Leaf analysis.

Average leaf concentrations of all nutrients except S were within or above the recommended leaf sufficiency range during 2005 and 2008. Average leaf N concentration was similar in both years of the study (Table 3). While no orchards were deficient in leaf N during the 2005 survey, 12% of mature orchards in the 2008 survey were deficient. The upper and lower ends of the sample range were lower in 2008 than in 2005 as well (Table 3). This may have been due to a sharp increase in N fertilizer prices occurring between 2005 and 2008, and a desire to save input costs by reducing N application in an anticipated OFF crop in 2008. Additionally, the record-tying 2007 Georgia pecan crop of 150 million pounds (U.S. Department of Agriculture, 2008) may also have somewhat depleted tree N storage pools.

Table 3.

Synopsis of pecan leaf elemental analysis for 2005 and 2008 pecan orchard surveys including mean leaf elemental concentration, desired sufficiency range, percentage of orchards with leaf elemental concentrations below the desired sufficiency range, percentage of orchards with leaf elemental concentrations above the desired sufficiency range, and leaf elemental concentration range of sampled orchards.

View Table

Leaf P concentrations from all orchards were within the recommended leaf sufficiency range (0.12%–0.3%) during 2005. The 2008 survey showed leaf P from 12% of sampled orchards to be lower than the recommended sufficiency range (Table 3). Pond et al. (2006) found leaf P from Georgia orchards to be higher than those from Arizona or New Mexico. The authors suggested that because all levels were near the bottom or below recommended ranges (Jones et al., 1991; Robinson et al., 1997), the published ranges for pecans may be too high. Sparks (1989) suggested that P deficiency can be expected to occur when leaf P falls below 0.11%, and that maximum nut growth occurred at some point above 0.14%. Leaves are believed to be the principal source from which late season P demand of developing fruit are met. The fruit's great demand for P late in the growing season can induce leaf necrosis and premature defoliation (Sparks, 1988); however, this was not observed in either year of the survey.

Nutrient deficiency symptoms occur when leaf K is below 0.70% (Alben, 1947; Sparks, 1978). The minimum leaf K recommendation may vary from one geographic region to another. However, Plank (1988) suggests that although good yields can be produced with a minimum leaf K of 0.75%, leaf K concentration for optimum production and tree vigor should be maintained between 1.25% and 2.50%. Wells and Wood (2007) suggested that the leaf N:K ratio should be maintained at or near 2:1 for optimum yields in the ON crop year. With an average leaf N of 2.77% and 2.65% for 2005 and 2008, respectively, leaf K would need to be maintained between 1.3% and 1.4%. Average leaf K concentration was 1.26% and 1.39% for 2005 and 2008, respectively. In 2005, 45% of sampled orchards were below the recommended minimum leaf K of 1.25%. Only 35% of sampled orchards were below 1.25% in 2008. The upper end of the leaf K sample range was higher in the 2008 survey than in the 2005 survey (Table 3).

Leaf Ca was high in about half the leaf results from sampled orchards in 2005 and in almost 90% of orchards in 2008. One orchard was low in leaf Ca during 2008 (Table 3). Leaf Mg was low in 7% and 6% of orchards sampled in 2005 and 2008, respectively. Leaf Mg concentration was high in 12% of sampled orchards in 2008 (Table 3). Leaf Ca is commonly found to be relatively high as a result of the liming of acidic soils on which orchards are grown in most of the southeastern U.S. coastal plain. Dolomitic lime, containing Mg, is often used as the source of liming material in most orchards. As a result, Mg is rarely deficient in southeastern U.S. orchard trees.

While leaf S concentration was marginal for most orchards during 2005, leaf S was deficient in only one pecan orchard that year; however, in 2008, leaf S was deficient in almost 90% of the orchards surveyed. Average leaf S was below the recommended leaf sufficiency range for S in both years of the study (Table 3). Southeastern U.S. coastal plain soils on which orchards are established are inherently low in S. In addition, sulfur leaches readily through the soil profile. As a result, S availability does not appear adequate for many southeastern U.S. pecan orchards. Soil S may be immobilized by a variety of factors, including soil organic matter, temperature, moisture, and soil pH. Where immobilization is a problem, foliar S applications may benefit commercial production.

