Zinc Fertilization in California Orchard Crops

Zinc Fertilization in California Orchard Crops

Phoebe Gordon, UCCE Madera and Merced Counties

Zinc is the micronutrient that is most commonly deficient across the state of California.  This is primarily due to its low availability in Central Valley soils due to the high pH and sometimes presence of carbonates, which zinc will adsorb to at high pH.  Just a 1 unit increase in pH will decrease zinc availability 100-fold . 

Symptoms of zinc deficiency differ from crop to crop, but there are some symptoms that are common across species.  Reduced yields are the primary symptom and the one we care most about, but that can be difficult to diagnose if there are no zinc sufficient trees in your orchard to compare.  A classic symptom is new growth exhibiting small leaves and extremely short internodes that result in a rosetting effect .  In almonds and other Prunus species (Figure 1), these new leaves can look lance-like. Walnuts and pistachios (Fig. 1) can exhibit interveinal chlorosis in addition to smaller leaves with short internodes, and pecans (Fig. 2) will also exhibit necrotic margins or spotting.  However it is important to remember that once you see deficiency symptoms, your orchard is already losing yield. 

The most ideal way to remediate any nutritional deficiency is with a soil application, however soil conditions in California orchards necessitate foliar applications to fix zinc deficinecies.  This is because zinc is basically immobile in soils, and while it can be banded, the required rates can be so high that it can be uneconomical or even possibly cause phytotoxicity.  This also doesn’t take into account that any factor that makes zinc unavailable in soils.  High pH and the presence of carbonates will eventually make any applied zinc unavailable as well.  Chelated forms of zinc are more mobile in soils, but they can be expensive.  All these add up to foliar zinc being the most practical way to remediate zinc deficiencies in California orchards.

Zinc has limited mobility within plants, though the degree to which it is mobile differs depending on the crop plant and the study.  Two studies examined the absorption amount and fate of foliar applied zinc in California orchard trees.  Zhang and Brown (1999) examined walnut and pistachio leaves in laboratory experiments and found only 3.5 and 6.5% was actually absorbed into plant tissues and moved away from the site of application, respectively.  Sanchez et al (2006) painted zinc sulfate on the top surface of all leaves of one year old peach trees to simulate a foliar spray and found that 7% of foliar applied zinc was recovered the next spring during leaf break, indicating a small fraction of zinc that was taken up (which was not tested) was remobilized and reused the following season.  This study also included nitrogen in the foliar application; nitrogen has been shown to increase absorption of zinc in some species.

The lab work done by Zheng and Brown also showed that in walnuts and pistachios, zinc is absorbed more readily in young pistachio leaves than older ones, but absorption did not differ in walnut leaves based on leaf age.  However, looking at the paired research trials which included Bob Beede in pistachios and Joe Grant in walnuts, both showed that foliar sprays were best applied during early spring flush in pistachios and late spring flush for walnuts.  The difference in timing is due to the reduced ability of mature pistachio leaves to absorb zinc.  Because mature walnut leaves absorb as much as young leaves, the late spring flush period results in increased surface area and therefore better absorption.  This work also showed that the combination of zinc and nitrate resulted in phytotoxicity in pistachios and walnuts, and this combination is not recommended. 

Figure 1: Zinc deficient walnut (top left, source K. Uriu), zinc deficient almond (top right, source W. Asai), zinc deficient pistachio (bottom, source: B. Beede)

Figure 2: Zinc deficient pecan (source: R. Heerema)

In almonds and peaches, fall sprays with zinc are adequate for addressing zinc deficiency as well as spring sprays.  Work done by Franz Niederholzer has shown that low rates of foliar applied zinc in October (5 lbs of Zn sulfate per acre) are as effective as much higher rates applied in November (20 lbs of Zn sulfate per acre).  In the past it was suggested that these heavy, late applications, which defoliate the trees, may have the side benefit of reducing rust inoculum in an orchard.  However, work done by Macej Zweinecki at the Carbohydrate Observatory at UC Davis has recently shown how important carbohydrates are to tree health, and we now recommend that growers and PCAs do not do this.  Spring applications, applied once leaves reach nearly full size, of zinc on Prunus species may result in phytotoxicity symptoms, so it is best to use products that are safe for this application.  However, if rains follow these applications, shot-hole symptoms may occur.   I visited several orchards in the spring of 2020 with this issue.  The trees retained the leaves and I didn’t hear back about any further issues (and in my world, no follow-up news is typically good news!)

