It’s time for leaf tissue analysis for pecans

Annual leaf tissue analyses are currently the best thing out there to get a true answer. Soil analyses are important too, but for a lot of the nutrients, they just don’t give as clear of an answer as to the trees’ nutritional needs. The soils give us a lot of information about salinity or sodicity, and about issues we’re having with irrigation or root health. They can also tell us some things about why certain nutrients are out of whack, but when it comes to actual tree nutrition, they don’t give the whole story. Leaf tissue analysis, on the other hand, offers complete insight.

There are two kinds of leaflet tissue samples that can be collected. Sometimes you might collect tissue samples when you already suspect some sort of nutritional problem in your orchard and want to confirm it or figure out exactly which nutrient is out of balance. Maybe you already see a suspicious symptom in your orchard. Or maybe you know from experience that a particular part of your orchard is underperforming in nut production.

In these cases, you should sample the problem trees separately from other parts of the orchard where you don’t think there’s a problem. You should do this kind of diagnostic sampling as soon in the growing season as you can—as soon as your suspicions of a problem arise and as soon as there’s enough leaf growth to sample. Unless you collect the diagnostic samples in late July or August, you should really not use the published tables with recommended nutrient levels when evaluating the lab results from these kinds of diagnostic samples. Rather, in that case, you should compare the results of the sick or underperforming tree leaflet samples to that of healthy trees.

We’ll call the other kind of leaflet tissue sampling: the “routine” sampling. This routine sampling should be done every single growing season, whether you think there’s problem or not. As mentioned above, the timing of this sample is more critical because “normal” nutrient levels change throughout the season. It is perfectly normal for leaf nitrogen concentrations to start off higher in the early growing season and drop off over time. And it is perfectly normal for boron and calcium levels to increase in leaves over time up to the end of the growing season. That is why the published tables with recommendations have to be based on a particular time in the season.

Some producers question the timing for the routine sampling because August is so late in the growing season. The argument is that by that point in the season, so much is already lost if there’s a problem. There is a certain truth to this argument, but it is also important to bear in mind the purpose of the routine sample—to get a clear and accurate measure of the current orchard nutritional status. Earlier sampling is pretty weak in this regard, in part because the leaf concentrations for many of the nutrients are rapidly changing and are more variable during the early part of the growing season; they’re much more stable during the middle part of the growing season. Differences in springtime temperatures or even previous winter chilling, for example, can make meaningful comparisons of nutrient status from year to year impossible. That’s why you need to collect the leaves from healthy trees for comparison when doing early season diagnostic sampling. August may indeed be a bit late to make significant adjustments for some nutrients. Still, I would argue that the routine sampling’s purpose primarily has the orchard’s long-term (big picture) nutritional health in mind and secondarily begins the process of dealing with nutritional imbalances in the current season.

The actual collecting and processing method is also crucial when conducting routine tissue sampling for some of the same reasons that timing matters. It lets you compare apples-to-apples with the technique used when the recommendations were established. Briefly, the sampling protocol involves collecting uniform leaflets from each orchard block—with a minimum of 60 leaflets per block from multiple trees so that the sample is representative of the entire block and the lab has sufficient material to do their tests.

Sample the middle leaflet pair from the middle compound leaves of each sampled shoot. Wash and rinse the leaves thoroughly before shipping the samples to the analytical agricultural lab. You should include one washing step in a mild hydrochloric acid solution if the leaves have been previously sprayed with foliar fertilizers.

When the laboratory sends you the results, they will sometimes send an interpretation along as well. If you decide to use the lab-generated interpretation of the results, make sure they use pecan and not some other crop species as the standard for their interpretation. If not, you can also just do the interpretive step yourself by comparing the raw leaf tissue mineral concentration data to the recommendations developed by your university.

For more detailed information about the leaflet tissue sampling technique and interpretations of the actual nutrient concentration analyses, you can go online to the University of Arizona Cooperative Extension’s article on “Zinc Management in Arid Region Pecan Orchards” or New Mexico State University’s publication titled, “Diagnosing Nutrient Disorders of new Mexico Pecan Trees.”

