Approximately 3.1 million kg (6.8 million lb) of pecans were produced in Alabama in 2013 (Brown, 2013). The majority of production occurred in the southwest corner of the state in Mobile and Baldwin counties. Soils in those counties are mostly sandy loams and are typical of pecan orchards throughout the southeastern United States. Growers often experience difficulties maintaining recommended foliar phosphorus (P) levels in their orchard trees due to naturally low P in the soils and the nature of the movement and adsorption of P in those soils.
Recommendations for adequate foliar P concentrations in pecan vary; however, observations have shown that visual symptoms of P deficiency can be expected at foliar concentrations less than ≈0.11% (Alben, 1947; Sparks, 1978, 1986). The current standard recommendation for adequate foliar P is 0.14% (Smith, 2010; Smith et al., 2012). Pecan fertilizer recommendations are often based on a combination of the current year’s soil test data and the previous year’s foliar nutrient concentration data, with the latter considered more important. When correcting P deficiency, other essential plant nutrients must be considered because high concentrations of P can inhibit the uptake of nitrogen (N), iron (Fe), zinc (Zn), and copper (Cu) in pecan (Sparks, 1988; Wells, 2007).
In most soils, P is relatively immobile, and although it is often applied in combination with N and potassium (K), it is required in much lower quantities by plants. Therefore, a single broadcast application of 29.4 to 49 kg·ha−1 (26.2 to 43.7 lb/ac) P incorporated at planting can be adequate for several years during orchard establishment (Wells, 2007). The immobility of P in soils may result in the observed ineffectiveness of broadcast applications for correcting short-term deficiencies in established orchards (Alben and Hammar, 1964; Hunter and Hammar, 1947, 1952, 1957; Smith et al., 1960; Sparks, 1988; Worley, 1974). As a result, Sparks (1988) reported that 2.2 kg P per 6 m2 per tree was required to significantly increase P concentrations in pecan leaves. Due to tree spacing, the rate reported by Sparks was not easily interpreted in the initial publication, and later interpretations ranged from 3670 kg·ha−1 (3274 lb/ac) (Smith and Cheary, 2013) to 14,985 kg·ha−1 (13,369 lb/ac) P per year (Worley, 2002). Broadcast applications at extremely high rates should be discouraged due to potential environmental contamination.
Previous research indicated that banded applications of P increase foliar P when applied annually at the rate of 127.3 kg·ha−1 (113.6 lb/ac) P (Smith and Cheary, 2013). Banded P applications increased leaf P concentrations, ameliorated foliar deficiency symptoms, and increased return bloom. However, Smith and Cheary (2013) also reported kernel darkening in response to repeated P banding. The reported kernel darkening is in contrast to the results of a previous study that reported that P application improved the color quality of pecans (Smith, 2010). Whether the reported kernel darkening was due to cultivar response, drought stress conditions, P banding, or some combination thereof is unknown (Smith and Cheary, 2013). The reported positive benefits of P banding have outweighed the negative for many growers in the southeast who have adopted it as a standard practice. Thus far, there have been no reports of observed kernel darkening due to P banding in Alabama.
The positive effects of irrigation on pecan are well-known and include precocity, increased nut size and yield, and improved nut quality (Alben, 1957; Brison, 1974; Daniell et al., 1979; Stein et al., 1989; Wells, 2015; Worley, 1982). The current recommended irrigation schedule for pecan in the southeast was established by Wells (2007) based on the data of Daniell (1985). Adequate soil moisture is important for pecans during the nut filling stage in August and September (Wells 2015). The adoption of irrigation practices among Alabama pecan growers has been slow, but some larger growers have started to invest in the required infrastructure.
The P banding study by Smith and Cheary (2013) was conducted in an irrigated orchard, but under drought conditions. Irrigation efficacy was compromised at the height of the drought because the irrigation source became unusable (M.W. Smith, personal communication). According to a separate and unrelated meta-analysis of the effects of drought stress on plant P concentrations, He and Dijkstra (2014) reported that drought stress may reduce P concentrations in plants by up to 9.18%. Despite the negative effects of drought stress, increased foliar P was observed with annual banded applications of 127.3 kg·ha−1 (113.6 lb/ac) P (Smith and Cheary, 2013).
Phosphorus banding has yet to be replicated in an irrigated or nonirrigated environment like that present in the southern portion of Alabama. Additionally, the effects of a one-time band application have not been observed, nor have those of lesser, more sustainable application rates. An experiment was designed to determine the efficacy of a single P band at selected rates on soil test P and P uptake by plants over multiple years in a typical nonirrigated and irrigated Alabama pecan orchard.
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