogged soils often create reducing conditions that convert manganese to its more soluble Mn²⁺ form, increasing its availability. Soil pH is a critical determinant of manganese availability; acidic soils (pH <6.0) generally enhance Mn solubility, whereas alkaline soils (pH >7.5) significantly limit its availability, often leading to deficiencies. The total manganese content in cropping soils varies widely, typically ranging from 200 to 3,000 ppm, but only a small fraction is in a plant-available form.

For more information on soil testing and reporting please visit N.C. Department of Agriculture & Consumer Services Agronomic Division.

The NCDA&CS soil test employs the Mehlich-3 extractant to measure levels of phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), manganese (Mn), copper (Cu), zinc (Zn), and sodium (Na). However, results may vary from those of other laboratories due to differences in extraction methods and testing procedures. It is recommended that growers rely on a laboratory whose recommendations are supported by field research conducted within their region.

The manganese availability index (Mn-AI) is determined using the soil-test manganese index (Mn-I), crop sensitivity, and soil pH. Among micronutrients, manganese availability is most strongly influenced by soil pH: as pH increases, manganese availability decreases. The critical Mn-AI value is set at 25. If the soil's Mn-AI meets or exceeds this value, a "0" will appear in the manganese column, indicating that manganese levels are sufficient. When the Mn-AI falls below 25, a manganese application rate of 10 pounds per acre is generally recommended, as most field crops show a positive response to manganese supplementation under such conditions.

Vegetable crops differ in their manganese requirements, with leafy greens, onions, and legumes being particularly sensitive to manganese deficiencies. Symptoms of deficiency, such as interveinal chlorosis (yellowing between veins), stunted growth, and poor root development, are most commonly observed in calcareous soils or regions with neutral to high pH. Excess manganese, although less common, can occur in highly acidic soils or waterlogged conditions and may lead to toxicity symptoms such as brown spots on leaves and reduced overall vigor. Manganese sufficiency levels in soil tests are generally considered adequate when extractable Mn levels range from 3 to 30 ppm, but specific thresholds vary depending on the crop and soil conditions.

Manganese fertilizers are available in various forms, including manganese sulfate [28-32% Mn], and chelated manganese [5-12% Mn]. Manganese sulfate is the most commonly used source due to its high solubility and effectiveness in addressing deficiencies. Chelated manganese products are also widely used, particularly in alkaline soils, where they offer improved solubility and availability compared to inorganic sources. Foliar manganese fertilizers, often applied as manganese sulfate or chelates, provide a quick and effective method for addressing acute deficiencies during the growing season.

Soil and foliar applications of manganese serve different purposes and should be used based on the specific needs of the crop and soil conditions. Soil applications, either broadcast or banded, are typically performed before planting to establish baseline manganese levels in the root zone. In systems with drip irrigation, manganese fertilizers can be applied through fertigation, allowing precise delivery to the root zone. This method is particularly effective in sandy soils prone to leaching, as it minimizes nutrient losses and ensures consistent manganese availability. Foliar applications, on the other hand, are ideal for correcting mid-season deficiencies or providing supplemental manganese during critical growth stages, such as flowering and fruiting. Foliar sprays are absorbed quickly through leaf tissues, offering a rapid response to visible deficiency symptoms.

Soil pH management is critical for maintaining manganese availability, especially in North Carolina, where soils often vary widely in pH. Liming acidic soils to increase pH can inadvertently reduce manganese availability, necessitating careful monitoring and adjustment of manganese fertilization. Nutrient interactions also play a significant role in manganese uptake. High levels of calcium and magnesium, common in limed soils, can inhibit manganese absorption, while excessive phosphorus can lead to the precipitation of manganese as insoluble compounds. Balancing these nutrients is essential for effective manganese management.

Irrigation practices influence manganese availability, particularly in sandy soils with low nutrient-holding capacity. Consistent soil moisture enhances manganese uptake, as plants rely on water to transport Mn²⁺ ions to root surfaces. However, over-irrigation can exacerbate leaching in sandy soils or create waterlogged conditions in poorly drained soils, increasing the risk of toxicity. Drip irrigation systems offer an efficient solution by maintaining uniform soil moisture and delivering manganese directly to the root zone.

Farmers and extension agents should prioritize regular soil and tissue testing to guide manganese management decisions. Soil tests provide baseline manganese levels and help determine the need for fertilization, while tissue tests during critical growth stages can detect deficiencies early, allowing timely interventions. Visual scouting for deficiency symptoms, such as interveinal chlorosis and stunted growth, is also a valuable tool for in-season management. Education about the interactions between manganese and other nutrients, as well as the effects of soil pH and irrigation, is essential for promoting informed decision-making.

Manganese is an essential micronutrient critical for the growth and productivity of tomatoes, peppers, and brassicas.

• Tomatoes and Peppers: Manganese is particularly important for fruit set and development. Deficiencies can cause interveinal chlorosis (yellowing between the veins) in younger leaves and may lead to weakened stems and reduced flowering (Figure 1).
• Brassicas: Crops like broccoli, cauliflower, and cabbage require manganese for cell wall formation and lignin synthesis, which are vital for structural integrity. Deficiency symptoms in brassicas often include pale, chlorotic leaves with stunted growth.