Mean leaf Fe concentration was adequate in both years of the survey. All sampled orchards were within the recommended leaf sufficiency range in 2005; however, 18% of sampled orchards had low leaf Fe concentrations in 2008 (Table 3), although no visible symptoms of Fe deficiency were observed. In Georgia, symptoms of iron chlorosis appear occasionally in commercial orchards, most often in the early spring. However, as soils warm and Fe becomes more readily available to tree roots, the symptoms are often corrected. Our samples were taken in midsummer, therefore, cold soils should not have led to low leaf Fe. It is unclear why leaf Fe was low in some orchards. The most likely explanation is the amount of excess Zn in orchard soils. Metal cations such as Cu, Mn, Zn, and molybdenum (Mo) can interact with Fe to induce such deficiencies in certain situations (Havlin, et al., 2005).

Rosette, or Zn deficiency, was one of the first major nutritional problems associated with commercial pecan production. As a result, Zn has been and continues to be commonly applied to most commercial pecan orchards as a foliar spray, via ground application, or both. Mean leaf Zn concentration was above the recommended sufficiency range in 2005 and only slightly within the upper limit of this range in 2008. Among orchards sampled, 7% and 2% were below the recommended leaf concentration for Zn in 2005 and 2008, respectively, while 34% and 41% of orchards were high in leaf Zn during the same years (Table 3). Most commercial pecan orchards in Georgia have adequate or high leaf Zn concentrations as a result of multiple years of annual Zn application.

Foliar B application has been recently shown to increase pecan kernel quality and fruit retention under certain conditions (Wells et al., 2008). Mean leaf B was well within the recommended sufficiency range in 2005 and 2008 (Table 3). Although no orchards were below the recommended range in 2005, 12% of sampled orchards were below the minimum recommended leaf B concentration in 2008. About 21% and 6% of orchards had excessive levels of leaf B in 2005 and 2008, respectively, when compared with the recommended range (Table 3). The slightly lower B concentrations observed in 2008 are likely a result of fewer B applications in 2008. Before 2007, most commercial pecan producers were applying a foliar boron application with each fungicide spray. Since that time, most producers only apply three boron applications per season. Increases in percentage of kernel and fruit retention have been observed with properly timed foliar boron sprays at leaf B concentrations below the current recommended minimum concentration (Wells et al., 2008).

Mean leaf Cu concentration was within the recommended sufficiency range in both study years (Table 3). All orchards were within the recommended range for leaf Cu in 2005. In 2008, leaf Cu was low in 35% of sampled orchards, while no orchards had leaf sufficiency ranges above the recommended maximum of 30 ppm (Table 3). High soil Zn concentrations can suppress Cu absorption by roots (Havlin et al., 2005). Because many orchard soils have high Zn levels, the potential for Cu deficiency does exist in commercial pecan orchards, especially during the early portion of the growing season.

Mean leaf Mn concentrations were within the recommended leaf sufficiency range in both years of the study (Table 3). While no orchards were below the recommended leaf sufficiency range in either survey year, 21% and 24% of orchards had leaf Mn concentrations higher than the recommended range in 2005 and 2008, respectively (Table 3). Manganese deficiency has been reported from Texas (Smith et al., 2001); however, it is rare on the acidic soils of the southeastern U.S. coastal plain.

Soil analysis.

A total of 10 soil types were included in the orchards sampled in both the 2005 and 2008 surveys (Tables 1 and 2). These include Clarendon (fine-loamy, siliceous, thermic Plinthaquic Paleudult), Dothan (fine-loamy, siliceous, thermic Plinthic Paleudult), Faceville (fine, kaolinitic, thermic Typic Kandiudult), Fuquay (loamy, sliliceous, thermic Arenic Plinthic Paleudult), Greenville (clayey, kaolinitic, thermic Rhodic Paleudult), Lucy (loamy, siliceous, thermic Arenic Paleudult), Norfolk (fine-loamy, siliceous, thermic Typic Paleudult), Orangeburg (fine-loamy, siliceous, thermic Typic Paleudult), Red Bay (fine-loamy, siliceous, thermic Rhodic Paleudult), and Tifton (fine-loamy, silicieous, thermic Plinthic Paleudult). Skinner et al. (1938) estimated that 95% of the pecan acreage on the upland soil series was located on Ruston, Norfolk, Tifton, Orangeburg, Greenville, Red Bay, and Cecil soils. About 66% and 60% of the soils surveyed in 2005 and 2008, respectively, were among these soil groups (Tables 1 and 2).

