Zinc is the 24th most abundant element in the earth’s crust, the eighth metal known to man, a component of ancient brass artifacts, and an ingredient of batteries, gutters and galvanized screws.
Zinc is a relative newcomer to plant fertilizer, having not been discovered as essential to plant growth until 1926. Not long afterward, zinc was implicated as the cause for “pecan rosette” — a condition of stunted, bunchy-appearing shoots, with under-sized, wavy-margined and chlorotic leaves (Figure 1) that has plagued orchards (then and now) throughout the pecan belt.
The negative effects of zinc deficiency extend well beyond appearances. Research from the 1940s to present day has linked zinc deficiency in pecan trees to negative impacts on photosynthesis, flower set, nut size and erratic shuck opening. Its role in plant production is increasingly understood to be complex, including protein metabolism, auxin synthesis, pollen formation, cell membrane integrity, and disease infection resistance.
Natural soil Zinc levels fluctuate widely around the world, with approximately half being deficient for agricultural production. The majority of North American soils are considered moderate to severely deficient, with the greatest problems occurring on soils that are sandy and low in organic matter; or high pH with abundant calcium carbonate; or contain high levels of phosphates. Pecan growers in the southeastern production region, from East Texas to North Carolina, in general, tends to be less concerned with zinc than their counterparts in the arid Southwest because acid soils facilitate the availability of zinc for root uptake. Nonetheless, zinc is not something that can be taken for granted in the Southeast. Orchards can become deficient if zinc is not monitored and managed properly each year.
Regular soil testing is an important component of Zinc management in the Southeast. Soil tests identify the amount of zinc present in the soil and will identify rates to apply for preventing/correcting zinc deficiency. These rates typically call for rates of approximately 5 to 50 pounds of 36-percent zinc sulfate per acre, depending on pre-existing soil levels and tree age/size. Soil tests also indicate any need for limestone to raise soil pH.
Herein is a major dilemma of soil-applied zinc fertilizer. Liming the soil is appropriate for pecan tree root growth if the pH is less than 6.0, but it is counter-productive to soil zinc management. Liming with magnesium carbonate or calcium carbonate causes the formation of insoluble zinc carbonate ions. So if there is a soil deficiency of zinc, it can only be corrected if the soil pH is below neutral and has not received lime applications in the previous two growing seasons. Additionally, broadcast-type applications of Zinc to the soil surface take years to work their way into the tree unless some type of light disking is used to get the zinc below the surface and in contact with the feeder roots.
Banding may be a better approach to soil management of Zinc than broadcast application over the entire orchard floor. Wood (2007) demonstrated maintenance of leaf Zinc levels above the 50 ppm sufficiency level for 4 consecutive years by placing a 4-inch by 12-foot band of zinc sulfate or zinc oxide over 2 drip irrigation lines positioned on either side of the tree row. The trees in Wood’s study were 4 years old at application, and the lowest effective rate was 2.3 pounds actual zinc per tree. This approach may allow growers to manage soil pH (lime application) and zinc levels concurrently without a conflict. Effective total per tree rates for older trees would need to be worked out for those growers interested in adopting this approach.
Foliar application has been shown in numerous studies to be an effective way to keep pecans from experiencing a Zinc deficiency in the current growing season. July leaf samples should be taken the preceding year to determine whether trees are responding adequately to zinc in the soil. Leaf levels of 50 ppm have been widely used as a sufficiency level for many years. Recently, Smith, et al. (2012) called for a 60 ppm sufficiency threshold. If July leaf levels are less than 50 or 60 ppm, foliar sprays should be made beginning at leaf burst (aka “parachute stage”) in March or April, with 2-3 additional applications, according to orchard spray practices and timing of other disease or insect control sprays. To understand spray timing, it should be understood that Zinc is not mobile in pecan leaves, so where trees are not getting enough from the soil, each significant leaf flush should be sprayed while they are approximately one-third to one-half expanded.
The most common foliar Zinc fertilizer used in the Southeast and elsewhere is zinc sulfate wettable powder. It should be applied at 1 to 2 pounds of product per 100 gallons of water, targeting an application rate of 3 to 6 pounds of 36-percent Zinc sulfate per acre with each spray. Zinc nitrate liquid may also be used in the Southeast at one-half to 1 quart per 100 gallons of water. Zinc nitrate is more reactive than zinc sulfate, and thus carries more risk for phytotoxicity, which is usually not a problem in humid growing regions, unless too little water is used during the application.
The third form of foliar zinc that may be used is zinc oxide. Worley (1970, 1972) and Wood and Payne (1997) showed equal performance between the oxide and sulfate forms of Zinc, with oxide forms sometimes being less expensive. Liquid nitrogen (32-percent urea-ammonium nitrate or “uan”) acts as a catalyst for the uptake of zinc through the foliage and is used by growers in the arid regions to improve zinc uptake. Some growers in the Southeast use it, but it is not as necessary for successful zinc uptake as it is out west.
One challenge in Zinc management today is dealer/distributor marketing of proprietary blends of zinc and nitrogen products. These are usually liquid products developed by fertilizer or agricultural chemical suppliers for various agronomic or horticultural crops. With many of these products, the true amount of zinc being supplied per unit volume of product is obscure if not impossible to decipher on the label.
With still other products, the amount of liquid product needed to apply zinc equal to 3 to 6 pounds of 36-percent zinc sulfate per acre in a pecan orchard would require a tremendous amount of product and expense. I don’t believe there is any intentional misrepresentation or attempt to take advantage of pecan growers by these companies. Rather, most if not all of such products are developed for general crop production where the amount of zinc needed per acre is much lower than it is for pecans.
Pecan is a unique crop with regards to its zinc needs, and thus using zinc sulfate wettable powder is the most affordable and reliable way to go. Growers using some of these pre-mixed products may be unknowingly under-applying zinc in their orchards. The final proof of effectiveness certainly rests with the presence of zinc in July leaf samples.
Good spray practices are important to foliar zinc fertilization. Best absorption of zinc occurs when foliage is wet thoroughly and dries slowly. Ideally, Zinc sprays would be made on damp days with little wind, yet many growers tank mix Zinc with fungicides and or insecticides and such conditions are troubling for application of those products. Therefore if spraying zinc to pecan foliage, water application rates should be kept high enough to give good coverage at modest ground speeds. Avoid spraying zinc during high winds, which impede coverage and speed leaf drying. Good sprayer calibration and coverage evaluation should be practiced.
Zinc is the mighty micronutrient. Human bodies contain only 2-3 grams of zinc, yet it is implicated in all kinds of health benefits, including cancer prevention, immune system boosting, sleep regulation, and preventing Alzheimer’s disease, to name a few. Pecan growers need to think about zinc in the same way — an important micronutrient linked to many physiological processes and ultimately orchard profitability.