Arboriculture & Urban Forestry 42(6): November 2016 into the planting-hole backfill soil have typically shown no consistent improvement in the growth and establishment of woody plants (Schulte and Whitcomb 1975; Hummel and Johnson 1985; Gil- man 2004; Cogger 2005). When organic amend- ments are mixed into the entire landscape planting area, woody plant growth responses have been more positive. For example, amending planting beds by incorporating yard waste compost into the top 18 cm of a sandy soil improved azalea shoot and root growth (Beeson and Keller 2001). Incor- poration of peat moss or pine bark into the entire planting bed to a depth of 20 cm improved growth and survival of one-year-old seedling moun- tain laurel in a clay soil (Bir and Ranney 1991). Organic mulches applied to the surface of the soil have been shown to have a beneficial effect on soil properties and plant growth (Chalker-Scott 2007; Scharenbroch 2009). Because there are sites where incorporation of soil amendments by mixing is dif- ficult if not impossible (steep roadsides, rocky sub- soils), research has begun to focus on the potential for improving degraded urban soils and hence plant survival and growth by applying organic matter to the soil surface. Scharenbroch and Watson (2014) compared a compost topdressing to a wood-chip mulch treatment and found both improved tree growth and soil quality in a compacted urban soil. They called for future research to examine the effectiveness of combining those two treatments. Research with redosier dogwood in an agricultural soil (Cogger et al. 2008) compared surface-applied compost (topdressed) to soil-incorporated compost with and without a bark mulch layer and deter- mined that surface application of compost could improve soil and plant qualities when incorpora- tion was not feasible. In that study, application of bark mulch over either the incorporated or surface- applied compost was beneficial, but the efficacy on a compacted urban soil had yet to be tested. The current study is a follow-up to Cogger et al. (2008). It is focused on the compacted, highly degraded soil environment typically found along urban highway roadsides. Two systems used by the Washington State Department of Transporta- tion (WSDOT) were compared: amendment incor- porated into the soil and covered by bark mulch, and amendment applied to the undisturbed soil surface, then covered by bark mulch. The objec- 419 tives of this research were 1) to compare the effects of organic waste-derived soil amendments on woody landscape plant survival and growth and soil properties on a highly compacted high- way roadside landscape; and 2) to determine if amendment incorporation benefitted plant growth and survival compared with surface application. MATERIALS AND METHODS Site The study site was along a recently graded highway roadside in Tacoma, Washington, U.S., following construction but before landscaping. The A and B soil horizons were removed during construction, and the site was graded, leaving a highly compacted glacial outwash substratum with a texture of very gravelly sand (30% gravel and rocks). The site had a southwest exposure adjacent to a sound barrier and received runoff from the surrounding landscape, resulting in wet conditions following winter rains. Tacoma has a pacific northwest maritime cli- mate, with cool, wet winters and mild dry sum- mers. Mean December temperature is 5°C, mean July temperature is 19°C, and mean annual pre- cipitation is 996 mm (Tacoma, WA 1981–2010 normals). July and August are the driest months, averaging 20 mm of precipitation each month. Organic Amendments Organic amendments used in the experiment includ- ed yard debris compost, a biosolids-sawdust blend, and worm castings (Table 1) obtained from commer- cial sources. The yard debris compost was produced in intensively managed, aerated turned piles under cover, with a six-week composting time. Feedstocks included woody trimmings, leaves, weeds, and grass clippings. The biosolids blend was a mixture of one part Class A biosolids cake, from the City of Tacoma, and four parts sawdust. The worm castings were produced using separated dairy solids as feed- stocks and redworms (Eisenia fetida) for processing. All plots were mulched with bark, follow- ing standard WSDOT highway roadside land- scape practices. The bark mulch was Douglas-fir (Pseudotsuga menziesii) bark, ground to meet the WSDOT (2006) mulch specification of 95%, passing a 3.8 cm (U.S. No.2) sieve and ≤55% ©2016 International Society of Arboriculture
November 2016
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