Soil pH averaged 5.96 and 6.07 in 2005 and 2008, respectively (Table 4). In 2005, pH of orchard soils ranged from 5.3 to 7.0, with ≈41% of orchard soils with a pH of less than 6.0. The sample range of soil pH in 2008 was from 5.2 to 6.8, with a pH of less than 6.0 in 55% of sampled orchards. Growth of pecan is sensitive to soil pH. Low soil pH suppresses root growth (White et al., 1982). Sparks (1989) suggested that pecan orchard soil pH should be maintained at 6.0 to 6.3.

Table 4.

Synopsis of soil elemental analysis and pH for 2005 and 2008 pecan orchard surveys including mean elemental soil levels, recommended soil sufficiency range, and range of elemental soil levels for sampled orchards.

View Table

Orchard soils averaged 98 and 95 lb/acre P for 2005 and 2008, respectively (Table 4). Uptake of P may be poor in orchard soils even though soil P is adequate or high in most orchards. This results from the slow mobility of applied P on the undisturbed soil surface. For this reason, it can be difficult to raise leaf P concentration when levels fall below the minimum required. Therefore, soil P should be monitored closely. Foliar fertilization with P is seldom recommended as a substitute for soil fertilization practices, due in large part to the inconsistent response of crops to foliar P (Barker and Pilbeam, 2007).

Surveyed orchards averaged 153 lb/acre K in 2005 and 148 lb/acre K in 2008. Soils ranged from 94 to 361 lb/acre K in 2005 and from 53 to 298 lb/acre in 2008 (Table 4). Orchards with soil levels below 100 lb/acre K were found on soils with a higher sand content. K is easily lost to leaching when soil pH falls below 6.0 (Sparks, 1989). K levels were high in some clay soils and still relatively low in leaf samples, indicating a lack of available K in those soils. K tends to hold tightly to clay particles, making soil K unavailable to plants.

Orchard soils averaged 988 lb/acre Ca and 2404 lb/acre Ca in 2005 and 2008, respectively (Table 4). Many pecan orchards in the southeastern U.S. coastal plain have high soil Ca levels as a result of liming. Mean soil level averaged 184 lb/acre Mg in 2005 and 307 lb/acre Mg in 2008 (Table 4). As mentioned earlier, dolomitic lime is commonly used to raise soil pH of most orchard soils. As a result, most Georgia pecan orchard soils are rarely deficient in Mg. Sparks (1989) suggests that replacing dolomitic lime with calcitic lime when leaf Mg reaches 0.45% or greater can help to minimize the effect of Mg on K uptake.

In 2005, orchard samples had a mean of 26.6 lb/acre S, with levels ranging from 4 to 41 lb/acre S (Table 4). Soil S was measured as percentage of total S in the 2008 survey. In 2008, the percentage of total S averaged 0.007%, and ranged from 0.001% to 0.01% (Table 4). As mentioned earlier, southeastern U.S. coastal plain soils contain low natural levels of S, which leaches readily from the sandy and sandy loam soils found throughout the region. In the humid conditions of the southeastern United States, ≈90% of total S is found in organic matter (Havlin et al., 2005). Therefore, much of the S that does not leach from the soil is bound in organic matter and thus is likely to be unavailable to the trees. Soil C:S ratio may also influence the availability of S to plants. The mean C:S ratio of Georgia pecan orchard soils was 504:1 (Table 5), which may further explain the low leaf S concentration observed here in the face of low soil S levels. Sulfur is generally immobilized in soils with a C:S ratio >400:1 (Havlin et al., 2005), even at adequate soil S levels.

Table 5.

Means and sample ranges for soil carbon (C), soil sulfur (S), soil nitrate-nitrogen (NO₃-N), soil organic matter (SOM), soil C:N ratio, and soil C:S ratio for the 2008 pecan orchard survey.

View Table

Soil Fe was not measured in 2008; however, 2005 orchard samples revealed a mean of 22.6 lb/acre Fe, with samples ranging from 8 to 76 lb/acre (Table 4). Soil pH in orchards with low leaf Fe was above 6.0, therefore, low leaf Fe could have been related to high soil pH.

