Pecan trees growing in alkaline and calcareous soils are prone to zinc (Zn) deficiency (Malstrom and Fenn, 1981; Smith et al., 1980). Soluble Zn compounds such as Zn sulfate applied to alkaline soil react with hydroxides and carbonates and are converted to compounds unavailable to plants (Essington, 2003; Lindsay, 1972; Sadiq, 1991; Udo et al., 1970). Acidification using large quantities of strong acids such as sulfuric acid can increase Zn uptake by pecan (Fenn et al., 1990). Acid-forming compounds such as sulfur (S) and organic matter can increase Zn availability (Essington, 2003). Humic acids can also complex metals, including Zn, increasing mobility and solubility in calcareous soil (Bunluesin et al., 2006; Chien et al., 2006; Ozkutlu et al., 2006). Addition of manure, S, and Zn can decrease rosette symptoms caused by Zn deficiency in pecans growing in heavy-textured alkaline soils (Alben and Hammer, 1944). Fertilization of alkaline soils with Zn, in combination with sulfuric acid, was evaluated by acidifying a shallow trench making up less than 1% of the effective root zone of a mature Texas pecan trees by applying a mixture of 9 kg ZnSO4 and 113 L of 36 N H2SO4/tree. Leaf Zn did not change in the first 3 years, but 4 years after application, leaves of the treated trees contained 54 μg·g−1 Zn versus 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 (Fenn et al., 1990).
Increased soil organic carbon increased levels of DTPA-extractable Zn in tropical desert soils in Nigeria; DTPA-extractable Zn rose 0.17 mg·kg−1 with each percent increase in soil organic matter (Agbenin, 2003). It is not surprising then that addition of organic matter to soil can increase Zn levels and plant growth in annual plants such as rice, sorghum, and soybean (Battacharyya. et al., 2006; Pinto et al., 2004; Warwick et al., 1998; Zheljazkov and Warman, 2004). Even without incorporation into the soil, use of organic mulch at planting increased the trunk cross-sectional area (TCSA) in young ‘Desirable’ pecan trees. After 3 years, TCSA was 31 cm2 in untreated trees versus 39 cm2 in trees with 10 cm of mulch applied to the soil surface (Foshee et al., 1996). In another study, incorporation of pecan pruning wood chips into the soil improved soil tilth and aggregation and increased volumetric water content 20 d after irrigation (Tahboub et al., 2008) while not affecting nitrogen, phosphorus, or potassium availability (Tahboub et al., 2007).
Increased use of animal manure in pecan orchards has recently accompanied a shift toward “organic” production. The purpose of the manure applications is primarily to supply nitrogen in this production system. The relatively large quantities of manure used for this purpose (≈5 to 12 Mt·ha−1) might reasonably be expected to have an impact on levels of soil organic matter and metal solubility and bioavailability. Manure fortified with supplemental Zn might provide additional available Zn for trees growing in calcareous soils.
Zinc uptake and pecan tree performance were studied after the application of manure alone or in combination with Zn sulfate. The purpose of this study was to determine the effect of the addition of manure on Zn uptake, growth, yield, and nut quality of pecan trees growing under alkaline and calcareous conditions and to ascertain whether these effects can be enhanced by providing extra Zn with applied manure.
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