Abstract
Site selection is key to successful establishment of fruit and nut trees. The upland soils on which pecan [Carya illinoinensis (Wangenh.) K. Koch] trees are commonly planted in the southeastern United States consist of sites that have recently been in row crop cultivation or pine or hardwood timber. Anecdotal observation suggests that orchards planted to land converted from cultivated row crop fields tends to result in better tree growth and survival than those on land recently converted from timber plantations or wooded areas. The objective of this experiment was to compare growth of first- through third-leaf pecan trees planted on sites with varying land-use history [row crop cultivation or pine (Pinus spp.) tree production up to the year before planting] and to determine the effects of supplemental addition of phosphorus (P), potassium (K), and zinc (Zn) at planting on the two sites. These results suggest that the soil conditions of sites recently in pine timber production limit the growth and development of pecan trees planted to those sites. These limitations result from soil acidity and an exhaustion of soil nutrients and loss of organic matter on pine sites, making the uptake of nitrogen (N), P, K, and calcium (Ca) challenging during the establishment phase unless soils are improved before planting.
Site selection is key to successful establishment of fruit and nut trees. Pecan trees are relatively nonprecocious, requiring 5–6 years before commercial harvest is possible and 8–10 years before trees reach mature production. The loamy bottomland ridges to which pecans are adapted in their native range along the Mississippi River Valley and its tributaries along with streams in Oklahoma, Texas, and Mexico, typically consist of deep, well-drained soils high in organic matter and natural fertility (Sparks, 2005). Upland soils of the southeastern U.S. Coastal Plain, by contrast, are highly acidic, have relatively low cation exchange capacity, and are naturally low in various macronutrients and micronutrients required for pecan growth and production (Skinner et al., 1938). Southeastern soils do, however, respond well to management in the form of liming, fertilization, and the addition of organic matter (Blackmon and Ruprecht, 1934; Fowler et al., 1933).
The upland soils on which pecan trees are commonly planted in Georgia consist of sites that have recently been in row crop cultivation or pine or hardwood timber. A downturn in timber prices coinciding with an increase in pecan prices from 2009 to 2018 led to the clearing of loblolly pine (Pinus taeda L.) and slash pine (Pinus elliotti Engelm) plantations for the planting of pecan. Anecdotal observation suggests that orchards planted to land converted from cultivated row crop fields tend to result in better tree growth and survival than those on land recently converted from timber plantations or wooded areas (L. Wells, personal observation). Pecan trees grown immediately following the clearing of timber often exhibit a high degree of marginal leaf scorching, early defoliation, stunted growth, and tree loss. These issues are especially notable on sites with poorly drained soil. Additionally, wooded sites where hardwood species such as hickory (Carya spp.) or walnut (Juglans spp.) are present may also harbor population of pecan root-knot nematode (Meloidogyne partityla Kleynhans) and other plant pathogenic nematodes, which may infect pecan (Starr et al., 1996). Root-feeding pests such as the Prionus root borer may also be present on such sites where alternative hosts have been recently removed.
The objective of this experiment was to compare growth of first- through third-leaf pecan trees planted on sites with varying land-use history (row crop cultivation or pine tree production up to the year before planting) and to determine the effects of supplemental addition of P, K, and Zn at planting on the two sites.
Materials and Methods
The experiment was conducted from 2019 to 2021 in a commercial pecan orchard located in Dooly County, GA, on Tifton loamy sand (fine-loamy, silicieous, thermic Plinthic Paleudult). The orchard was located at lat. 32°09′ N and long. −83°68′ W. All trees were irrigated with microsprinklers at a rate of 56.8 L·h−1. Microsprinklers were placed ≈0.3 m from the tree trunk. Trees were irrigated 4–6 h every other day in the absence of rain from April to September. Trunks were protected with corrugated tree guards (A.M. Leonard, Piqua, OH). The orchard was managed under commercial conditions according to University of Georgia Cooperative Extension recommendations (Wells, 2007). A 3.7-m-wide, vegetation-free strip was maintained with the herbicide glyphosate along the tree row in all plots. Row middles consisted of bermudagrass (Cynodon dactylon L.) sod.