Our research has resulted in a few adjustments to our interpretation of the pecan zinc tissue concentrations. We now have good evidence from several studies that growers can use a somewhat lower leaf tissue zinc concentration in orchards that receive chelated zinc fertilizers by soil application than the previously recommended 40-50 ppm.

One study showed that pecan root uptake of zinc from soil-applied (by fertigation) zinc EDTA fertilizers actually occurs even in Arizona’s alkaline, calcareous soils and that this approach can successfully eliminate zinc deficiency symptoms in pecan trees, at least immature ones (Walworth et al., 2017).

A second related study showed pecans trees with only soil-applied zinc fertilizers (i.e., no foliar sprays) experience a decline in leaf photosynthesis levels only when leaf tissue concentrations drop below 15 ppm zinc (Heerema et al., 2017).

A third study showed that when foliar zinc fertilizer (zinc sulfate) solutions were applied to leaves of bearing pecan trees receiving soil applied zinc EDTA fertilizers via fertigation, the leaflet zinc concentration rose from around 20 ppm to over 120 ppm, but photosynthesis rates did not increase (Smith et al., 2021). The bottom line on all this is that it seems that the threshold for zinc deficiency for pecan (at least for individual trees and as indicated by leaf function, i.e., photosynthesis) may be much lower than 40 or 50 ppm—probably around 15 or 20 ppm.

The problem with this is that a leaf tissue sample is a composite sample that comes from many trees in a block. If the composite sample’s average zinc tissue concentration is only 15 ppm, then it stands to reason that some trees will be at or above this average (i.e., sufficient for zinc) while others will be below it (i.e., deficient for zinc). So, the question for managing orchard zinc nutrition becomes: how high does this average zinc concentration in the composite sample need to be to ensure that there are not a lot of trees below the 15 ppm zinc deficiency threshold?

A fourth related study was conducted in a zinc-EDTA fertigated (no foliar zinc applied) ‘Western’ and ‘Wichita’ pecan orchard in Cochise County, Arizona, to answer this question by characterizing the tree-to-tree variability in leaf tissue zinc concentration across the block over two growing seasons (Smith et al., 2022). This study was funded by the USDA Specialty Crops Research Initiative (USDA-NIFA-SCRI 2016-51181-25408) and research donations from the Arizona Pecan Growers Association.

In 2018, the first year of the study, leaflets were sampled and analyzed for zinc from 119 individual trees (17 ‘Western’; 102 ‘Wichita’). In 2019, leaflets were sampled and analyzed for zinc from 153 individual trees (51 ‘Western’; 102 ‘Wichita’). The average zinc concentration for ‘Wichita’ was 19 ppm in 2018 and 18 ppm in 2019, while it was a little higher in ‘Western’ at 23 ppm in 2018 and 25 ppm in 2019. Some of the trees were well below the 15 ppm deficiency threshold, especially in the ‘Wichitas.’ The tree-to-tree variability in zinc was not related to differences in soil properties in the orchard, which suggests that genetic differences among the seedling rootstocks may have caused these patterns in zinc nutrition.

Assuming that an orchard block has the same variability in leaf zinc concentration as this Arizona study orchard, and assuming a composite leaf sample collected from 35 trees in the block, the minimum average leaflet tissue zinc concentration necessary to be sure that fewer than 5% of trees were zinc deficient (<15 ppm) was 28 ppm. These results confirm the earlier suggestion by Walworth and Heerema (2019) that “…30 ppm is a safe minimum ‘orchard-level’ leaf zinc concentration” for pecans fertilized with zinc EDTA applied to the soil via the irrigation water.

Sources and Further Reading

Heerema, R. 2013. Diagnosing nutrient disorders of New Mexico pecans. NMSU Extension Guide H-658.

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., D. VanLeeuwen, R.J. Heerema, J. D. Sherman, M. J. Comeau, and J. L. Walworth. 2022. Zinc variability in pecan orchards: Implications for leaf sampling and nutrient recommendations. HortScience, 57(4), 550-557.

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. and R. Heerema. 2019. Zinc management in arid region pecan orchards. University of Arizona Extension Guide 1789.

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.