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Let’s Learn from Zinc Studies this Growing Season

We are rapidly coming upon spring. And for many in the pecan industry, that means zinc. Pecans are uniquely sensitive among nut crops in their susceptibility to zinc deficiencies. This might be a consequence of a higher zinc requirement than some other plant species. Or perhaps it’s due to the kinds of soils that we often use for pecan orchards—soils with low native zinc content or calcareous, alkaline soils (pH greater than 7) that render the zinc very poorly available. Or maybe it’s because pecan roots don’t have root hairs that help other kinds of plants get the zinc out of the soil more easily. In any event, pecan producers, especially in the southwestern growing region, spend a LOT of time and effort (and a fair bit of money, too!) on applying zinc fertilizers each spring. 

Here, I will summarize the take-home messages of pecan zinc EDTA research from a series of four related studies conducted in the semi-arid growing region over the past 10 years or so. All of these are published in open-access journals, meaning you can acquire and read them freely online from the journals. There are many other excellent articles about research on zinc nutrition in pecans. If, after you’ve read these articles, your interest is at all piqued, I would encourage you to look for others to read on the subject!

The first study that we’re going to talk about, Walworth et al. (2017), was conducted from 2011 to 2015 in a newly planted orchard in San Simon, Arizona, with a calcareous, alkaline soil as is typical in the southwestern U.S. pecan growing area. ‘Wichita’ was the main variety (75%), and ‘Western’ was the pollinizer variety (25%). The orchard had a planting pattern of 20 feet between trees in the row and 40 feet between rows. The trees were watered with a microsprinkler irrigation system. 

The objective of that study was to learn whether zinc (chemical symbol Zn) applied through irrigation water (fertigation) as chelated Zn EDTA fertilizer can be effectively taken up by pecan trees. Trees were randomized into three different treatment groups according to the amount of actual zinc applied as Zn EDTA fertilizer that was applied annually: 1) 4 pounds per acre Zn (4.4 kg/ha Zn), 2) 2 pounds per acre Zn (2.2 kg/ha Zn), and 0 pounds per acre Zn. No additional foliar zinc was applied in this study. 

Over the five years of the study, we learned several important things. First, every year of the study, the zinc concentration in the leaf tissue increased with increasing soil Zn EDTA application rate. This is contrary to the conventional wisdom that zinc can only be effectively applied as a foliar spray in the southwestern growing region. 

Second, the untreated control (0 pounds per acre zinc treatment) had, as expected, visual zinc deficiency symptoms worsening over time, especially for the more susceptible ‘Wichita’ variety. On the other hand, visual zinc deficiency symptoms were basically absent in both of the Zn EDTA treatments by the third and fourth years of the study (again, contrary to conventional wisdom). 

Third, we found significantly higher zinc concentrations in the kernels and dormant-season shoots and roots with increasing Zn EDTA application rates. This, along with some other older research showing the immobility of foliar-applied zinc, suggests that zinc taken up by the tree from the soil may be more effective than zinc taken up by the leaves in meeting the zinc needs of other tree tissues besides the leaves. Additional research is needed to learn how this might impact the physiology of those organs and whole-tree physiology over the long term and potentially benefit pecan tree health and nut production

Lastly, we learned tree growth (as trunk diameter) and early nut yields were significantly greater for the Zn EDTA fertigated trees than for the untreated control trees. Interestingly, regarding nut production, the low Zn EDTA (2 pounds of zinc per acre) treatment was significantly greater in years 4 and 5 of the study than the high Zn EDTA (4 pounds of zinc per acre) treatment. 

The bottom line is that zinc fertigated as Zn EDTA is effectively taken up by pecan trees, even in the alkaline, calcareous soils we have in the southwestern United States.

The second study that we will cover, Heerema et al. (2017), was conducted in 2012 through 2013 in the same orchard as the first study. The treatments were the same as in the last study, but only the ‘Wichita’ variety was evaluated. This second study aimed to describe how soil-applied zinc affects pecan trees’ ability to produce carbohydrates (which are basically the “food” for the trees’ function, survival, growth, and nut production) through leaf photosynthesis. Leaflet tissues were analyzed for zinc concentration, and a portable photosynthesis meter was used in this study to measure photosynthesis on individual leaflets. Leaf greenness (related to chlorophyll content) was measured with a SPAD meter.

Over the three years of this study, we learned three important things. First, leaf photosynthesis rates were significantly higher, often much higher, for trees treated with Zn EDTA than for those that were not treated with Zn EDTA. Still, the trees treated with the high Zn EDTA application rate did not have any significant increase in leaf photosynthesis rates compared with that of trees given the low Zn EDTA application rate.

Additionally, leaf photosynthesis rates in August were unaffected by differences in leaf zinc concentration above about 15 parts per million (and around 20  parts per million in June or July). However, leaf photosynthesis rates began to drop when leaf tissue zinc fell below these concentrations. These “breakpoint” concentrations are much lower than the concentrations usually suggested as the minimum levels for zinc in pecan leaf tissue (usually about 50 parts per million). 

Lastly, we found leaf greenness (SPAD) in August followed a very similar pattern to photosynthesis, beginning to drop off below approximately 15  parts per million. The bottom line of these findings is that August zinc leaf tissue levels below approximately 15  parts per million impede leaf photosynthesis rates in pecan, but fertigation with Zn EDTA fertilizer can resolve this problem, even in alkaline, calcareous soil of the desert Southwest.

