The effect of cattle manure or combined manure and zinc (Zn) application on Zn uptake, mineral composition, and yield and nut quality in pecan trees [Carya illinoinensis (Wangenh.) C. Koch] was evaluated. In 2006, treatments evaluated were: manure (12 ton/ha; M), manure plus Zn (12 ton/ha plus 129 kg Zn/ha as ZnSO4; MZ), and an untreated control. During 2007, two more treatments were added with doubled manure and Zn rates. New treatments were manure 2× (24 ton/ha; M2) and manure 2× plus Zn 2× (24 ton/ha plus 258 kg Zn as ZnSO4; M2Z2). Manure was broadcast on the soil in a 2.5-m wide band 2 m from the trunk. Zinc sulfate was broadcast over the manure, and then manure and Zn were disked into the top 10 cm of soil. In 2008, in five of nine sampling dates, significant treatment effects were detected on leaf Zn concentrations. On all of the dates, M2Z2 had the highest foliar Zn levels. During the Summer 2008 (17 July) foliar Zn in M2Z2, treatment reached 66 μg·g−1; the control treatment level was 45 μg·g−1. Nut yields were higher in treatments receiving manure, with or without Zn in the first year, and highest in the untreated control the second year. No differences were observed in trunk growth, leaf area, leaf weight, or nut growth. Kernel percentages were over 61.4 in the 3 years of study in all treatments. Largest differences among treatments in nut size were found in 2007; nut weight in the control treatment was 7.5 g per nut and in M was 8.0 g per nut. Nut weight was smaller during 2008 when nut yield was high, and the untreated control nuts were smaller than those from treated trees. The manure and manure plus Zn treatments increased foliar Zn levels in pecan trees after 3 years of annual applications. In 2008, significant differences in leaflet Zn concentration among treatments were detected with M2Z2 having the highest concentrations.
The effect of soil banding zinc sulfate and zinc (Zn)-EDTA was evaluated over a period of 4 years on established ‘Wichita’ pecans [Carya illinoinensis (Wangenh.) K. Koch] growing in alkaline, calcareous soil. Treatments evaluated were ZnSO4 applied at 74 kg Zn/ha and Zn-EDTA at 19 kg Zn/ha. These materials were applied just once on 23 Mar. 2005. Fertilizers were injected in two bands placed 1.2 m from either side of the trunk of the tree and 18 cm deep. Treatments were replicated four times in a randomized complete block design. Data collected included foliar Zn concentrations throughout the season, midseason foliar nutrient concentrations, leaflet growth, nut yield, and nut quality. Significant differences in foliar Zn levels were found 1 month after application of Zn-EDTA. Differences also were noted during the next 3 years on ≈25% of the sampling dates. Yields of in-shell pecans averaged 2800 kg·ha−1 during the 3 years of harvest but were unaffected by treatments. Nut quality also was unaffected. Nut kernel percentage was very high, ranging from 61.2% to 63.6% during the study. Growth, measured as leaflet area and trunk cross-sectional area, was unaffected by Zn application. Chlorophyll index ranged from 47.5 to 48.0 in 2007 and from 44.7 to 45.4 in 2008 and was unaffected by applied treatments. Zn-EDTA increased Zn uptake slightly by ‘Wichita’ pecan trees in alkaline, calcareous conditions during 3 years after one soil band injection. Ongoing research on potted pecan trees (with the same soil used in the present study) suggests that Zn-EDTA can very effectively increase Zn uptake if placed in close proximity to the tree roots. Additional research is needed to refine application and placement methods in these types of soils to produce a more effective and consistent response.
A field study was conducted to evaluate efficacy of soil-applied zinc (Zn) fertilizer on young pecan [Carya illinoinensis (Wangenh.) K. Koch] trees growing in alkaline, calcareous soils. Chelated Zn ethylenediaminetetraacetic acid (ZnEDTA) was applied at rates of 0, 2.2, or 4.4 kg·ha−1 of Zn via injection into irrigation water (fertigation) in microsprinkler irrigated ‘Western’ and ‘Wichita’ trees. Over the 5-year duration of the study, leaf Zn levels were increased from 22 to 35 µg·g−1 in the highest rate of ZnEDTA treatment compared with 7 to 14 µg·g−1 in unfertilized trees. Zn concentrations in shoot and root tissues were also elevated in Zn-treated trees. Zn treatments largely eliminated visible Zn deficiency symptoms, and increased trunk diameter growth compared with untreated trees. Nut yield (in the third through fifth seasons) were also increased as a result of Zn fertilization. No additional benefit in terms of trunk diameter growth or nut yield was observed by adding a higher rate of Zn (4.4 kg·ha−1) vs. the lower rate (2.2 kg·ha−1). ‘Western’ and ‘Wichita’ trees responded similarly to Zn fertigation.
