Spraying zinc (Zn) solutions onto the tree canopy is the standard method for supplying this nutrient to pecan trees [Carya illinoinensis (Wangenh.) K. Koch]. Zinc applied to soil reacts with hydroxyls and carbonates in alkaline and calcareous soils forming compounds of low solubility, limiting its plant availability (Agbenin, 2003; Lindsay, 1979; Udo et al., 1970). 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, uses fuel, repeated traffic through the orchard causes soil compaction, and spray schedules interfere with management in the orchard (mainly with irrigation). 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). Because of these disadvantages, growers would benefit from an effective and efficient method of applying Zn fertilizers to the soil.
Soil application of Zn has been successful in the acidic soils of the southeast United States (Sparks, 1976; Wood, 2007) but is much less likely to be effective in alkaline and, particularly, calcareous soils.
Response of pecan to soil Zn application can take several years depending on application rate, form of fertilizer, and method of application. On an acid soil in Georgia, using zinc sulfate (35 kg Zn/ha), zinc oxide (35 kg Zn/ha), and Zn-EDTA (3.5 kg Zn/ha), recovery of a Zn-deficient orchard was very slow, and it took 3 to 4 years to reach the optimum Zn levels in the leaf and eliminate deficiency symptoms (Worley et al., 1972). Zinc application rates from 0 to 4224 g Zn/tree (as either ZnSO4 or ZnO) placed over drip irrigation lines were evaluated on 4-year-old ‘Desirable’ trees in an acid soil in Georgia. In the first year of evaluation, 4 months after application, trees that received rates higher than 528 g Zn/tree had leaf Zn levels over 50 μg·g−1. In the second year, only those with rates of at least 2112 g Zn/tree had leaf Zn levels over 50 μg·g−1, and in the fourth year after application, treatments with rates higher than 132 g per tree had greater than 50 μg·g−1 of Zn in the leaves (Wood, 2007). In contrast, extremely high rates, as much as 126 kg of ZnSO4/tree, were needed to provide adequate Zn to pecan trees in a calcareous soil in Texas (Storey et al., 1971).
In a study of several forms of Zn fertilizer, ZnSO4, ZnO, and Zn-EDTA were broadcast or placed in holes (six holes, each 2.5 cm in diameter × 30 cm depth) in a Georgia soil that had been limed to a pH of 7.4 (Worley et al., 1972). Broadcast applications of either ZnO (applied at 22 g·cm−1 of trunk circumference, applied annually over 5 years) or Zn-EDTA (3.3 g·cm−1) increased leaf Zn relative to the untreated control; ZnSO4 placed in holes did not. Response to broadcast Zn-EDTA was significant in Year 2, whereas ZnO effects were not apparent until Year 5. Zinc sulfate had no significant effect regardless of placement. Extractability of Zn2+ in soils treated with EDTA at rates of 0.1 and 0.2 mg·kg−1 soil was increased as EDTA rate was increased (Karaca et al., 2000). DTPA-extractable Zn was 6.29 for the 0.1 mg·kg−1 rate and 7.57 mg·kg−1 for the 0.2 mg·kg−1 rate versus 2.07 mg·kg−1 in the control. In a field demonstration study in Texas, Zn-EDTA was applied through a drip irrigation system in 1974 at annual rates of 0.8, 1.6, and 2.5 kg Zn/ha (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 Zn-EDTA elevated leaf Zn levels; however, no unfertilized controls were included, and the data were not statistically analyzed.
Zinc fertilizer placement is also critical. 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 that. A single application of 160 kg Zn/ha of either material increased tissue Zn above 50 μg·g−1 in Year 2 when disked into the soil and in Year 4 when not incorporated (Wood and Payne, 1997). In another study, an application of ZnSO4 was banded (0 to 391 kg·ha−1) or broadcast (0 to 448 kg·ha−1) on a Georgia soil with pH ranging from 4.8 to 5.2 in a single application. Leaf Zn levels increased over the next 5 years. Applications of at least 112 kg ZnSO4/ha broadcast or 391 kg ZnSO4/ha banded increased Zn levels to over 50 μg·g−1. Greater broadcast application rates increased leaf Zn sooner; 448 kg ZnSO4/ha increased Zn above the 50 μg·g−1 threshold in the second year, 224 kg ZnSO4/ha in Year 4 to 5, and 112 kg ZnSO4/ha in Year 5. In banded treatments, 391 kg ZnSO4/ha raised leaf Zn to 50 μg·g−1 in Year 5 (Payne and Sparks, 1982b).