Soils averaged 25 lb/acre Zn and 67 lb/acre Zn in 2005 and 2008, respectively. The sample ranges were much higher in 2008 than in 2005 (Table 4). Because most pecan producers are aware that Zn deficiency is the most well-known micronutrient deficiency of pecan, it is not surprising that soil Zn levels would be relatively high in many commercial orchards. This likely results from multiple Zn applications over many years. Although liming of an acid soil can reduce Zn availability (Hunter, 1965), tree Zn needs can usually be maintained if foliar Zn is also applied (Brooks, 1964; Hunter, 1965). Most forms of Zn become relatively immobile soon after soil application. As a result, soil applied Zn is often slow to move into the tree, sometimes taking as much as three years before an increase in leaf Zn is observed (Hunter, 1965).

Soils averaged 0.99 lb/acre B in 2005 and 1.22 lb/acre B in 2008 (Table 4). Coarse-textured, well-drained soils are generally low in B. In addition, B is highly mobile in the soils of the southeastern U.S. coastal plain. Less than 5% of total soil B is available to plants (Havlin et al., 2005). Orchard soils in 2005 averaged 1.1 lb/acre Cu, and ranged from 0.20 to 7.2 lb/acre Cu (Table 4). Soil Cu was not measured in 2008. The Cu requirements of pecan have been poorly studied; however, requirements are believed to be relatively low, with a minimum leaf Cu sufficiency concentration of only 6 ppm. Still, as mentioned previously, in 2008, 35% of sampled orchards were marginally low in leaf Cu (Table 3). The potential for Cu deficiency is highest in coarse-textured soils with high levels of Zn, Fe, and P (Havlin et al., 2005). Average Soil Mn levels were identical in 2005 and 2008 (Table 4).

Organic matter levels of pecan orchard soils have received little attention in the literature. This is surprising, in light of the valuable role played by SOM and the attention it has received regarding its contributions to agricultural productivity. Sparks (2005) suggests that the humus-topsoil-root association is key to the overall nutrition of pecan in nature via nutrient recycling, mainly from decaying leaves and shucks. SOM of Georgia pecan orchards from the 2008 survey averaged 3.63% (Table 5), and ranged from 1.74% on coarse-textured sandy soils to over 5% on sandy loam soils. Most Georgia soils vary in organic matter content from 0.5% to 2.0% (Kissell and Sonon, 2008). The warm, humid climate typical of the region is conducive to microbial activity during much of the year, preventing extensive accumulation of SOM. Consequently, SOM rarely exceeds 3% (Giddens, 1957). The relatively high level of shallow subsurface SOM from pecan orchards is comparable to that of natural hardwood forests occurring within the Georgia coastal plain (Giddens, 1957; M.L.Wells, unpublished data), and higher than that observed on row crop fields (Giddens, 1957). The high level of SOM found in pecan orchard soils is a result of the annual addition of leaves, shucks, and woody debris to the surface of orchard soils and the decomposition of this material. It is likely that SOM would be lower in the vegetation-free herbicide strips than was found between tree rows.

Soil NO₃ levels averaged 14.05% (Table 5), and ranged from 3.5% on sandy soils to 44% on loamy soils. Orchard soil C averaged 2.05%, ranging from 1.12% to 3.96% (Table 5). Skinner (1924) reported an average soil C percentage of 1.5% for orchards in Alabama. Most southeastern U.S. pecans at the time of Skinner's study were grown under clean cultivation and were usually disked at least once per year, which may explain the reduced soil C levels at shallow soil depths compared with the current study.

Soil C:N ratio affects N availability, total organic matter, and the rate of organic decay. For this reason, it is considered a highly valuable factor in developing sound soil management schemes (Brady, 1974). Orchard floor covers using organic materials with low C:N ratios of 15:1 to 20:1, such as legumes, will release N more rapidly, whereas materials with high C:N ratios of 40:1 to 80:1, such as grasses, may actually immobilize N during their decomposition before it would become available to tree roots. Mean C:N ratio of pecan orchard soils was ≈13:1, and ranged from 12:1 to 16:1 (Table 5). Lewis and Hunter (1940) reported a more narrow C:N ratio of 8:1 on southeastern U.S. coastal plain soils on which cotton (Gossypium hirsutum) and corn (Zea mays) were grown.

