Pecan trees require a continuous supply of micronutrients for robust growth and productive nut yields, with zinc being particularly important. Zinc acts as a cofactor for numerous enzymes, influencing processes like protein synthesis and energy production. Adequate zinc levels are necessary for the development of new leaves and flowers, directly affecting the tree’s ability to photosynthesize and produce a crop. Because pecan trees have a high demand for this nutrient, deficiencies are common. Zinc sulfate is the industry standard material used to correct these deficiencies, offering a readily available source of the nutrient.
Identifying Zinc Deficiency in Pecan Trees
Before applying treatment, growers must accurately identify zinc deficiency symptoms, often called “pecan rosette.” The most obvious signs appear on new growth, as zinc is not easily mobilized from older tissues. Leaves on affected shoots are noticeably smaller, narrower, and may have wavy or rippled margins. This reduction in leaf size decreases the tree’s ability to produce energy, leading to overall poor vigor.
The characteristic “rosette” appearance is caused by a severe shortening of the internodes—the stem sections between leaves—which results in a crowded cluster of small leaves at the shoot tip. Leaves often display interveinal chlorosis, appearing yellow between the veins while the veins themselves remain green. As the deficiency progresses, chlorotic areas may turn necrotic, and terminal shoot dieback can occur. Proper identification of these visual cues confirms the need for zinc sulfate application.
Foliar Application Rates and Timing
Foliar spraying is the most common and effective method for rapidly correcting zinc deficiency. This method ensures the nutrient is absorbed directly by the newly expanding leaves. Zinc must be applied to these soft, young tissues because it is largely immobile once absorbed. Therefore, multiple applications are necessary throughout the growing season to protect all new growth flushes.
For mature, bearing pecan trees, the standard application rate for zinc sulfate monohydrate (36% elemental zinc) is 2 to 3 pounds per 100 gallons of water. This solution is applied to ensure thorough coverage, often around 100 gallons per acre in commercial orchards. Typically, three to five sprays are applied over the season, beginning at bud break and continuing at two-week intervals. The total annual application of elemental zinc for mature orchards usually falls between 4 to 6 pounds per acre.
Young, rapidly growing non-bearing trees require a more frequent and prolonged spray schedule because they produce new growth flushes over a longer period. For these trees, the concentration is usually lower, around 1 pound of zinc sulfate per 100 gallons of water. They may be sprayed weekly or every 10 to 21 days, beginning at bud break and continuing until the end of July or until vegetative growth slows considerably. This intensive schedule is necessary to ensure every new flush of leaves receives zinc before it fully expands.
When mixing the solution, special care is needed because dry zinc sulfate is not highly soluble and requires good agitation in the tank, or it should be premixed before adding. To maximize the absorption of the zinc through the leaf surface, it is recommended to add an adjuvant such as urea ammonium nitrate (UAN32) to the spray tank. Adding approximately 3 pints of UAN32 per 100 gallons of water can significantly enhance the uptake of the zinc sulfate. Ensuring complete and uniform coverage of the expanding canopy is the most important factor for a successful foliar treatment.
Soil Application Methods and Dosage
Soil application of zinc sulfate is an alternative method, though it is generally less efficient for immediate deficiency correction, particularly in Western pecan-growing regions. The primary challenge is that zinc sulfate rapidly reacts with soil particles, especially in alkaline or calcareous soils with a high pH (above 7.0). This reaction renders the zinc insoluble and unavailable to the tree roots. Soil treatments are most effective in acidic soils, specifically those with a pH below 6.0.
When soil application is utilized, the treatments are typically applied during the late dormant season, such as in February or March. The dosage is often calculated based on the tree’s size to provide a long-term reservoir of the nutrient in the root zone. A common recommendation is to apply one-half pound of 36% zinc sulfate per inch of trunk diameter, with an annual maximum of 10 pounds per tree. For young trees in acidic soil, a general guideline is to apply one pound of zinc sulfate per tree for the first three years after planting.
The granulated material is usually broadcast or banded around the tree’s drip line, corresponding to the area of active feeder roots, while avoiding contact with the trunk. Soil injection places the material deeper into the root zone but is less common than surface application. For long-term management, especially in high-pH soils, chelated zinc products like Zn-EDTA are preferred over zinc sulfate. The chelation process protects the zinc from being tied up by the soil, allowing it to remain available for root uptake.