Urban Highway Roadside Soils and Shrub Plantings Enhanced by Surface-Applied and Incorporated Organic Amendments
Andy Bary, Rita L. Hummel, and Craig Cogger
Abstract: Degraded, highly compacted soils along roadsides present an inhospitable environment for trees and shrubs and lead to the failure of urban landscapes. Developing and testing practices to ameliorate urban soils, thereby improving plant growth and survival, is essential. This research compared the effects of waste-derived soil amendments on woody landscape plants and soil properties on a compacted highway roadside in Tacoma, Washington, U.S., and compared surface application versus incorporation of amendments. Treatments included yard debris compost (surface-applied and incorporated), biosolids blend (surfaceapplied and incorporated), and worm castings (surface-applied only), plus a control with no amendments. Amendments were applied 8 cm deep, and incorporated to a 10–15 cm depth on the tilled plots. An 8-cm layer of bark mulch was blown onto all plots, including the control. Rhus aromatica, Symphoricarpos orbiculatus, and Mahonia aquifolium were transplanted from 3.8 L containers in March 2007. One year later, soil under the mulch/amendments was analyzed for bulk density, total carbon, and nitrogen. Plant growth and survival was evaluated for three years. Incorporating soil amendments reduced bulk density by >50% and increased soil C and N tenfold in the incorporated zone. Soil properties within the surface and control treatments or within the incorporated treatments were not different. All amendments significantly improved plant growth in comparison to the bark mulch control, in the order worm castings = biosolids blend = yard debris compost. Neither plant growth nor plant survival was affected by surface application versus incorporation, and plant roots remained confined to the amended zone.
Keywords: Bark; Biomass; Biosolids; Compost; Coralberry; Fragrant Sumac; Highway; Mahonia aquifolium; Mulch; Oregon Grape; Rhus aromatica; Roadside; Symphoricarpos orbiculatus; Total Soil Carbon; Total Soil Nitrogen; Urban Soils; Vermicompost; Washington; Worm Castings; Yard Debris