Iron is critical for energy production and the formation of chlorophyll, making it indispensable for photosynthesis and plant health. In tomatoes, peppers, and eggplants, iron supports fruit set, uniform ripening, and vegetative growth. For broccoli and cabbage, iron is essential for leaf and head development, impacting yield and marketability. However, excess iron can cause toxicity, leading to reduced plant vigor and interference with other essential nutrients like manganese and zinc. Sensitive crops like tomatoes and peppers are particularly prone to iron-related imbalances, highlighting the need for precise management.

Figure 1. Tomato field

Photo by Emmanuel Torres

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Figure 1. Tomato field

Photo by Emmanuel Torres

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Iron constitutes up to 5% of soil weight, primarily found in minerals like ferromagnesian silicates (e.g., olivine, augite, hornblende, and biotite) and oxides such as hematite and magnetite. However, due to the low solubility of these primary minerals and oxides, the total iron content in soil has little correlation with the amount of iron available to plants.

To ensure healthy crop growth, maintaining adequate iron levels in both soil and plant tissues is critical.

• Soil Iron Levels: Iron availability is typically measured through a DTPA soil test, with levels between 2.5 and 10 ppm considered adequate for most vegetable crops. However, availability is strongly influenced by soil pH and other factors.
• Foliage Iron Levels: Adequate iron levels in leaf tissue range from 50 to 200 ppm, depending on the crop. Regular tissue testing during the growing season helps monitor iron status and detect potential deficiencies or toxicities.

Iron deficiencies, often referred to as "iron chlorosis," are characterized by yellowing of young leaves (interveinal chlorosis) while veins remain green. Severe deficiencies can lead to stunted growth and poor fruit or head formation. Deficiencies are more likely in alkaline soils (pH > 7.5), compacted soils, or areas with high levels of competing cations such as calcium and magnesium. Iron has low mobility within plants, so deficiency symptoms typically appear first in the youngest leaves. When corrected with fertilizer, greening begins along the veins. Calcareous soils and anaerobic conditions often promote iron deficiency, which can also be triggered by excessive levels of heavy metals. Plant species and varieties differ in their ability to uptake iron, with some being Fe-efficient and others Fe-inefficient. Fe-efficient plants can acidify the rhizosphere or release siderophores, enhancing iron uptake.

To correct deficiencies:

1. Soil Applications: Use chelated iron (e.g., Fe-EDDHA) for alkaline soils, as it remains available over a wide pH range. Apply at rates of 1-5 pounds per acre, depending on deficiency severity.

1. Foliar Sprays: Most iron deficiencies are due to low availability of iron in an unfavorable pH range, not to low total iron levels. The mobility of soluble iron does not exceed 1.5 cm from the site of placement. Hence, foliar fertilization with FeSO4, FeCl2, or Fe-chelates at 2.7 to 4.5 lb/acre is sometimes preferable to soil applications.
2. Preventive Measures: Incorporating organic matter and avoiding over-liming can improve iron availability in the soil.

While deficiencies are more common, excess iron can occur in acidic or poorly drained soils. Toxicity symptoms include bronzing and speckling of older leaves, reduced growth, and nutrient imbalances. To address excess iron, improve soil drainage, reduce iron-containing fertilizer applications, and manage soil pH to prevent solubilization of excessive iron.

Soil pH has a profound effect on iron availability. Iron is most soluble and available to plants in acidic soils (pH < 6.5) but becomes increasingly unavailable as pH rises above neutral. In alkaline soils (pH > 7.5), iron precipitates into insoluble forms, leading to deficiencies even when total soil iron levels are adequate. Maintaining soil pH within the range of 6.0 to 6.5 is ideal for most vegetables, ensuring sufficient iron availability without the risk of toxicity. Regular pH testing and proper lime application are essential for managing pH and preventing iron-related issues.

Ammonium-N applications generally result in higher iron content in tissues compared to nitrate-N applications. Nitrate-N sources can exacerbate iron deficiency in high-pH soils by increasing rhizosphere pH during nitrate absorption. Ammonium-N reduces iron deficiency by acidifying the substrate and rhizosphere through ammonium uptake and nitrification.

However, ammonium-N is not always beneficial for iron availability. In acidic soils or low-pH nutrient solutions, further pH reduction can harm plant growth, particularly root development, limiting the plant's ability to access iron. Additionally, nitrogen fertilization can increase iron deficiency by promoting rapid growth, diluting the available iron in plant tissues.

High soil phosphorus can inhibit iron uptake, and elevated phosphorus levels in plants are often linked to iron deficiency. This is likely due to the formation of insoluble iron phosphates in alkaline soils or within the plant itself. The optimal phosphorus-to-iron (P:Fe) ratio for most plants is approximately 29:1 (w:w).

Increased soil pH, often resulting from excessive application of calcium-containing liming materials or native calcium compounds, is a common cause of iron deficiency. Higher pH reduces the solubility of iron compounds. However, calcium itself does not directly alter soil pH; the accompanying anions, such as carbonate, oxide, or hydroxide, are responsible for the pH changes.

High copper concentrations in soil or nutrient solutions can exacerbate iron deficiency by competing with iron for uptake. Similarly, the addition of magnesium carbonate (MgCO₃) in agricultural limestone can reduce iron availability by increasing soil alkalinity and causing iron precipitation. Excess manganese has also been observed to induce iron deficiency. High zinc levels can further suppress iron uptake, with studies showing reduced iron content in the leaves of corn and soybean when exposed to elevated zinc levels compared to plants grown with lower zinc concentrations.

Irrigation management plays a key role in iron availability and uptake. Over-irrigation can lead to waterlogged soils, reducing oxygen levels and causing iron toxicity in anaerobic conditions. Conversely, insufficient irrigation in dry soils can limit iron solubility, leading to deficiencies. High-quality irrigation water with low bicarbonate content is critical, particularly in areas with alkaline soils, as bicarbonates exacerbate iron precipitation. Drip irrigation systems and soil moisture sensors help maintain optimal soil moisture levels, ensuring consistent iron availability to the root zone.

Iron fertilizers are available in various forms, each suited to specific conditions:

• Chelated Iron (e.g., Fe-EDDHA, Fe-DTPA): Ideal for soil applications, especially in high-pH soils, due to their stability and availability.
• Iron Sulfate (FeSO₄): Cost-effective but less effective in alkaline soils. Often used for foliar applications or soil amendment in acidic soils.
• Liquid Iron Fertilizers: Easy to apply through fertigation or foliar sprays, providing quick corrections for deficiencies.

Choosing the appropriate fertilizer source and application method depends on soil pH, crop requirements, and the severity of the deficiency.

• Regular Testing: Conduct soil and tissue tests to monitor iron levels and detect potential issues early.
• Integrated pH Management: Maintain soil pH between 6.0 and 6.5 for optimal iron availability.
• Balanced Fertilization: Avoid excessive applications of calcium, magnesium, or phosphorus, which can interfere with iron uptake.
• Efficient Irrigation Practices: Use precise irrigation techniques to prevent waterlogging and ensure adequate moisture for nutrient uptake.

Iron management is a critical component of vegetable production in North Carolina. Understanding the importance of iron, its interactions with soil pH, and the impact of water management allows growers to optimize crop health and productivity. By monitoring soil and tissue iron levels, selecting appropriate fertilizer sources, and implementing effective irrigation and pH management strategies, farmers can prevent deficiencies and toxicities, ensuring high-quality yields. For further guidance, consult your local N.C. Cooperative Extension office or soil testing laboratory.