Zinc is crucial for maintaining healthy growth and development in vegetables. In tomatoes, it supports fruit set, development, and ripening, while in peppers, it enhances fruit size and quality. For eggplants, zinc promotes vegetative growth, flowering, and uniform fruit formation. In broccoli and cabbage, it is vital for proper head formation and leaf expansion, ensuring uniform size and marketability. A deficiency in zinc can lead to stunted growth, delayed maturity, and poor-quality produce, which negatively impacts profitability for growers.

Figure 1. Tomato flower in North Carolina

Photo by Emmanuel Torres

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Figure 1. Tomato flower in North Carolina

Photo by Emmanuel Torres

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To optimize crop performance, it is essential to maintain sufficient zinc levels in both soil and plant tissues.

• Soil Zinc Levels: Zinc concentrations in the soil should range between 0.5 to 2.0 ppm, as determined by a DTPA-extractable soil test. Below 0.5 ppm, plants are likely to exhibit deficiency symptoms, while levels exceeding 10 ppm may lead to toxicity.
• The NCDA&CS soil test uses Mehlich-3 extractant for determining levels of phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), manganese (Mn), copper (Cu), zinc (Zn) and sodium (Na), and results may or may not agree with those of other laboratories depending on method of extraction and other procedural variables. Zinc application may be suggested for a specific crop if the Zn-availability index (Zn-AI) is 25 or below and, the crop is known to respond to Zn. The Zn-AI is calculated based on soil levels of Zn and other factors affected by soil class.
• Foliage Zinc Levels: Zinc concentrations in leaf tissue should range from 20 to 50 ppm, depending on the crop. Cabbage and cauliflower levels can be as high as 200 ppm. Regular tissue testing during critical growth stages is recommended to monitor zinc status accurately.

Zinc deficiencies often present as distinct visual symptoms, although confirmation through soil and tissue testing is essential for accurate diagnosis. Common symptoms include:

• Tomatoes and Peppers: Interveinal chlorosis on younger leaves, leaf curling, and stunted growth.
• Eggplants: Reduced leaf size, rosetting of foliage, and poor flowering.
• Broccoli and Cabbage: Paling of young leaves, shortened internodes, and reduced head size or incomplete head formation.

Deficiencies are more likely to occur in sandy soils with low organic matter, high pH soils (above 7.5), or areas where phosphorus fertilizers have been heavily applied, as excess phosphorus can interfere with zinc uptake.

Soil pH plays a pivotal role in determining zinc availability to plants. Zinc becomes less available as soil pH increases above 7.0, as it forms insoluble compounds that are not readily absorbed by plant roots. Alkaline soils, common in regions with excessive liming or naturally high pH, are particularly prone to zinc deficiencies. Conversely, in acidic soils (pH below 6.0), zinc solubility increases, enhancing its availability. However, extremely acidic conditions can lead to excessive zinc uptake, potentially causing toxicity and nutrient imbalances, particularly with phosphorus, iron, and manganese. Maintaining soil pH within the optimal range for vegetable crops, typically between 6.0 and 7.0, ensures balanced micronutrient availability, including zinc. Regular soil testing and appropriate lime applications can help manage pH, improving zinc uptake and overall plant health.

Proper irrigation management is essential for optimizing zinc availability in vegetable production. Zinc mobility in the soil is limited, and its uptake by plants depends on consistent soil moisture levels. Over-irrigation can lead to waterlogged conditions, which reduce oxygen availability in the root zone and hinder zinc uptake. Conversely, insufficient irrigation can dry out the soil, further limiting zinc solubility and root absorption. Maintaining even soil moisture through irrigation scheduling, drip irrigation systems, or soil moisture sensors ensures optimal conditions for zinc availability and uptake. Additionally, avoiding water stress during critical growth stages helps prevent nutrient imbalances and supports overall plant health and productivity. Integrating efficient irrigation practices with zinc fertilization strategies enhances nutrient use efficiency and promotes sustainable vegetable production.

The timing and placement of zinc fertilization are critical to ensuring efficient uptake and minimizing nutrient losses. If recommended by a soil test, for vegetables such as tomatoes, peppers, eggplants, broccoli, and cabbage, zinc should be applied early in the growing season when root development is most active and the demand for micronutrients is high.

The NCDA&CS current recommendation for Zn is 6.0 lb/acre broadcast or 3.0 lb/acre banded, regardless of soil type.

Potential zinc toxicity is indicated whenever the Z designation appears in the Zn column of the recommendations section of a report. For most crops, the Z designation indicates that Zn-AI is greater than 2000 (the critical toxic level being 3000).

Zinc fertilizers can be incorporated into the soil before planting, applied in bands near the root zone during planting, or included in fertigation systems for targeted delivery. Band applications are particularly effective as they place zinc within the root zone, where it is more readily absorbed. For crops already showing deficiency symptoms, foliar applications of chelated zinc can provide a quick corrective measure during active growth. When using foliar sprays, apply during cooler parts of the day to avoid leaf burn and ensure uniform coverage. Combining proper timing and strategic placement of zinc fertilizers enhances nutrient efficiency and supports optimal growth and yield in vegetable crops.

There are several zinc fertilizer options available to address soil or foliar deficiencies:

• Zinc Sulfate (23% zinc): A commonly used source for both soil and foliar applications.
• Chelated Zinc (e.g., Zn-EDTA): Ideal for foliar applications due to its high availability to plants.
• Zinc-Containing Blends: Some fertilizers, such as NPK blends, include zinc and are suitable for broadcast or band applications.

The approach to correcting zinc deficiencies depends on the soil and crop conditions:

1. Soil Applications: Zinc sulfate can be applied directly to the soil or through fertigation systems. For band applications, concentrate zinc near the root zone to improve uptake efficiency.
2. Foliar Sprays: In-season foliar applications are effective for correcting deficiencies quickly. Use chelated zinc, ensuring uniform coverage of the foliage.
3. Preventive Measures: Incorporating zinc-containing fertilizers during pre-planting fertilization or using micronutrient blends can help prevent deficiencies. Rotating crops and managing soil pH can also enhance zinc availability.

• Regular Testing: Conduct soil tests before planting and tissue tests during the growing season to monitor zinc levels and identify potential issues early.
• Integrated Nutrient Management: Balance zinc applications with other nutrients, particularly phosphorus, to avoid interactions that may reduce zinc availability.
• Application Timing: Apply zinc fertilizers during early growth stages when the demand for micronutrients is highest.
• Avoid Overapplication: Excessive zinc can lead to toxicity, which damages plants and disrupts the balance of other nutrients.

Effective zinc management is critical for optimizing the growth and productivity of tomatoes, peppers, eggplants, broccoli, and cabbage in North Carolina. By understanding the importance of zinc, maintaining adequate levels in soil and foliage, and using appropriate fertilizer sources, growers can minimize deficiencies and improve their yields. Regular soil and tissue testing, coupled with timely interventions, will ensure sustainable vegetable production and support long-term soil health.

For additional information or assistance, contact your local N.C. Cooperative Extension office or horticulture advisor.