Manganese deficiency in tomato

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Manganese deficiency in tomato

©2024 NSW Department of Primary Industries.

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Soil type and pH heavily influence manganese availability for tomatoes, peppers, and brassicas:

• Soil pH: Manganese is most available in acidic soils (pH <6.0). Alkaline soils (pH >7.5) significantly limit manganese availability, often requiring corrective actions.
• Soil Type: Sandy soils, common in parts of North Carolina, have limited manganese-holding capacity and are prone to leaching, especially under high rainfall. Conversely, clayey or organic-rich soils may have higher manganese reserves, but availability is reduced if pH levels are neutral or alkaline.

Testing soil pH and extractable manganese levels before planting is crucial for determining whether supplemental manganese applications are needed.

Manganese deficiencies in tomatoes, peppers, and brassicas are most common in high-pH or sandy soils. Key symptoms to monitor include:

• Tomatoes and Peppers: Yellowing between leaf veins (interveinal chlorosis) on new growth; slowed growth; brittle stems; and reduced flowering and fruit set.
• Brassicas: Chlorotic young leaves with mottled yellowing; poor root development; and weak structural integrity, which can affect head or flower formation in crops like cabbage and broccoli.

Early detection is critical, as deficiencies can severely impact yields and marketability.

Managing manganese effectively requires selecting the right fertilizer source and application method. Common manganese fertilizers include:

• Manganese Sulfate: Highly soluble and effective for both soil and foliar applications.
• Chelated Manganese: Offers greater availability in alkaline soils and reduces the risk of nutrient tie-up.
• Organic Sources: Products derived from natural materials are slower-acting and less predictable but suitable for organic systems.

Soil Applications
Soil applications are most effective for pre-planting corrections. In tomatoes, peppers, and brassicas, manganese sulfate can be applied as a broadcast or banded treatment. In systems with drip irrigation, manganese can be applied via fertigation, delivering nutrients directly to the root zone and minimizing leaching.

Foliar Applications
Foliar sprays are ideal for correcting in-season deficiencies, especially during critical growth stages like flowering and fruit set in tomatoes and peppers or head formation in brassicas. Manganese sulfate or chelated formulations are common choices. Foliar applications provide rapid results, making them suitable for addressing visible deficiency symptoms.

An adequate manganese application rate for tomato crops depends on soil test results, plant tissue analysis, and specific environmental conditions. General guidelines suggest that soil applications of manganese sulfate range from 20 to 25 pounds per acre for broadcast applications, and around 5 pounds per acre for band applications [equivalent to 10 lb/a of Mn]. Foliar applications are typically more economical and effective, with recommended rates of 1 to 2 pounds per acre using manganese sulfate or chelated manganese formulations. It's important to base application rates on current soil and tissue analyses to avoid over-application, which can lead to toxicity issues. Regular monitoring and adherence to local extension recommendations are essential for optimal nutrient management.

Effective manganese management for tomatoes, peppers, and brassicas requires attention to soil pH, nutrient balance, and irrigation practices:

• Soil pH: Keeping soil pH in the range of 6.0–6.8 is optimal for manganese availability without risking toxicity. Over-liming to correct soil acidity can inadvertently reduce manganese uptake.
• Nutrient Interactions: Excessive calcium or magnesium in limed soils can inhibit manganese uptake. High phosphorus levels may precipitate manganese, reducing its availability. Balanced fertilization is key.
• Irrigation: Consistent soil moisture promotes manganese uptake, as plants rely on water to transport nutrients. Over-irrigation can leach manganese in sandy soils, while waterlogged conditions in heavy soils may cause manganese toxicity.

Practical Recommendations

For tomatoes, peppers, and brassicas in North Carolina:

1. Test Soil and Tissue: Conduct soil tests to measure extractable manganese and monitor pH. Use tissue tests during the growing season to assess manganese status.
2. Correct Soil pH: Maintain pH within the optimal range for each crop, avoiding over-liming in acidic soils.
3. Apply Manganese as Needed: Use manganese sulfate for soil applications or foliar sprays during critical stages if deficiencies arise. In sandy soils, consider split applications through fertigation to maintain consistent availability.
4. Monitor Symptoms: Train farm workers to identify manganese deficiency symptoms, such as interveinal chlorosis, and address them promptly with foliar applications.
5. Irrigate Effectively: Use drip irrigation to maintain consistent soil moisture and deliver manganese directly to the root zone.

By managing manganese effectively, growers can enhance the health, productivity, and quality of tomatoes, peppers, and brassicas, ensuring high yields and marketable produce while maintaining soil sustainability.

Manganese is a vital micronutrient that significantly impacts vegetable crop health and productivity. In North Carolina’s diverse agricultural systems, effective manganese management requires a nuanced understanding of its role in plant physiology, behavior in soils, and fertilization strategies. By integrating soil and tissue testing, precise fertilization, and tailored irrigation practices, growers can optimize manganese availability, enhance crop yields and quality, and contribute to the sustainability of vegetable production systems.