Bare-root ‘Oconee’ pecan trees grafted to ‘Elliott’ rootstock were planted from nursery stock in Jan. 2019 at a spacing of 10.6 m × 10.6 m. Treatments were arranged in a split-plot design with site history as the main effect and fertilizer supplement as the split effect. Main plot effects were arranged within a single orchard row, with 16 trees planted in the area under loblolly pine production for the previous 35 years and 16 trees planted in the area under row-crop cultivation [cotton (Gossypium hirsutum L.)/peanut (Arachis hypogea L.)/corn (Zea mays L.) rotation] for the previous 35 years. At planting, one of the following fertilizer treatments was mixed into the backfill soil of the planting hole of pecan trees planted within each of the two sites: 1) 0.91 kg monoammonium phosphate (11N–52P–0K), 2) 0.91 kg muriate of potash (0N–0P–52K), 3) 0.91 kg zinc sulfate (35.5% Zn), and 4) nontreated control. Split-plot effects were arranged so that each fertilizer treatment was replicated four times in each planting site. Measurements were taken from each tree within each plot.
After planting, all nutrients were applied uniformly throughout the fertigation system in April, June, and August of each year of the study. Nitrogen was applied at 8.7 kg·ha−1 in April and August and at 11.6 kg·ha−1 in June. Phosphorus was applied at 0.13 kg·ha−1 with each fertigation, and K was applied at 0.23 kg·ha−1 with each fertigation. Boron (B) and copper (Cu) were each applied at 0.04 kg·ha−1 with each fertigation. Iron (Fe), manganese (Mn), and Zn were each applied at 0.12 kg·ha−1 with each fertigation.
Stem diameter at 72.6 cm above the soil surface was measured on 5 Mar. 2019, 17 Oct. 2019, 28 Sept. 2020, and 20 Oct. 2021. Foliage was sampled in late July each year by collecting one leaflet pair from 20 compound leaves per tree. All leaflet samples were taken from the middle leaf of sun-exposed terminals. Leaflet samples were washed in a dilute phosphate-free detergent solution (0.1% detergent) followed by rinsing with deionized water. Leaves were then dried to a constant weight at 65 °C and ground in a Wiley mill (Wiley, Philadelphia, PA) to pass a 1-mm screen. Leaves for nutrient analysis were ground with a mortar and pestle. Samples were analyzed for N by combustion using a Leco FP528 protein/N determinator (Leco Corp., St. Joseph, MI). Samples were analyzed for K, P, and Zn by an inductively coupled plasma spectrophotometer coupled to a Digiblock 3000 (SCP Science, Baie D’Urfé, Quebec, Canada).
Soil samples at 0- to 20.3-cm depth with the surface 2.54 cm removed were taken from both the pine and row-crop planting sites on 5 Mar. 2019 and on 23 Oct. 2021. Four composite samples consisting of four cores each at each depth were taken per site. Soil was dried and analyzed for pH, P, K, Ca, Mg, Mn, Fe, and S. Soil pH was determined in a 0.01 M calcium chloride (CaCl2) solution using a LabFit AS-3000 (Labfit, Perth, Australia) dual pH analyzer. Soil P, K, Ca, Mg, Mn, Fe, and S were determined by inductively coupled plasma spectrophotometry.
Leaf area, leaf length, and leaf width were measured on 2 Aug. 2019, 28 Sept. 2020, and 27 Sept. 2021 using a portable leaf area meter (LI-3000C; LI-COR Technologies, Lincoln, NE). Five leaves per tree were measured at each sampling date. All leaves measured were fully expanded and selected from one pair of middle leaflets of compound leaves.
Statistical analyses of data were performed with SAS (version 9.4; SAS Institute, Cary, NC). Two-way analysis of variance was used to compare site and fertilizer treatment effects. Means were separated using Tukey’s least significant difference test (P < 0.05).
