Zinc Deficiencies and Fertilization in Corn Production
Crop Insights by Steve Butzen, Agronomy Information Manager
Crop Insights by Steve Butzen, Agronomy Information Manager
Zinc is an element used by crops in small quantities (usually less than 0.5 pounds per acre), yet is essential to normal plant growth and development. Zinc has several important functions in plants, including major roles in enzyme reactions, photosynthesis, DNA transcription and auxin activity.
Zinc is sufficient in most soils to supply crop needs, but may be deficient in sandy soils, other low-organic soils (e.g., those with topsoil removed by erosion), or soils with high pH. When deficient, zinc can be supplied by fertilizer in several forms. In North America, zinc is one of the micronutrients most often supplied by fertilizer.
Of all micronutrients, zinc is the one most often deficient in corn production and most likely to elicit a yield response when applied as fertilizer. However, yield responses are only attainable when zinc is deficient and therefore limiting yield. Soil and plant tissue tests can determine if zinc is deficient in the soil or plant. This Crop Insights will describe zinc requirements, deficiency symptoms, soil and plant sampling, and fertilization practices in corn production.
Most zinc in soils is held in unavailable forms, such as metallic oxides and other mineral complexes. Plants derive zinc that is 1) dissolved in the soil solution, 2) adsorbed to the surface of clay particles and 3) adsorbed by and chelated or complexed with organic molecules in soil organic matter. Zinc from these various soil pools is taken up by plants predominately in the divalent cation form (Zn2+) or under high pH soil conditions, the monovalent cation (ZnOH+) as well.
Zinc availability to plants largely depends on soil texture, organic matter, pH, phosphorus levels and weather conditions.
Soil texture and organic matter: Soils with at least moderate levels of clay and/or organic matter are generally sufficient in zinc. Conversely, sandy or low organic matter soils tend to be more prone to deficiencies. Muck or peat soils may also show deficiencies, as strong natural chelation can make zinc unavailable.
Soil pH: Zinc is most soluble and therefore available to the plant at a pH of 5 to 7. In alkaline soils (pH above 7.0), zinc may form insoluble compounds, making it unavailable to the plant.
Phosphorus (P): High P levels such as those created by excess manure application or high soil P combined with high rates of P in row starter can reduce zinc availability, resulting in deficiencies to the plant.
Weather conditions: Cool, wet conditions result in less zinc in available form. If roots are not well established at this time, deficiencies can result. This explains zinc deficiencies sometimes observed on corn seedlings early in the spring.
Fields showing zinc deficiency are seldom affected uniformly. Zinc deficiency symptoms may also vary from field to field, depending primarily on the timing and severity of the deficiency.
Very early zinc deficiency may be induced by cold, wet soil conditions that limit corn root growth and available zinc. In such cases, zinc deficiency may be exhibited on early leaves, but not on later leaves that develop when the soil begins supplying and the roots begin extracting more zinc.
Moderate deficiencies in seedlings may result in white to pale-yellow longitudinal areas in the newest leaves that are usually more pronounced in the lower half of the leaf. Severe deficiencies at this time may result in entire plants being pale yellow to white in color, and stunted.
As plants grow beyond the seedling stage, the demand for zinc becomes greater and deficient soils may be unable to supply the need. In such cases, the earliest developing leaves may be normal, but the newest growth will show deficiency symptoms. This pattern occurs because zinc is not readily translocated within the plant.
Moderate deficiencies may result in interveinal chlorosis (white or yellow) in the newest growth. This chlorosis is not always uniform across the width of the leaf, but may appear as longitudinal "bands" of more chlorotic tissue (Figure 2). Areas of the leaf near the stalk may develop a general white to yellow discoloration (Figure 1).
Figure 1. Zinc deficient corn plant.
More severe zinc deficiencies may result in bands of pale yellow to white tissue running longitudinally along both sides of the midrib, particularly in the lower half of the leaf. This tissue may eventually wilt and become necrotic (Figure 3). Plants may appear stunted due to shortened leaves and internodes, sometimes called "rosetting."
Figure 3. Severe zinc deficiency in V12 corn plant. Note pale yellow and white chlorotic areas becoming necrotic.
Both soil and plant analysis can be used to determine if zinc application is needed. Because soil tests for zinc are considered among the most reliable of the micronutrients, this method is most often recommended. Using both tests together can help to arrive at a firm recommendation for zinc application.
Your state extension service or soil test laboratory may have specific instructions for soil sampling for zinc. In general, a composite soil sample should be taken from the area of the field suspected of being low in this nutrient. In collecting samples, avoid using anything (tools or containers) galvanized or made of rubber, as these materials contain zinc.
Soil Test Interpretation / Fertilizer Recommendations
Soil test results will be reported in parts per million (ppm) zinc. Specific recommendations for your field may depend on level of zinc in the soil, the zinc extraction method used by the laboratory, soil pH, knowledge of local soils and history of zinc deficiency and remediation in your area. For this reason, follow local recommendations for zinc application. General guidelines are shown below.
