320 Scharenbroch and Watson: Wood Chips and Compost Improve Soil Quality and Increase Growth impact soil properties and tree growth [see reviews by Chalker-Scott (2007) and Scharenbroch (2009)]. Beneficial effects of these organic materials include improved soil moisture, reduced erosion and com- paction, maintenance of optimal temperature, reduced salt and pesticide use, increased binding of heavy metals, reduction of weeds, improved soil fer- tility, improved plant establishment and growth, and reduction of disease (Chalker-Scott 2007). Current standards do not exist for application rates of organic materials. Wood-based mulch is oſten applied annu- ally to depths of 5 to 15 cm around the tree, but not against the base of the tree. Saebo and Ferrini (2006) suggest no more than 1.0 to 1.2 kg of plant avail- able N 100 m-2 yr-1 to be delivered from compost. Clients and circumstances oſten dictate that turf- grass remain under urban trees in lieu of mulch. Furthermore, mulch rings rarely cover the full extent of the rooting area, which has been esti- mated to be 38 times the tree diameter (Day et al. 2010). Liquid-based organic products and materials (e.g., compost tea, humic acids, mycorrhizal spores) are gaining popularity for applying nutri- ents and organisms to soils for landscape trees. Aerated compost tea (ACT) is a mixture of compost, organisms, additives, and aerated water (NOSB 2004). According to the National Organic Program (NOP), the predominant ACT produc- tion method in the United States involves one part compost in 10-50 parts water, constant aeration for 12 to 24 hours, and immediate application (NOSB 2004). NOP standards specify that compost used to make ACT must be made from allowable feed- stock materials and the entire pile must undergo an increase in temperature to at least 131ºF for at least three days (NOSB 2002). ACT additives, such as molasses, yeast extract, and algal powders are used to encourage growth of beneficial microbes. No standards exist for application rates of ACT in agriculture or horticulture. Suggested ACT appli- cation rates for urban landscape plants range from 500 to 5,000 L ACT 100 m-2 yr-1 (E. Ingham of Soil Foodweb, Inc., July 2008, pers. comm.), albeit these rates are not based on scientific evidence. It is suggested that ACT will increase nutrient availability and retention via microbial mineraliza- tion and immobilization, build soil structure and decrease the effects of compaction, detoxify soil and water, and suppress disease by inducing com- ©2014 International Society of Arboriculture petition among disease (anaerobic) and beneficial (aerobic) organisms (e.g., Ingham 2003a; Ingham 2003b; Ingham 2004; Lowenfels and Lewis 2007). However these claims are unsubstantiated and few peer-reviewed controlled, replicated scien- tific studies have been performed on the impacts of ACT on plants, soil, and the environment (Scheuerell and Mahaffee 2004; Duffy et al. 2004; Scheuerell and Mahaffee 2006; Larkin 2008; Segarra et al. 2009). Even fewer studies have examined ACT with urban trees (Scharenbroch et al. 2011; Scharenbroch 2013; Scharenbroch et al. 2013). The objective of this research was to evaluate inorganic fertilization (FERT), wood chips (WC), compost (COMP), aerated compost tea (ACT), a commercial biological product (CBP), and a water control (NULL) for impacts on tree growth (Acer rubrum and Betula nigra) on a disturbed site. Although a relatively large body of literature sug- gests beneficial impacts of inorganic fertilizers and organic materials, few studies have examined these six typical urban soil treatments in isolation for their effects on soil quality and tree growth in a controlled experiment. The primary research hypothesis was that due to more comprehensive and dramatic impacts on soil quality, the organic materials (COMP and WC) will have significantly greater impacts on tree growth compared to liquid-based treatments (FERT, ACT, and CBP). Because COMP is relatively more labile compared to WC, improvements in soil quality will be greater with COMP compared to WC. MATERIALS AND METHODS Soil and Site Preparation In spring of 2007, an urban soil research plot (1.5 ha) was created in the research grounds of The Morton Arboretum in Lisle, DuPage County, Illinois, U.S. (N 41°48’52.5” and W 88°04’15”). The native soil (fine illitic, mesic, Oxyaquic Hapludalfs Ozaukee series) on site is moderately well drained, slowly permeable, and moderately deep to a densic contact with till. During site preparation, attempts were made to mimic the activities of a typical urban site development. The site preparation was performed when the soil was near field capac- ity in an attempt to maximize the impact of the disturbance. The topsoil (0 to 20 cm) on the site was stripped with a bulldozer, and the underlying
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