Results and Discussion
Planting site had a significant effect on leaf N, P, K, Ca, and Zn during the study (Table 1). Pecan leaf N was significantly (P < 0.05) higher on the cultivated site than on the pine site in 2019, 2020, and 2021 (Fig. 1A). Pecan leaf P was significantly (P < 0.05) higher on the cultivated site than on the pine site in 2019, 2020, and 2021 (Fig. 1B). There was a significant site × fertilizer treatment interaction for leaf P in 2021, in which leaf P concentration was higher in the at-planting Zn fertilizer treatment on the cultivated site than on the pine site (Table 1).
Pecan leaf concentrations of (A) nitrogen (N), (B) phosphorus (P), (C) potassium (K), (D) calcium (Ca), and (E) zinc (Zn) during 2019, 2020, and 2021 on sites previously under row-crop cultivation and on sites previously under pine timber.
Citation: HortScience 57, 7; 10.21273/HORTSCI16627-22
Mean leaf N, leaf P, leaf K, and leaf Zn concentrations of ‘Oconee’ pecan trees planted in row-crop fields and in recently cleared pine land receiving P, K, and Zn and control treatments during the first (2019), second (2020), and third (2021) growing seasons following transplant.
Pecan leaf K was significantly (P < 0.05) higher on cultivated site than on the pine site in 2020 and 2021 (Fig. 1C). Pecan leaf Ca was significantly higher (P < 0.05) on the cultivated site than on the pine site in 2020 and 2021 (Fig. 1D). Pecan leaf Zn was significantly higher on the former pine site than on the cultivated site in 2019, 2020, and 2021 (Fig. 1E). There was a significant fertilizer treatment effect on pecan leaf Zn in 2021, in which at-planting P and K increased leaf Zn over that of the control and the at-planting Zn application (Fig. 2). This is puzzling because P can be antagonistic to Zn uptake by the tree. There was also a significant site × fertilizer interaction for leaf Zn in 2021. On the pine site, leaf Zn was significantly (P < 0.05) higher in the at-planting Zn and K treatments, whereas on the cultivated site, leaf Zn was higher (P < 0.05) in the at-planting P treatment (Table 1).
Effect of at-planting fertilizer treatments of phosphorus, potassium, zinc sulfate, and untreated control on pecan leaf Zinc (Zn) concentration in 2021.
Citation: HortScience 57, 7; 10.21273/HORTSCI16627-22
During each year of study, pecan leaf concentrations of the macronutrients N and P were higher on the cultivated site than on the pine site (Fig. 1). Leaf Ca was higher on the cultivated site than on the pine site in 2020 and 2021 (Fig. 1). Leaf Zn concentration was consistently higher on the pine site than on the cultivated site (Fig. 1). The contrast between leaf concentrations of macronutrients and that of Zn is likely explained by increased macronutrient availability on the cultivated site as a result of higher soil pH. The tendency for a lower soil pH on the pine site would favor enhanced tree uptake of Zn and explains higher leaf Zn concentrations on the pine site. Higher leaf P on the cultivated site than on the pine site can also be partially attributed to higher soil P concentration on the cultivated site throughout the study (Table 2). Coastal Plain soils are generally low in P as a result of inherently low mineral P in the soil parent material, the soil’s advanced stage of weathering, and the tendency of orthophosphate to adsorb on hydrous metal oxides or become occluded in the secondary mineral structure (Scott and Bliss, 2012). As a result, many pine stands throughout the southeastern United States are chronically low in P (Albaugh et al., 1998), whereas land cultivated for row crops generally receives generous annual applications of P to enhance crop production.
Synopsis of soil elemental analysis and pH taken in 2019 and 2021 for orchard sites previously cultivated or in pine timber.
