Palms are an important component of landscapes in subtropical and warm temperate climates of the United States as a result of their bold leaf texture, small footprint in the landscape, and the tropical or Mediterranean look they impart to a landscape. Although palms have some of the highest nutrient requirements of any plants, most palms in the southeastern United States are grown in sandy, nutrient-poor soils (Broschat, 2009). The result is that most landscape palms within the southeastern United States are deficient in one or more elements. These deficiencies result in chlorosis, necrosis, or malformation of leaves; reduction in canopy size (number of leaves supported by the palm); trunk tapering or bending; and even death of the entire palm (Broschat and Elliott, 2005; Elliott et al., 2004). Because palms have only one apical meristem per trunk and no aerial lateral meristems, nutrient deficiencies that might cause only a twig dieback in broadleaf trees can be fatal in palms (Broschat, 2009). Also, because palms have such large leaves, deficiency symptoms are highly conspicuous and unsightly for plants that are grown primarily for aesthetic purposes. Six nutrient elements are commonly deficient in palms in the southeastern United States [N, K, magnesium (Mg), iron (Fe), manganese (Mn), and boron (B)], and a deficiency of any one of these elements will affect palm aesthetic quality and health (Broschat, 2009).
Broschat (1999) determined that fertilizers having a N:K2O:Mg ratio of 2:3:1 are required to grow deficiency-free palms in the sandy and calcareous soils of the southeastern United States. Significant deviations from this analysis cause nutrient imbalances that can induce or exacerbate deficiencies of K and/or Mg. For example, turf fertilizers with N:K2O ratios of 2:1 or 3:1, which contain no Mg or micronutrients except for Fe, have been shown in Florida to exacerbate K deficiencies and induce Mg deficiencies (Broschat, 2009). Similarly, too much K will also induce Mg deficiency. Currently, the recommended fertilizer for palms in these soils has an analysis of 8N–0.9P–10K–4Mg plus micronutrients (hereafter 8–2–12) (Broschat, 2005a).
Because sandy and calcareous fill soils in southeastern United States have very low cation exchange capacities and are highly leached, there are serious concerns about the effects of N and P fertilizers applied to landscapes on the quality of surface and groundwaters. Excess P has been blamed for inland and coastal algal blooms, eutrophication of lakes, and proliferation of invasive cattails in the Florida Everglades (Anderson et al., 2002; Conley et al., 2009; Paerl, 2009). Nitrate-N is considered a human health hazard to children at concentrations greater than 10 ppm in drinking water (U.S. Environmental Protection Agency, 1995) but has also been implicated in the degradation of the near shore marine environments. As a result, a Florida statute (Urban Turf Fertilizer Rule RE-1.003) prohibits application of P in excess of 1.6 g P/m2/year to home lawns unless justified by a soil test documenting a P deficiency in the soil. The recommended palm fertilizer application rate of 73 g·m−2 of canopy area four times per year results in 23 g of N and 2.5 g of P being applied per square meter per year (Broschat, 2005a). These palm fertilization recommendations also state that turfgrass growing within 15 m of a palm should be fertilized only with the 8–2–12 palm fertilizer because palm roots can extend 15 m or more from the trunk. Thus, the recommended palm fertilizer rate would result in P applications in excess of those allowable for home lawns in Florida. Furthermore, ordinances in some Florida counties prohibit the application of any N or P fertilizers during the rainy months of June through September as a result of concerns about runoff into coastal waters.
Palm requirements for P are unknown. Phosphorus deficiency in palms is believed to be rare, but it is not unknown. It has been experimentally induced in several species of palms in sand culture (Broschat, 1984; Bull, 1958) and has been observed on extremely P-deficient soils (P < 0.25 ppm) in south Florida (Broschat and Elliott, 2009). Symptoms of P deficiency are uniform yellow–green discoloration of the foliage that could easily be confused with N deficiency or other problems (Broschat, 1984; Elliott et al., 2004). The most characteristic symptom of P deficiency, however, is a complete cessation of growth, something that will only be apparent if the palm is observed over time. This suggests that palm P requirements are much lower than those of most other crops where soil P concentrations in sandy soils lower than 20 ppm are considered deficient in this element (Anon, 2007). Because most soils in southeastern United States where palms are grown probably contain sufficient P for normal palm growth without supplemental P provided by fertilizers, it may be possible to eliminate all P from routine palm fertilization programs, thus eliminating one potential source of environmental pollution.
Similarly, N requirements for palms in landscapes are largely unknown. Nitrogen deficiency is uncommon among palms growing in landscapes in southeastern United States but is occasionally observed in areca palm (Dypsis lutescens), queen palm [Syagrus romanzoffiana (Cham.) Glassman], and Veitchia H. Wendl. spp. (Broschat, 2009). Still, although N deficiency is rare, N is the element that most strongly affects growth rate and thus N fertilization is needed for field production of palms and establishment and early growth phases in the landscape (Broschat and Moore, 2010, 2012). Soils containing organic matter appear to release sufficient N for good palm growth and quality and thus these soils may not require supplemental N to the degree that sand or calcareous soils do. In st. augustinegrass (Stenotaphrum secondatum) lawns, Broschat et al. (2008) showed that during the rainy summer months in south Florida, unfertilized turf had quality equal to turf receiving 23 g N/m2/year, presumably as a result of release of N from decomposing thatch and clippings. Thus, it may be possible to eliminate all N from landscapes fertilized during those rainy summer months of June through September in areas where N applications are prohibited. Because K, Mg, and micronutrient requirements of palms are greatest during the summer months as a result of rapid growth, and these elements are not considered to be environmental pollutants, applications of an 8N–0P–10K–4Mg plus micronutrients (hereafter 8–0–12) palm fertilizer in February, May, and November but a no-N equivalent product such as 0N–0P–13.3K–6Mg (hereafter 0–0–16) in August should theoretically provide adequate nutrition for palms while reducing N inputs and eliminating all P inputs into the environment.
Finally, as discussed previously, application of a typical turfgrass fertilizer that is relatively high in N, but low in K, and contains no Mg or micronutrients, either directly to palms or indirectly through application to turf as far away as 9 m from a palm, has been shown to induce or exacerbate K and Mg deficiencies in several species of palms, sometimes fatally (Broschat, 2005a; Broschat et al., 2008; Broschat and Moore, 2010). As a result, Broschat (2005a) recommends fertilizing any turf within 15 m of a palm only with an 8–2–12 palm fertilizer to prevent such problems. However, in some landscape situations, landscape professionals may not be able to control what is being applied to adjacent turf in the landscape. If the 8–2–12 palm fertilizer is applied to the palm at its recommended rate and turf under or near the palm receives a typical turf fertilizer that is high in N but low in K and contains no Mg or micronutrients, the combined N from these applications could result in excessively high N:K and N:Mg ratios and exacerbate K and Mg deficiencies in the palms. Under such circumstances, application of a palm fertilizer that contains no N such as 0–0–16 to the area under the palm canopy could potentially mitigate the negative effects of the nearby turf fertilization.
The objectives of this experiment were to determine if: 1) areca palms can be grown without supplemental P in sandy or calcareous landscape soils; 2) areca palms can grow without supplemental N during the summer rainy months of June through September in southern Florida; and 3) a 0–0–16 fertilizer can be used to mitigate potential damage to palms caused by application of high N fertilizers to palms or nearby turfgrass.
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