Pecan alternate bearing is typified by high production 1 year followed by 1 year or more of low production (Sparks, 1986). All segments of the pecan industry consider alternate bearing as the greatest factor limiting expansion (Smith and Weckler, 2010). Low crop years are normally associated with a lack of return bloom rather than flower abortion (Rohla et al., 2007).
Alternate bearing trees show distinct differences in leaf K and P accumulation and depletion between large and small crops (Krezdorn, 1955; Smith, 2009). During large crop years, trees had higher concentrations of leaf K and P shortly after budbreak, perhaps resulting from greater stored reserves after a small crop (Krezdorn, 1955), or greater absorption from soil triggered by a signal from a large number of developing flowers (Smith, 2009). Shortly after the fruit begins rapid volume increase, phloem-mobile K (Marschner et al., 1996) in leaves drops and shuck K accumulates rapidly (Diver et al., 1984; Kim and Wetzstein, 2005; Pe’er and Kessler, 1984; Smith, 2009) presumably associated with carbohydrate transport (Haeder, 1977; Mengel, 1980; Mengel and Haeder, 1977; Vreugdenhil, 1985) to the developing cotyledons supporting lipid accumulation (Pe’er and Kessler, 1984). Accumulated shuck K acts to increase shuck hydration assisting in shuck opening (Pallardy, 2008; Thor and Smith, 1935). Potassium shortages have been associated with leaf margin chlorosis/necrosis and partial defoliation (Smith, 2010; Sparks, 1977), reduced kernel oil content (Hunter and Hammar, 1956), poor kernel development (Smith, 2010), greater fruit abortion (Wood et al., 2010), lower tree yield (Blackmon and Ruprecht, 1934; Smith et al., 1985), and reduced return bloom (Smith, 2010).
Pecan tree yield was unresponsive to applied K in many studies (Alben and Hammar, 1963, 1964; Gammon and Sharpe, 1959; Hunter, 1956; Hunter and Hammar, 1947; Sharpe et al., 1950, 1952; Worley, 1974). However, four studies reported a yield increase with applied K (Smith et al., 1985; Wells and Wood, 2007; Wood et al., 2010; Worley, 1994). Inconsistent response to applied K was largely the result of native K availability or lack of plant uptake indicated by leaf K concentration. Frequently, multiple years of annual K application were required to elicit a response (Smith et al., 1985; Worley, 1994). Typically, applied K was more readily available in soils with a sandy texture than those with high clay content. Wood et al. (2010) demonstrated that banding K within the root zone near a drip irrigation source resulted in substantial K uptake by trees.
Worley (1994) conducted a 20-year study to establish the minimum pecan leaf K concentration to achieve maximum production. He concluded that 0.75% was adequate for non-irrigated, old ‘Stuart’ trees. However, other work reported fruit drop of ‘Desirable’ could be reduced if fruit K was ≈1.25% during Stage II fruit drop and suggested minimum leaf K concentrations as high as 1.5% might be needed for irrigated ‘Desirable’ (Wood et al., 2010). Another study suggested that K concentrations should be adjusted based on nitrogen (N) management to yield a leaf N:K ratio of 2:1 for maximum pecan yield (Wells and Wood, 2007).
Phloem-mobile P in leaves decreased rapidly as it was transported to developing cotyledons during the latter part of the growing season (Diver et al., 1984; Kim and Wetzstein, 2005; Krezdorn, 1955; Smith, 2009). Rapid P transport was later than K to developing fruit. Phosphorus accumulation was primarily in the cotyledons in the form of phospholipids used as a substrate for unsaturated fatty acid synthesis and stored as inositol hexaphosphate (Chesworth et al., 1998). The rapid accumulation of P in the fruit and concurrent depletion from the leaves resulted in leaf necrosis and partial defoliation when P was limiting (Smith, 2010; Sparks, 1977).
Smith (2010) reported that leaf, kernel, and shuck P concentrations were positively correlated with weight/nut and the percentage of number-1 kernels and recommended a minimum leaf P concentration of 0.14% in July. Sparks (1988) suggested that a leaf P concentration of 0.16% may be required for pecan. However, correcting a P shortage using traditional broadcast application was frequently unsuccessful (Alben and Hammar, 1964; Hunter and Hammar, 1948, 1952, 1957; Smith et al., 1960; Worley, 1974). Sparks (1988) increased P absorption of pecan by broadcasting massive quantities of P (2.2 kg/6 m2/tree P; equivalent to broadcasting 7491 lb/a P2O5). Such large amounts of P are not economically practical to correct a shortage.
This study was conducted in a pecan orchard with documented shortages of P and K (Smith, 2010). The objective of this study was to correct these deficiencies using banded fertilizer applications and to determine tree response. Because broadcast applications were usually ineffective unless exceptionally large quantities of P were applied and K shortages are difficult to correct in soils with high clay content, banding the fertilizer at the edge of the vegetation-free area surrounding the tree seemed a reasonable approach.
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