Crimson clover (Triflolium in-carnatum), ‘Durana’ white clover (Trifolium repens), or ball clover (Trifolium nigrescens) were used as orchard floor covers on nearly 15% of surveyed orchards in 2005, and on 47% of orchards in 2008 (Tables 1 and 2). SOM and soil NO₃ were higher in orchards using clover and sod as a cool season orchard floor covering than those using sod only (Fig. 1). Skinner et al. (1938) reported an increase in SOM where kudzu (Pueraria montana), velvetbeans (Mucuna pruriens), austrian winter peas (Pisum sativum), or rye (Secale cereale) were used as winter cover crops in pecan orchards; however, soil N was maintained at about the same level as the control. Smith, et al. (1996) reported occasionally higher soil NO₃ levels with a legume/bermudagrass (Cynodon dactylon) cover crop than for bermudagrass and March-applied N for orchards in Oklahoma. Ware and Johnson (1958) also suggested that orchard soils on which winter legumes were cut and left on the soil surface had nitrate levels that increased later in the season and were higher than control plots. No differences in soil C or soil C:N ratio were observed between orchards with clover as a floor covering and those with only sod in the current study. Skinner et al. (1938) suggested that use of a soluble commercial N fertilizer in conjunction with a green manure crop maintained a lower soil C:N ratio, resulting in a higher level of soil fertility.

Soil organic matter (SOM) and soil nitrate-nitrogen (N) levels of pecan orchard soils utilizing clover (crimson clover, ball clover, or ‘Durana’ white clover) and sod (bermudagrass) or sod only as an orchard floor covering in 2008. Columns with different letters are significant according to analysis of variance at P < 0.05.

Citation: HortTechnology hortte 19, 2; 10.21273/HORTSCI.19.2.432

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Soil organic matter (SOM) and soil nitrate-nitrogen (N) levels of pecan orchard soils utilizing clover (crimson clover, ball clover, or ‘Durana’ white clover) and sod (bermudagrass) or sod only as an orchard floor covering in 2008. Columns with different letters are significant according to analysis of variance at P < 0.05.

Citation: HortTechnology hortte 19, 2; 10.21273/HORTSCI.19.2.432

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Soil organic matter (SOM) and soil nitrate-nitrogen (N) levels of pecan orchard soils utilizing clover (crimson clover, ball clover, or ‘Durana’ white clover) and sod (bermudagrass) or sod only as an orchard floor covering in 2008. Columns with different letters are significant according to analysis of variance at P < 0.05.

Citation: HortTechnology hortte 19, 2; 10.21273/HORTSCI.19.2.432

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In conclusion, many southeastern U.S. pecan producers could likely reduce input costs by applying only those nutrients required according to the results of annual leaf and soil analysis. The results presented here suggest that the greatest potential for nutrient deficiencies in the region currently exist with N, K, S, and Cu compared with P, Ca, Mg, Fe, Mn, and B. Nitrogen is the most important nutrient required for tree growth and nut production. Due to the wide variability among locations and years, practitioners should closely monitor leaf N concentration of commercial orchards. It is apparent that utilization of clover in combination with grass sod on orchard floors can enhance the availability of N to pecan trees. Although there was no difference in leaf N between orchards with and without clover, midsummer soil NO₃ levels were higher where clover was grown. This suggests that utilization of clover in orchards can reduce N applied to orchards, leading to considerable savings for producers. These results agree with Smith et al. (1996), who demonstrated that N fixed by clover can meet much of the N demand by pecan trees in Oklahoma. Pecan orchard soils also maintain relatively high levels of SOM, which appears to be enhanced with the addition of clover as an orchard floor cover. A relatively high percentage of the region's pecan orchard soils have low soil pH. Producers should be aware of the value of maintaining soil pH in the range of 6.0 to 6.5. However, because Ca and Mg levels where high in leaf and soil samples from many orchards, producers may want to continue applying lime only when soil analysis indicates a low pH. Also, it would appear that many growers could likely forego the soil application of P and Zn until leaf or soil analysis indicates a need, as long as foliar Zn applications are applied as recommended.