There is a lack of studies of Zn applied to irrigated calcareous soils. We evaluated Zn uptake, tree growth, and nut yield after a one-time band application of ZnSO4 and Zn-EDTA in a calcareous soil over a period of 4 years. The objective was to determine if these fertilizers can effectively enhance Zn uptake in young ‘Wichita’ pecan trees in an irrigated, alkaline desert soil.
Agbenin, J.O. 2003 Zinc fractions and solubility in a tropical semiarid soil under long-term cultivation Biol. Fertil. Soils 37 83 89
Andersen, P.C. & Brodbeck, B.V. 1988 Net CO2 assimilation and plant water relations characteristics of pecan growth flushes J. Amer. Soc. Hort. Sci. 113 444 450
Grauke, L.J., Storey, J.B., Emino, E.R. & Reed, D.W. 1982 The influence of leaf surface, leaf age, and humidity on the foliar absorption of zinc from 2 zinc sources by pecan HortScience 17 474
Karaca, A., Turgay, O. & Arcak, S. 2000 Effect of EDTA on the extractability of zinc, cadmium, and nickel in soils Proceedings of International Symposium on Desertification 422 428 13–17 June 2000 Konya, Turkey
Kim, T., Mills, H.A. & Wetzstein, H.Y. 2002 Studies on the effect of zinc supply on growth and nutrient uptake in pecan J. Plant Nutr. 25 1987 2000
Lane, R., Perkins, H.F. & Johnstone, J.F.E. 1965 Studies on the relationship of calcium, zinc, and pH in pecan production Proc. S.E. Pecan Growers Assn. 58 21 24
Lombardini, L., Harris, M.K. & Glenn, D.M. 2005 Effects of particle film application on leaf gas exchange, water relations, nut yield, and insect populations in mature pecan trees HortScience 40 1376 1380
O'Barr, R.D., Rachal, S., Koonce, K. & Kowalzuk, J. 1987 Influence of manganese applications on nine elements in pecan leaves, trunk and roots Louisiana Agricultural Expt. Sta. Circ. Circ. 102 112
Olsen, S.R. & Sommers, L.E. 1982 Phosphorus 403 430 Page A.L. & Miller R.H. Methods of soil analysis. Part 2. Chemical and microbiological 2nd ed Ameri. Soc. Agron.-Soil Sci. Soc. Amer Madison, WI
Payne, J.A. & Sparks, D. 1982b Zinc levels in pecan leaflets from broadcast and band applications over a 6-year period HortScience 17 235 236
Sharpe, R.R. & Marx, D.H. 1986 Influence of soil pH and Pisolithus tinctorius ectomycorrhizae on growth and nutrient-uptake of pecan seedlings HortScience 21 1388 1390
Sparks, D. 1978 Nutrient concentrations of pecan leaves associated with deficiency symptoms and normal growth HortScience 13 256 257
Uddling, J., Gelang-Alfredson, J. & Piikki, K. 2007 Evaluating the relationship between chlorophyll concentration and SPAD-502 chlorophyll meter readings Photosynth. Res. 91 37 46
Wadsworth, G. 1970 Absorption and translocation of zinc in pecan trees [Carya illinoensis (Wang.) K. Koch] TAMU College Station, TX MSc thesis.
Worley, R.E., Carter, R.L. & Harmon, S.A. 1972 Effect of zinc sources and methods of application on yield and leaf mineral concentration of pecan, Carya-Illinoensis Koch. J. Amer. Soc. Hort. Sci. 97 364