Arboriculture & Urban Forestry 40(4): July 2014 While mulching has many benefits for soil quality and root health, there are some potential drawbacks. One concern about mulching is that it creates conditions ideal for certain disease-causing fungi. Fraedrich and Ham (1982) did not find any enhancement of the soil-borne pathogenic fungi, Pythium spp. and Fusarium spp. during their one-year study. Austrian pine saplings that were mulched with fresh needles and shoot tips from Sphaeropsis tip blight diseased trees devel- oped more than twice the percentage of blighted tips. There was no Botryosphaeria canker or Armil- laria root rot disease development when redbud (Cercis canadensis) and red oak (Quercus rubra) saplings, respectively, were mulched with wood chips from diseased trees (Jacobs 2005). A decrease in growth the first year aſter mulching, and an increase in the second year has been attributed to nitrogen immobilization in the first year fol- lowed by release the next (Hensley et al. 1988; Truax and Gagnon 1993; Erhart and Hartl 2003). A layer of mulch can intercept rain water before it reaches the roots if the amount of water is small or the mulch is thick (Gilman and Grabosky 2004; Arnold et al. 2005; Johansson et al. 2006). Although 25 cm or more of coarse textured organic mulch does not adversely affect soil oxygen or fine root development (Watson and Kupkowski 1991; Greenly and Rakow 1995), as little as 5 cm of fine- textured organic mulch, or compost, can reduce soil oxygen to less than 10% under wet conditions, which can affect root function (Hanslin et al. 2005). Aeration Compressed air soil injection treatments have generally been ineffective in relieving compac- tion or increasing soil aeration (Yelenosky 1964; Smiley et al. 1990; Hodge 1991; MacDonald et al. 1993; Rolf 1993). Soil texture may have a strong influence on the results. Reports of success in reducing bulk density or increasing porosity were in loamy soils (Rolf 1993; Lemaire et al. 1999). A traditional approach to aeration of compacted soil around trees is vertical mulching (i.e., drilling a pattern of holes in the root zone soil). Research on vertical mulching has provided mixed results. Holes 5 cm diameter, 45 cm deep, with or without sand-bark mix backfill, provided no benefit to Chinese wingnut trees (Pterocarya stenoptera) 205 (Pittenger and Stamen 1990). Similar results were seen in sugar maple (Acer saccharum) when the holes were filled with perlite backfill (Kalisz et al. 1994). However, roots of Monterey pine (Pinus radiata) were able to utilize 10 mm diameter vertical perforations to grow the same depth as uncompacted controls, while root growth of trees on compacted soil without perforations was sup- pressed (Nambiar and Sands 1992; Sheriff and Nam- biar 1995). Largeleaf linden (Tilia platyphyllos) and planetree (Platanus × Acerifolia) roots colonized the majority of the depth of 10 cm diameter, 60 cm deep holes filled with a mix of coarse sand, composted organic materials, and fertilizer, and grew deeper than in adjacent site soils (Watson et al. 1996). Root growth in larger trenches filled with compost-amended soil was increased relative to undisturbed soil, but root growth was not increased in the soils adjacent to the trenches aſter 2, 4, and 14 years. Soil aeration was not measured and may not have been limiting in the undisturbed and not compacted soil adjacent to the trenches (Watson et al. 1996; Watson 2002). pH Adjustment Neutral to slightly acid pH is optimum for most plants. Applications of lime are used to raise soil pH. Aluminum sulfate and sulfur can help to lower pH, although high rates of aluminum sulfate may cause injury to some plants, particularly in broad- leaf evergreens. The injury is believed to be caused by excessive aluminum. Ammonium sulfate may be as effective as aluminum sulfate, but neither is as effective as granular sulfur (Messenger 1984). Ammonium sulfate is sometimes used if nitrogen application is needed along with pH reduction, but applying enough to lower the pH would likely apply a quick release form of nitrogen in excess of best management practices (Smiley et al. 2007). Enhancing root development may improve uptake of available nutrients. Improving soil qual- ity using methods such as cultivation, addition of organic amendments, and mulching can enhance root systems (see above). Basal drench applica- tion of paclobutrazol, a tree growth regulator, increased fine-root development interveinal chlorosis commonly to iron deficiency of pin oak (Quercus palustris) on alkaline soils (Watson and Himelick 2004). ©2014 International Society of Arboriculture and relieved attributed
July 2014
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