With the exception of soil P, soil macronutrient concentrations were similar for both sites in 2019 and in 2021 (Table 2), suggesting that soil pH was the major factor in availability of soil macronutrients. Although soil pH was similar in 2019 for both sites, pH was higher on the cultivated site than on the pine site during the final year of study. This rapid change in soil chemistry is also evident in the effect of site on pecan leaf K concentration. In the year of planting (2019), there was no difference in leaf K between sites. Leaf K was significantly higher on the cultivated site after the initial year of planting (Fig. 1C). Soil availability of many nutrients, including N, K, and Ca, following timber harvest is often initially high because the soil disturbance by equipment and burning provides suitable conditions for rapid decomposition and the release of nutrients from the accumulated forest floor and slash material (Vitousek and Matson, 1985). This pulse of nutrients is relatively short lived, and the effect of soil disturbance and organic matter removal disrupts the soil air/water balance and depletes soil fertility (Burger and Kelting, 1998).
The effect of these disturbances by the timber harvest on soil nutrient availability is also reflected in our observations of pecan tree growth. Planting site had a significant effect (P < 0.05) on mean trunk diameter, leaf area, and maximum leaf width each year of the experiment (Table 3). Mean leaf width and mean leaf length were influenced by planting site in 2 out of the 3 years of study (Table 3). Mean trunk diameter growth was 114%, 42%, and 19% greater on the cultivated site than on the pine site in 2019, 2020, and 2021, respectively (Fig. 3A). Trees grown on the cultivated site had a 53%, 42%, and 21% increase in leaf area over those on the pine site in 2019, 2020, and 2021, respectively (Fig. 3B). Maximum leaf width was 25%, 20%, and 7.7% greater on trees growing on the cultivated site than on the pine site in 2019, 2020, and 2021, respectively (Fig. 3C). Mean leaf width and leaf length were greater (P < 0.05) for trees planted in the cultivated site vs. the pine site in 2019 and 2020 (Fig. 3D and E). Leaf area for 2021 was greater in the P and K treatments than in the Zn and control treatments (Fig. 4A). Mean and maximum leaf widths were higher in the P treatment than for the control in 2021 (Fig. 4B and C).
Effect of planting site on (A) trunk diameter growth, (B) leaf area, (C) mean leaf width, (D) maximum leaf width, and (E) leaf length in 2019, 2020, and 2021.
Citation: HortScience 57, 7; 10.21273/HORTSCI16627-22
Effect of at-planting application of phosphorus (P), potassium (K), zinc (Zn), and untreated control on (A) pecan leaf area, (B) mean leaf width, and (C) maximum leaf width in 2021.
Citation: HortScience 57, 7; 10.21273/HORTSCI16627-22
Mean trunk diameter growth, mean leaf area, mean leaf width, maximum leaf width, and mean leaf length for ‘Oconee’ pecan trees as affected by planting site and fertilizer treatment in 2019, 2020, and 2021.
There was a significant site × fertilizer interaction for trunk diameter growth, leaf area, mean leaf width, and maximum leaf width in 2020 (Table 3). Trunk diameter growth in the P and Zn at-planting treatments was greater than the control on the cultivated site in 2020 (Table 3). Leaf area, mean leaf width, and maximum leaf width of trees on the cultivated site were greater where K and Zn were applied at-planting in 2020 (Table 3).
Tree and leaf growth differences between the two sites are likely a result of enhanced uptake of macronutrients on the cultivated site compared with the pine site, partially as a result of the lower pH on the pine site. This may also explain why at-planting fertilizer applications of P and K both led to growth responses on the cultivated site. Although leaf Zn was higher on the pine site, likely as a result of lower pH, this did not result in overall increases in tree or leaf growth response. Growth responses to at-planting P, K, and Zn treatments on the cultivated site were likely more related to overall improved availability of P and K where soil pH was higher by the end of the study.
Our results suggest that the soil conditions of sites recently in timber production limit the growth and development of pecan trees planted to those sites. These limitations result from soil acidity, an exhaustion of soil nutrients, and loss of organic matter, which has been previously documented following timber harvest and clearing activities and has also been shown to limit early growth of pine trees (Burger and Kelting, 1998). Producers planting pecan trees on such sites should focus attention on raising soil pH, building soil P levels, and enhancing soil organic matter before planting of pecan trees.
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