Correct zinc deficiency in pecan nut trees with soil banding

Pecan nut trees are heavy users of zinc (Zn). It is an essential nutrient for good growth and nut production. However, zinc deficiency is quite a common problem in commercial pecan nut orchards, especially those established on sandy, well-drained, acidic soils, and on soils from phosphatic rocks. In other words, the soil pH is important in determining the availability of zinc in pecan trees. In acidic soils of a pH less than six, zinc treatment is essential. Zinc is commonly applied by means of a foliar spray. Although multiple annual foliar spray applications, up to six per season, are effective and potentially a rapid method for correction of Zn-deficient foliage, it is expensive. Efficacy also depends on complete canopy coverage and repetitive application, as the foliar canopy expands during spring. Also, foliar sprays probably do not result in sufficient mobilisation and transport of endogenous zinc to alleviate deficiency in the non-foliage tree organs such as the roots. There is evidence that the zinc needs of pecan trees might be best satisfied through root uptake. Zinc can also be applied once in several years in a dry form by broadcasting it underneath the trees, or placed in trenches, but correction by soil application is also expensive and usually impractical or not effective. Trunk implants or injections are also possibilities. There is a need for a more effective, economical, and long-lasting method for satisfying Zn needs. The US Department of Agriculture, Agricultural Research Service, South-eastern Fruit and Tree Nut Research Laboratory did some extensive research on a better methods to solve this problem. The research was conducted over four years on trees growing in acidic soil, which exhibited moderate Zndeficiency symptoms. The trees were entering their fourth growing season.

Foliar application of zinc is the norm, but extensive research proves that there is a better method to pursue. Band placement of zinc on top of the soil under the trees proves to be an inexpensive and effective method of application. Image: tpga.org

Previous experiments proved that band placement methods were inferior to broadcasting, but this method hit the sweet spot. This fertilisation strategy offers an efficacious alternative to annual foliar Zn sprays for orchards established on acidic soils, and provides a means of ensuring rapid and long-term Zn absorption through soil application. It appears that under moist soil situations, the zinc needs of the tree will be satisfied for at least four years,

Another conventional method of applying zinc to pecan trees is by dissolving it into the irrigation water. Image: Hardus du Toit

Two parallel, underground drip irrigation lines were spaced 1,2 metres on either side of the tree trunks with the emitters raised above the soil level, one metre apart. Soluble zinc sulphate (ZnSO4.7H2O) or zinc oxide (ZnO) were applied under each experimental tree in a band of 10 cm by 4 metres, exactly over the drip lines. Different quantities of the nutrients were supplied to the different trees to find the perfect application. The dry weight of zinc in the leaves was measured to determine the outcome, and the results were significant. 50 milligram per gram dry weight of zinc in the leaves are needed for optimal tree functionality. This was obtained even four years after the application. Rates of Zn at 264 to 1 056 g per tree are efficacious and, as a bonus, increasing amounts of Zn also increased the foliar Mn concentration, and did not affect foliar concentration of other key micronutrients such as Fe, Co, Cu, or Ni. and potentially for several years thereafter, as applied zinc moves deeper into the soil profile with passage of time and more feeder roots penetrate the Zn-enriched zone.

Zinc oxide and zinc sulfate are very efficient providers of zinc when the banding application method is used.

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