The third study, Wang et al. (2020), was conducted again in the same orchard and with the same Zn EDTA treatments as the previous two studies. The objective of this experiment was to learn about the relationships between pecan tree zinc nutritional status and the human health-promoting components of pecan kernels. Nuts were hand-sampled from ‘Western’ and ‘Wichita’ trees at nut maturity in 2014 and 2015. The kernels of these nuts were analyzed for a range of human health-promoting components, including oils, tocopherols (Vitamin E-like biochemicals), and antioxidants.

In this study, we first learned that soil application of Zn EDTA by fertigation (which increases tree tissue zinc concentrations) resulted in significantly higher kernel oil content compared with the untreated control. This higher kernel oil content is desirable both because pecan oil is considered heart-healthy, and this increased content may enhance the consumer’s enjoyment of eating pecans. 

We also learned that the pecan oil of kernels sampled from trees subjected to the treatment with lower annual Zn EDTA application rate (2 pounds zinc per acre) had a significantly higher percentage of monounsaturated fatty acids than that of kernels sampled from the untreated control. A higher percentage of monounsaturated fatty acids is desirable because these are the fatty acids in pecan oil thought to be particularly good for consumers’ heart health, among other benefits. 

Lastly, the study found that both soil Zn EDTA treatments increased the gamma-tocopherol concentration of the pecan kernels compared with those from the untreated control. Gamma-tocopherol is the dominant kind of tocopherol in pecan, and enhanced levels of this biochemical are desirable for its Vitamin E activity and its function as an antioxidant, neutralizing harmful free radicals in the human body. 

The bottom line: Improving the zinc nutritional status of pecan trees (in this case by soil-applying zinc fertilizers in irrigation water) enhanced human health-promoting components of pecan kernels. 

The fourth study, Smith, et al. (2021),was conducted in 2018 and 2019 in another orchard near the San Simon orchard used in the three previously described studies. It also had a calcareous, alkaline soil, was planted in 2011 on a 20-by-40-foot pattern, was irrigated by a microsprinkler system, and had a variety mix of 75% ‘Wichita’ and 25% ‘Western.’ 

This study’s objective was to determine if foliar zinc applications further enhance the increased photosynthesis rates brought about by soil application of Zn EDTA. The orchard received Zn EDTA applications through fertigation at a rate of 6 kilograms of zinc per hectare ( about 5 lbs Zn/acre) in 2018 and 11 kilograms of zinc per hectare (about 10 lbs Zn/acre) in 2019. Trees did not show signs of zinc deficiency and had leaf tissue concentrations in late July above the expected threshold (“breakpoint”) of approximately 20 parts per million, below which photosynthesis begins to decline in pecan. Foliar applications of zinc sulfate fertilizer solutions (3.6g/L ZnSO4), both with and without the addition of urea ammonium nitrate (3.75 mL/L UAN), were made by hand twice in the first season (May and June) with small spray bottles to individual shoots in seven ‘Wichita’ tree canopies. On the same trees and on the same days, to serve as experimental controls, individual shoots were sprayed in the same way with distilled water, also both with and without the addition of UAN. In May and early July during the second year, the previous individual shoot foliar spray treatments were repeated, and we added a fifth foliar spray treatment to the experiment with a Zn EDTA solution (11 mL/L Zn EDTA). After the treatments, leaf photosynthesis was measured on the zinc-treated and control shoots on each tree twice each year, following a similar method to that used in the second experiment described. 

In this study, we learned this crucial thing: in this experiment where pecan trees were fertigated with Zn EDTA, none of the foliar zinc sprays applied had any significant effect on leaf photosynthesis rates. The bottom line? If sufficiently high levels of leaf zinc have been achieved through Zn EDTA fertigation, foliar zinc sprays do nothing to further enhance leaf photosynthesis.

And to tie it all together, based on the research work in these studies, we now know that soil-applied chelated Zn EDTA can supply sufficient zinc to pecan trees to eliminate visible deficiency symptoms and low photosynthesis rates caused by zinc deficiency. This finding is especially interesting because this approach is effective even in calcareous, alkaline soils. And applying foliar zinc sprays, in addition to the soil-applied Zn EDTA, does not improve the situation by further enhancing photosynthesis. Pecan farmers could benefit from this approach to zinc nutrition by the ease of management, and pecan consumers may benefit from this with even better human health-promoting aspects for pecans.

Heerema, R.J., D. VanLeeuwen, M.Y. Thompson, J.D. Sherman, M.J. Comeau, and J.L. Walworth. 2017. Leaf photosynthesis of immature ‘Wichita’ is increased by soil-application of zinc-EDTA. Journal of the American Society for Horticultural Science 142(1): 27-35
Smith, C.A., J.L. Walworth, M.J. Comeau, R.J. Heerema, and J.D. Sherman. 2021. Does foliar zinc application boost leaf photosynthesis of ‘Wichita’ pecan fertigated with zinc-EDTA? HortScience 56(5): 579-582.
Walworth, J.L., S.A. White, M.J. Comeau, and R.J. Heerema. 2017. Soil-applied Zn-EDTA: Vegetative growth, nut production, and nutrient acquisition of immature pecan grown in an alkaline, calcareous soil. HortScience 52(2): 301-305.
Wang, Y., R.J. Heerema, J.L. Walworth, B. Dungan, D. VanLeeuwen, and F.O. Holguin. 2020. Nutraceutical properties of pecan kernels are affected by soil zinc fertilizer application. HortScience 55(12): 2001-2007
Author Photo

Richard Heerema

Dr. Richard Heerema is the Extension Pecan Specialist at New Mexico State University, Las Cruces, New Mexico.