Table 1: Leaf critical values for some major orchard crops grown in the Central Valley

In pecans, the story is a bit different.  Pecans are regarded to be extremely zinc hungry plants, and their leaf sufficiency values are much higher than other tree species (Table 1).  In areas with low soil availability, sprays should start at bud break and continue until growth stops.  In mature trees, this will likely be 3-5 sprays, but in young rapidly growing trees, you may need to spray weekly for several months.  It has been found that nitrate or the inclusion of UAN32 increases the absorption of zinc in pecans, and applying zinc as a sulfate has been found to be as effective as a chelated form.  Because of the need for continual and frequent foliar applications, Walworth et al. (2017) examined soil applications of chelated zinc at either 2 or 4 lbs an acre, split up over the year in a young pecan orchard in a calcareous soil.  They found that soil applied zinc raised tissue levels in all parts of the plant, in contrast to foliar applications, which only raised leaf levels.  The soil applications were unable to raise leaf zinc values to the 40 ppm that was previously recommended pecan leaf sufficiency values, however, all visual symptoms of zinc deficiency were eliminated.  In a paired study looking at photosynthetic rates in this trial, leaf photosynthesis was maximized at 15 ppm of leaf zinc.  Because of this, suggested sufficiency values were decreased to 30 ppm.  An additional study examined applying 70 mL of ZnEDTA directly in planting holes at planting, which was found to mostly eliminate zinc deficiency symptoms for three years with no additional foliar applications.  While these studies were performed in Arizona, it is reasonable to assume, due to the similar climactic conditions and high pH soils, that sufficiency standards would be similar in California. 

As with other foliar applications (insecticides, fungicides), adequate spray coverage is essential, since a tiny fraction of applied zinc will be remobilized into other tissues, if at all.  The same rules that you should use for good spray coverage for pesticides apply for nutrient sprays; drive slowly (2 mph or less), don’t apply during a windy day, and make sure the spray distribution reflects the size of your trees.  Additionally, it is helpful to spray when humidity is high so that droplets do not evaporate before the zinc has time to be absorbed into leaf tissues; in our arid climate this can be at night or early in the morning. 

Leaf critical values for some major orchard crops are provided in table 1.  Something I would like to note: it can be tempting to think that your trees’ zinc levels have increased drastically after a zinc spray.  This is because foliar applied zinc binds very readily to the leaf cuticle, or the layer of wax that protects a leaf.  Unless the zinc is washed off with a dilute acid, that excess will remain and be included in leaf tissue values.  So, if you do a spring zinc spray in an orchard, you should not view your leaf tissue zinc levels from that orchard as an adequate measure of the trees zinc status.  Bag some leaves before a spray and mark the branches or spurs that you protect.  Remove the bags immediately after the sprays so the leaves don’t die and fall off, and come back and sample those leaves later for your July leaf tissue analyses to get a more accurate assessment of your orchard’s zinc levels

One last thing about zinc: high soil phosphorus levels in soil reduce zinc availability. While we do not heavily rely on phosphorus fertilizer in orchard agriculture because, to the best of our knowledge, phosphorus is not needed in large quantities in orchard crops and is abundantly available in soils, this can be seen in orchards that are established in old dairy corrals.

Literature used to write this article:

Brown, P., Zhang, Q. and Beede, B., 1993. Effect of foliar fertilization on zinc nutritional status of pistachio trees. California Pistachio Industry. Annual Report.

Brown, P., Zhang, Q. and Grant, J., 1995, IMPROVING WALNUT ZINC NUTRITIONAL STATUS BY FOLIAR SPRAYS. 1995 Walnut Research Reports.

Heerema, R.J., VanLeeuwen, D., Thompson, M.Y., Sherman, J.D., Comeau, M.J. and Walworth, J.L., 2017. Soil-application of Zinc-EDTA increases leaf photosynthesis of immature ‘Wichita’pecan trees. Journal of the American Society for Horticultural Science, 142(1), pp.27-35.

Sanchez, E.E., Weinbaum, S.A. and Johnson, R.S., 2006. Comparative movement of labelled nitrogen and zinc in 1-year-old peach [Prunus persica (L.) Batsch] trees following late-season foliar application. The Journal of Horticultural Science and Biotechnology, 81(5), pp.839-844.

Swietlik, D., 2002. Zinc nutrition of fruit crops. HortTechnology, 12(1), pp.45-50.

Walworth, J.L., White, S.A., Comeau, M.J. and Heerema, R.J., 2017. Soil-applied ZnEDTA: vegetative growth, nut production, and nutrient acquisition of immature pecan trees grown in an alkaline, calcareous soil. HortScience, 52(2), pp.301-305.

Walworth, J. and Heerema, R., 2020. Zinc management in arid region pecan orchards. Univ. Arizona Ext. Bul. az1789, 30.

Zhang, Q. and Brown, P.H., 1999. The mechanism of foliar zinc absorption in pistachio and walnut. Journal of the American Society for Horticultural Science, 124(3), pp.312-317.