If a deficiency is found, recommendations are generally to apply one to two pounds actual zinc per acre as starter, or 5 to 10 pounds as a broadcast application. This assumes that a soluble inorganic form of zinc (e.g., zinc sulfate) is used in the starter, and that soil pH is below 7. If an organic chelate is used, rates (on a zinc content basis) may be 1/5 that of inorganic sources. To determine the pounds of actual zinc, multiply pounds of material by the percent actual or "elemental" zinc in the material. For example, to apply 1 to 2 pounds of actual zinc when using zinc sulfate (33% zinc content) as the fertilizer source, 3 to 6 pounds of material should be applied.
The standard plant sampling technique for corn is to sample the ear leaf at initial silking. To obtain a representative sample, avoid border plants and collect ear leaves from a number of plants throughout the affected field area. Be sure to follow your diagnostic laboratory's specific sampling procedures. It may be beneficial to sample apparently deficient plants as well as those that appear normal. Keep these samples separate and indicate "symptomatic" and "nonsymptomatic" when submitting samples.
Plant Tissue Test Interpretation / Foliar Fertilizer Recommendations
Tissue test results will be reported as ppm zinc contained in the plant tissue. Recommendations based on these results will vary somewhat by state. Generally, within the range of 20 to 70 ppm, zinc content is considered sufficient; values of over 300 ppm are considered toxic. Two examples of soil test interpretation are shown in Table 1. However, because sufficiency levels vary by state, obtain and follow your local extension recommendations.
If a zinc deficiency problem is indicated by tissue analysis, zinc may be supplied by foliar application. When using zinc sulfate, apply 0.5 to 1 pound of actual zinc per acre (1.5 to 3.0 pounds of material). If a zinc chelate is used, apply at the rate of 0.15 pounds of actual zinc per acre. With either source or zinc, use at least 20 gallons of water per acre to ensure good coverage of the foliage. Some states do not recommend foliar applications due to inconsistent results in some studies.
The addition of Zn fertilizer to starter fertilizer banded two inches to the side of and two inches below the seed at planting is the most common approach to Zn fertilization of corn. This method places the nutrient near the plant roots for immediate uptake as seedling development begins. If starter fertilizer is not used or if growers wish to add enough zinc for several years of crops, then Zn fertilizers are typically broadcast and incorporated before planting. Foliar applications of zinc are not as common as soil applications, and are often reserved for unexpected zinc deficiencies discovered during plant development.
Figure 4. Zinc-deficient corn plants show general chlorosis of new growth as well as longitudinal bands on some leaves.
Choosing a Source of Zinc
There are many fertilizer sources of zinc effective in correcting zinc deficiencies. These sources can be grouped as:
The appropriate source depends on cost per unit of actual zinc, relative effectiveness of the product, method of application, soil pH, and remediation strategy (short or long term).
Cost per unit of zinc: Research studies have shown that many different formulations of zinc can do a good job of correcting deficiencies. For that reason, equally effective sources that cost less are preferred. One of the most cost-effective products available is zinc sulfate, which is considered an excellent source of zinc whether applied as a granular material in a bulk blend or incorporated into solid or liquid fertilizers.
Relative effectiveness: Research has shown that chelated forms of zinc are more plant available than inorganic forms, and that ZnEDTA chelate is the most effective source on the market. Based on comparisons with this standard, researchers have assigned Relative Availability Coefficients (RAC) to other zinc fertilizers. According to this system, ZnSO4 had an RAC of 23%, followed by Zn lignosulfonate at 22%. All other sources tested were less soluble and therefore less effective, such as Zn oxysulfate compounds, which ranged from 0.5 to 12%.
Method of application: The form of zinc chosen is often dependent on how it will be applied. For band application with dry starter fertilizer at planting, zinc sulfate is most often used. However, many other zinc fertilizers including finely-ground zinc oxide are also used with dry starter. If liquid starter is used, the chelated zinc forms or zinc-ammonia complex can easily be included. For foliar applications, zinc sulfate or chelates are most often chosen.
Soil pH: For alkaline soils (pH > 7.0) insoluble sources of zinc such as zinc oxide and zinc carbonate should not be used, as solubility and therefore plant availability is greatly reduced in alkaline soils.
Remediation strategy: If remediation is intended for only the current crop, lower rates of highly available zinc fertilizers can be used. However, in some cases growers may wish to broadcast higher rates of zinc fertilizer to supply several crop cycles. Zinc sulfate is often used for broadcast applications, and zinc oxide may also be used for this purpose on non-alkaline soils.
Table 2. Common zinc fertilizer sources.
Figure 5. Zinc-sufficient corn field. Yield responses from zinc fertilization are only attainable when zinc is deficient.