Fertilization requirements for palms growing in mineral soils differ greatly from those growing in largely organic container substrates (Broschat, 2009). Pine bark is a common component of container substrates in southeastern United States, but requires high N fertilization rates to compensate for N tie-up by microbes degrading this material (Ogden et al., 1987). Thus, fertilizers used in container production of palms and other ornamental plants contain large amounts of N relative to phosphorus (P), potassium (K), and other elements (Broschat, 2009; Poole and Conover, 1985). On the other hand, the highly leached, low cation-exchange capacity, sandy soils that predominate in the coastal plain of southeastern United States are typically deficient in N, K, magnesium (Mg), and often micronutrients such as iron (Fe), manganese (Mn), and boron (B) (Tisdale and Nelson, 1975). However, there is no published research on the optimum N and K fertilization rates and ratios for ornamental palms growing in landscape soils.
Potassium deficiency, which causes foliar discoloration, leaflet necrosis, and premature senescence of the oldest leaves, is nearly ubiquitous on palms growing in these soils and has been shown to be exacerbated by high N fertilizers (Broschat, 1990, 2009; Mills and Jones, 1996). While N deficiency symptoms are rarely observed on palms growing in these soils, growth rate is strongly affected by N fertilization rates. Thus, in field nurseries, where both visual quality and rapid growth are important, N and K fertilization rates must be properly balanced. It is not known if absolute amounts of N and K are critical for optimum growth and quality of field-grown palms or whether the ratio of these two elements is more important.
Since K deficiency was first identified as a serious problem in landscape and field-grown palms in southeastern United States in the late 1980s (Broschat, 1990), sequential trials with fertilizers containing increasingly lower N:K ratios and higher proportions of controlled-release K culminated in the recommendation of products having an analysis of 8N–0.9P–10K–4Mg plus 2% of Mn and Fe and trace amounts of zinc (Zn), copper (Cu), and B (Broschat, 2009).
Because of the severity of K deficiency on these soils, the need to reduce leaching losses, and the need to balance the ratios of N, K, and Mg over time, Broschat (2009) recommended that 100% of the N, K, and Mg should be in controlled-release form. Although a landscape maintenance application rate for this 8N–0.9P–10K–4Mg formulation of 73 g·m−2 of canopy or landscape area every 3 months has been recommended (Broschat, 2014), there is no research showing what the optimum fertilization rates are for either landscape maintenance or field production of palms. Commercial field production application rates of this product are typically twice the recommended landscape maintenance rates (Broschat, 2009). The purpose of this study was to determine the optimum N and K fertilization rates and ratios for field-grown areca and Mexican fan palms in southeastern Florida.
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