Arboriculture & Urban Forestry 40(6): November 2014 tor for the growth and pathogenesis of P. cinnamomi and should be a consideration with implementing a management strategy for control or prevention. SOIL AND PLANT NUTRITION The physical properties of soil are very important for the health of Taxus. As discussed above, satu- rated soils are detrimental to the health of Taxus and water-holding capacity is directly related to soil particle size. In general, the greater the clay con- tent of soil, the greater the water retention. As the sand component of the soil increases, the air-filled pore space increases and water-holding capacity decreases (Hillel 2008). Heavy clay soils and areas of poor drainage should be avoided when plant- ing Taxus. It seems logical that if clay soils exist, amending with sand will improve drainage. How- ever, this is not the case, and adding small amounts of sand to clay soils creates a soil structure similar to concrete and will not improve drainage (Da- vis and Whiting 2013). Amending clay soils with organic matter will improve drainage due to the flocculation and aggregation of clay particles. This occurs when humus, a product of organic matter decomposition, forms bridges that bond the clay particles together and forms particles sometimes referred to as pseudosand (Brady and Weil 2002). The chemical properties of soil are equally impor- tant for maintaining healthy Taxus. The suggested soil pH for Taxus ranges from 6.0 to 7.0 (Hibben and Cahilly 1996; Fraedrich 1999; Halcomb 2012; Kaiser and Ward 2013). In a study by Gilliam et al. (1985), Taxus cuspidata ‘Densiformis’ was grown in a fine sandy loam with three pH levels of 5.2, 5.8, and 6.2. At the highest pH, plants had greater growth indi- ces, and there was a significant effect of pH on tissue zinc (Zn) and manganese (Mn) concentrations. In the first year of the three-year study, when pH treatments decreased from 6.2 to 5.2, tissue nutrient concentration of Mn increased from 525 to 4150 ppm, which is a 690% increase. This trend also occurred in years two and three of the study at a similar magnitude. Tissue Zn concentrations were similarly affected by soil pH. However, the magni- tude was much lower, and concentrations ranged from 35 to 230 ppm. These types of results would be expected as the availability of Mn and Zn in soil increases as soil pH decreases (Marschner 1995). There was little to no effect of soil pH on all other 347 tissue elements tested [nitrogen (N), phosphorus (P), calcium (Ca), magnesium (Mg) and iron (Fe)]. Toxicity of Mn and/or Zn has been considered as a potential cause of decline and needle chlorosis of Taxus (Hibben and Cahilly 1996). In most plants, Zn and Mn toxicity cause leaf chlorosis of younger leaves (Marschner 1995). In the study at NYBG, soil pH was below suitable range for most of the Taxus tested and tissue Mn and Zn concentrations averaged 589 and 189 ppm, respectively. No correlation was found between chlorotic needles and the concentra- tion of either of these nutrients. However, the mani- festation of Mn or Zn toxicity specifically on Taxus has not been determined. Furthermore, the average tissue concentrations of Mn and Zn fell within the sufficiency range for T. × media ‘Hicksii’ published by Mills and Jones (1996). This publication is the only known reference for sufficient tissue nutrient concentration of Taxus, which provides the ranges for T. × media ‘Hicksii’ and average concentrations for T. baccata (Table 1). These averages and ranges can be helpful when making decisions on fertiliza- tion rates and practices. Further research to develop ranges for other species and cultivars is needed. Deficiencies of N, K, S, or Mg are also known to cause leaf chlorosis (Marschner 1995). For Taxus production and maintenance in the landscape, fertilizer recommendations are limited. Halcomb (2012) recommends high P and K fertilization rates during production and three to four applications of N through the growing season. The N should not be applied too late in the season to avoid the potential of winter injury on succulent tissue. A study with young Taxus × media ‘Hicksii’ in containers indi- cated fertilization with 380 ppm N at each irrigation reduced growth compared to 120 and 192 ppm N (Khatamian and Lumis 1982). This indicates high N rates should not be used when growing Taxus. In the landscape, fertilization rates should be lower unless maximum growth rates are preferred. Soil tests should be performed every one to two years to determine the type and timing of fertilizer applica- tions. These tests can also be done in conjunction with tissue testing, particularly in cases where plants are declining or when attempting to restore suffi- cient nutrient concentrations. Conducting these two tests at the same time will provide a more holistic view of the nutritional status of the plant and soil. The local soil testing laboratory should provide ©2014 International Society of Arboriculture
November 2014
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