Many growers fertigating their orchards with zinc–ethylenediaminetetraacetic acid (Zn-EDTA) are still using supplemental zinc foliar sprays because of a lack of confidence that soil-applied Zn-EDTA is supplying enough Zn to the trees. A field study was conducted in a pecan orchard located near San Simon, AZ, on 8-year-old ‘Wichita’ trees growing in an alkaline, calcareous Vekol loam soil to evaluate the effectiveness of supplemental foliar Zn sprays. All trees were fertigated with 6.0 kg⋅ha–1 Zn in the form of Zn-EDTA in 2018 and 11.0 kg⋅ha–1 Zn in 2019 and did not exhibit visible signs of Zn deficiency. Foliar treatments of 3.75 mL⋅L–1 urea–ammonium nitrate (UAN), 3.6 g⋅L–1 zinc sulfate monohydrate (ZnSO4·H2O), 3.6 g⋅L–1 ZnSO4·H2O with 3.75 mL⋅L–1 UAN, 11 mL⋅L–1 Zn-EDTA, and water alone were applied to individual fruiting shoot terminals of trees on two dates each in 2018 and 2019. Treatments were sprayed directly onto the leaves of the selected terminals. Zn-EDTA was included as a foliar treatment in 2019 only. Leaf photosynthesis was measured to determine the impact of leaf Zn concentrations on plant function. Midday stem water potential (MDSWP) was measured to verify that water stress was not limiting photosynthesis. Both measurements were taken about 2 to 4 weeks after the application of foliar treatments. MDSWP measurements indicated a lack of water stress and therefore no effect on photosynthesis. Leaf samples collected from untreated branches indicated that the average foliar Zn concentration of untreated leaves was 21.3 mg⋅kg–1 in 2018 and 15.7 mg⋅kg–1 in 2019. No differences were observed in photosynthesis rates of treated branches. No additional benefit to leaf photosynthetic function or appearance was observed from spraying Zn on foliage of trees fertigated with Zn-EDTA.
Pecan (Carya illinoinensis) has high kernel antioxidant activity and unsaturated fatty acid content, which contribute to its nutraceutical properties. In the western United States, where soils are typically alkaline, pecan trees require frequent zinc (Zn) fertilizer applications to maintain normal canopy growth and nut production. Our objective was to investigate the effects of tree Zn fertilization on nutraceutical properties of ‘Wichita’ and ‘Western’ pecan kernels. Trees were fertilized with ethylenediaminetetraacetic acid (EDTA) chelated Zn, which was applied to the soil at one of three seasonal rates for a total of three treatments: 0 (control), 2.2, or 4.4 kg·ha−1 Zn. Nut samples were collected and homogenized for analyses of kernel oil yield, hydrophilic antioxidant capacity, fatty acid profile, and γ-tocopherol content. Although soil Zn treatments did not significantly affect antioxidant capacity of defatted pecan kernels, Zn application had significant positive effects on both total kernel oil yield and γ-tocopherol content compared with the control. In conclusion, soil application of Zn fertilizer may increase the human health-promoting aspects of pecan kernels, a valuable attribute among health-conscious consumers.
Measurement of nutrients in leaf tissue is a practical method of monitoring the nutritional status of perennial crops such as pecan (Carya illinoinensis, Wang. C. Koch). Accurate interpretations require known standard concentrations for the crop and region. To determine standard concentrations for pecans, focusing on those grown in the desert southwest, we conducted a survey of 135 `Western Schley' pecan trees in Arizona for 2 years. Leaf nutrient concentrations and yield were collected for each tree. Leaf nutrient concentrations from the highest yielding trees (50th yield percentile) were used to calculate a mean and CV for each nutrient. Results were compared with data from New Mexico, Georgia, and Sonora, Mexico. Relatively large differences were noted in mean K, Ca, B, Cu, Fe, Mn, and Zn levels. Nutrient interpretation ranges were calculated based on Arizona population statistics using the balance index method.
