Pecan (C. illinoinensis) is the major tree nut crop grown in the desert southwestern states of Arizona, New Mexico, and Texas. Growing conditions vary throughout this region; however, the soils are generally calcareous, with pH levels of 8.0 or above. Pecan trees growing in these soils are prone to Zn deficiency due to limited soil Zn availability (Alloway, 2008; Malstrom and Fenn, 1981; Smith et al., 1980). Fenn et al. (1990) reported that water extractable Zn in a South Texas soil decreased from 394 to 12 µg·g−1 as soil pH increased from pH 4 to 8. Muhammad et al. (2006) found that the majority of Zn in calcareous soils is sorbed to calcium (Ca) carbonate.
Rosetting, a characteristic visual symptom of Zn-deficient pecan trees, occurs when leaf and internode size are reduced. Severe deficiency symptoms also include interveinal chlorosis, eventually developing into necrosis. Terminal growth dieback can occur in severe cases. Zn deficiency can reduce catkin length, number of fruits per shoot, fruit development, and when severe, it can inhibit production of inflorescences (Hu and Sparks, 1990, 1991). Published literature suggests that pecan leaf Zn concentrations should be greater than 40 to 50 µg·g−1 to avoid deficiency (Payne and Sparks, 1982; Pond et al., 2006; Reuters and Robinson, 1997; Sparks, 1993, 1994).
Foliar application of Zn solutions directly onto tree foliage is the standard method for supplying this nutrient to pecan trees. Foliar Zn sprays can reliably increase leaf Zn levels over threshold levels of 50 µg·g−1, but this method has several disadvantages. Foliar application is time consuming, requires investment in expensive equipment which requires fuel, and spray schedules may conflict with other management practices in the orchard (mainly with irrigation). Achieving adequate coverage via foliar application is difficult and only leaves actually contacted by spray are affected by foliar treatments (Wadsworth, 1970). Storey et al. (1971) noted that only 0.2% and 1.0% of applied Zn was absorbed by mature and young leaves, respectively. Additionally, repeated applications are required during the growing season as a result of low mobility of sprayed Zn within the tree (Grauke et al., 1982; Wadsworth, 1970). For these reasons, there is considerable interest among pecan producers in the potential of soil-applied Zn.
Soil application of Zn has been successful in the acidic soils of the southeastern United States (Sparks, 1976; Wood, 2007), but is much less likely to be effective in alkaline, and particularly, calcareous soils. Storey et al. (1971) found that a soil application of 126 kg/tree of zinc sulfate (ZnSO4) was needed to provide adequate nutrition to mature pecans growing in a calcareous Texas soil. Soluble Zn compounds such as ZnSO4 applied to alkaline soil react with hydroxyls and carbonates in alkaline and calcareous soils forming compounds of low solubility, limiting its plant availability (Agbenin, 2003; Essington, 2003; Lindsay, 1972, 1979; Sadiq, 1991; Udo et al., 1970). Smith (1934) reported that to correct rosette symptoms on pecan in an “alluvial silt loam soil high in lime” in Uvalde, Texas, a ZnSO4 rate of 1.8 to 2.3 kg/tree (applied in a trench 60 to 75 cm from the trunk) was necessary. An application of 91 kg/tree of ZnSO4 failed to bring soil Zn content to a critical level in a study by Lott (1938) because the Zn was converted to insoluble Zn carbonate.
Using Zn in combination with sulfuric acid (H2SO4) to fertilize pecans growing in alkaline soils was evaluated by acidifying a shallow trench making up less than 1% of the effective root zone of mature Texas pecan trees and applying a mixture of 9 kg ZnSO4 and 113 L of 36 n H2SO4 per tree (Fenn et al., 1990). Leaf Zn did not change in the first 3 years, but 4 years after application, leaves of treated trees contained 54 µg·g−1 Zn vs. 39 µg·g−1 in the untreated control. After 9 years, leaf Zn levels were 58 and 45 µg·g−1 for treated and untreated trees, respectively, and 56 µg·g−1 in trees receiving ZnSO4 alone. Soil pH was decreased to a depth of 60 cm; however, roots did not grow into the acidified soil, proliferating instead at the interface of the acidified and calcareous soil.
Chelated Zn fertilizers, such as those made with EDTA, can alter the soil mobility of Zn and more effectively increase levels of bioavailable Zn compared with inorganic Zn fertilizers (Alvarez, 1997). Chelated Zn is less subject to soil fixation reactions (Norvell, 1991). Naik and Das (2010) showed that ZnEDTA was more effective than ZnSO4 at keeping Zn in solution and Alvarez et al. (1997) found that compared with unamended soil, amendment with ZnEDTA increased soil Zn bioavailability to plants. In an alkaline, calcareous Arizona soil Núñez-Moreno et al. (2009) found that soil-banded application of ZnEDTA (19 kg·ha−1 of Zn) resulted in significant pecan leaflet Zn concentration differences 1, 3, 4, 18, and 30 months after application, but that banded ZnSO4 (74 kg·ha−1 of Zn) did not.
In a field demonstration study in Texas in 1974, ZnEDTA was applied to pecans through a drip irrigation system at annual rates of 0.8, 1.6, and 2.5 kg·ha−1 of Zn (Lindsey and Condra, unpublished data). Resulting leaf Zn levels were 39, 53, and 68 µg·g−1, respectively. In 1975, the corresponding leaf Zn levels were 49, 54, and 70 µg·g−1, respectively. These data suggest that drip irrigation-applied ZnEDTA increased leaf Zn concentrations; however, no unfertilized controls were included, treatments were not replicated, and the data were not statistically analyzed.
Zn fertilizer placement is also critical. Zinc oxide (ZnO) and ZnSO4 were broadcast on a limed Georgia soil with a pH of 7.3 in the top 2.5 cm and 6.2 in the 2.5 cm below (Wood and Payne, 1997). A single application of 160 kg·ha−1 of Zn supplied by either ZnO or ZnSO4 increased leaf tissue Zn above 50 µg·g−1 in the 2nd year after application if the Zn was disked into the soil and in the 4th year if it was not incorporated.
The purpose of this study was to evaluate the efficacy of applying ZnEDTA dissolved in irrigation water to immature pecan trees growing in an alkaline calcareous soil. This study reports on Zn uptake and plant growth response to fertigated ZnEDTA.
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