Nitrogen is the most commonly applied nutrient in orchard crops and there are multiple options with regard to the form of N applied (Weinbaum et al., 1992). The effect of N on pecan yield and nut growth has been studied since 1918 (Skinner, 1922). Numerous studies have assessed various rates (Hunter and Hammar, 1947; Skinner, 1922; Smith et al., 1985; Worley, 1974), application timings (Acuna-Maldonado et al., 2003; Smith, 1991; Smith et al., 1995, 2004), and areas of application within the orchard (Wells, 2013); however, little attention has been given to the suitability of various forms of N for pecan in Southeastern U.S. orchard systems characterized by acidic soils. Although N is a major component of pecan nutrient management, pecan tree response to N has been variable in multiple studies throughout the years (Hunter and Lewis, 1942; Smith et al., 1985; Storey et al., 1986; Worley, 1974, 1990).
Ammonium nitrate and urea are the most common forms of dry N fertilizer applied in southeastern U.S. pecan orchards. Ammonium nitrate is decreasing in popularity as a result of cost, and the risk potential for use in terroristic activities and storage-related problems.
Urea is the most widely used dry N fertilizer in the world. After application, urea is converted to ammonia (NH3), which is then held in the soil or converted into nitrate (NO3); however, NH3 can be readily lost to volatilization under certain conditions (Havlin et al., 2005).
In the arid southwestern United States, AS is commonly used in pecan orchards as the N source where dry fertilizer is used. Pecan producers in the humid southeastern United States more commonly use the AN or urea forms of dry N fertilizer. The disparity between N source in the regions is likely the result of the hyperacidifying properties of AS. Because of the alkaline soils of the southwestern United States, the acidification resulting from the application of AS can be beneficial and is only rarely a cause of concern regarding the lowering of soil pH. However, in the acidic soils of the southeastern United States, further acidification can potentially lead to interference with plant uptake of macronutrients from the soil.
Kim et al. (2002) evaluated the effect of N form on growth, development, and nutrient uptake of hydroponically grown pecan seedlings; however, comparisons of N form on yield and growth parameters of mature pecan trees under orchard conditions is lacking.
N fertilization is a major component of pecan nutrient management and is a significant cost for producers (Weinbaum et al., 1992). A better understanding of the efficacy of various N forms on pecan tree production would help growers make more sound decisions regarding the fertilization of their orchards. The objective of this study was to investigate the effects of various dry N fertilizer sources at varying rates on pecan leaf tissue nutrient concentration, leaf chlorophyll index, trunk circumference growth, pecan yield, nut weight, percent kernel, pecan tree yield efficiency, and alternate bearing.
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