Zinc deficiency is common in pecan (Carya illinoinensis) grown in alkaline, calcareous soils. Zinc (Zn)-deficient pecan leaves exhibit interveinal chlorosis, decreased leaf thickness, and reduced photosynthetic capacity. Low photosynthesis (Pn) contributes to restricted vegetative growth, flowering, and fruiting of Zn-deficient pecan trees. Our objectives were to measure effects of soil-applied ethylenediaminetetraacetic acid (EDTA)-chelated Zn fertilizer on gas exchange of immature ‘Wichita’ pecan and characterize the relationship between leaf Zn concentration and Pn. The study orchard had alkaline and calcareous soils and was planted in Spring 2011. Zinc was applied throughout each growing season as Zn EDTA through microsprinklers at rates of 0 (Control), 2.2, or 4.4 kg·ha−1 Zn. Leaf gas exchange and SPAD were measured on one occasion in the 2012 growing season, four in 2013, and five in 2014. Soil Zn-EDTA applications significantly increased the leaf tissue Zn concentration throughout the study. On all measurement occasions, net Pn was significantly increased by soil-applied Zn EDTA compared with the control, but Pn was not different between the two soil-applied Zn-EDTA treatments. Leaf Pn in midseason did not increase at leaf tissue Zn concentrations above 14–22 mg·kg−1. Leaf SPAD consistently followed a similar pattern to Pn. Soil Zn-EDTA application increased leaf stomatal conductance (gS) compared with the Control early through midseason but not after August. Intercellular CO2 concentration was significantly lower for Zn-EDTA-treated trees than the Control even on dates when there was no significant difference in gs, which suggests that soil application of Zn-EDTA alleviated nonstomatal limitations to Pn caused by Zn deficiency.
A field study was conducted to evaluate tolerance of pecan rootstocks to soil salinity and sodicity. Seven cultivars—Elliott, Giles, Ideal, Peruque, Riverside, ‘Shoshoni, and VC1-68—were selected from a range of geographic regions of origin. The soil of the experimental plot was a poorly drained, saline–sodic Pima silty clay variant. The irrigation water was a moderately saline mix of Gila River and local groundwater with an electrical conductivity of 2.8 dS⋅m–1, containing primarily ions of Na and Cl. Eighty seeds of each cultivar were planted in a greenhouse in late Feb. 2016; 48 seedlings of each cultivar were transplanted into field plots in Feb. 2017. Half the trees received a soil-based application of Zn–ethylenediaminetetraacetic acid (EDTA) at planting. The trees were observed and rated for both vigor and resistance to salt injury on seven separate occasions. Trunk diameter was measured each dormant season. Leaf samples were collected on 9 Oct. 2019 and 6 Oct. 2020, and were analyzed for nutrient content. Zn-EDTA was not found to have a significant effect on growth, vigor, or resistance to salt injury. ‘Elliott’ seedlings exhibited greater tolerance for the alkaline, saline–sodic soil conditions than other cultivars. ‘Giles’ and ‘Peruque’ were most severely affected. Resistance to salt injury (ranging from marginal leaf burn to necrosis of entire leaf), vigor, and growth correlated more strongly with foliar concentrations of Na than Cl or K during 2019. Vigor and growth were not significantly correlated with foliar Na, Cl, or K concentrations in 2020. The foliar K:Na ratio had a nearly equal correlation with resistance to salt injury and a greater correlation with growth than that of Na alone in 2019. However, although the correlation of the K:Na ratio with vigor was stronger than that of Cl or K, Na had the strongest correlation with vigor in 2019. In 2020, the only significant correlation of growth and vigor was with the K:Na ratio. The strongest correlation with resistance to salt injury in 2020 was with foliar Na concentration.
Analysis of composite pecan leaf samples typically used to determine need for nutrient applications does not account for variability among trees in the sampled area. To account for this unmeasured variability, pecan orchard block nutrient standards are greater than actual single tree nutrient requirements. In 2018 and 2019, we measured variability in a pecan orchard block by evaluating nutrient status of all trees in a study area consisting of two cultivars (Wichita and Western) grafted on open-pollinated ‘Ideal’ seedlings. Foliar zinc (Zn) coefficient of variation (cv) ranged from 0.186 to 0.255 within individual cultivars and years but was as high as 0.30 when combining cultivars within a year. The ‘Western’ cultivar had higher foliar Zn concentrations than ‘Wichita’, but Zn concentrations were not consistently associated with other leaf nutrient levels, soil Zn status, or other soil properties. Using observed foliar Zn variability, we determined that it is necessary to sample 35 trees for a composite sample to achieve a relative margin of error of 10% and 95% confidence level in a pecan orchard block with more than 1000 trees. We developed field scale foliar Zn recommendations based on individual tree research that indicates a minimum acceptable leaf Zn concentration of ≈15 mg·kg–1 is needed to maintain optimal photosynthetic function in Zn chelate fertigated pecan trees. Assuming a Zn cv of 0.30 and a composite sample comprised of leaves from 35 trees, the minimum acceptable orchard block Zn level to ensure that less than 5% of trees had suboptimal levels of Zn was 27.6 mg·kg–1. An orchard block Zn level below 23.4 mg·kg–1 indicates that more than 5% of trees in the block had suboptimal foliar Zn concentrations.