Soil and Woody Plant Response to Incorporated and
Surface-applied Compost
C. Cogger, R. Hummel, J. Hart, and A. Bary
Although compost can improve soil properties related to plant growth and water quality, the value of amending landscape beds for trees and shrubs has been questioned. This research assesses short and mid-term effects of compost application and bark mulch on soils and plants in landscape beds, and compares the effects of compost applied to the surface or incorporated. Trees and shrubs were established in 2001 in a replicated field experiment with the following treatments: 1) unamended control, 2) compost (7.6 cm depth) applied to the surface, 3) 7.6 cm compost incorporated by rototolling to a depth of 20 cm, 4) bark mulch (7.6 cm), 5) compost surface applied (7.6 cm)+ bark mulch (7.6 cm), and 6) compost incorporated + bark mulch. Soil measurements were made one or more times between 2001 and 2007, including bulk density, compaction, infiltration, aggregate stability, soil moisture tension, total carbon (C) and nitrogen (N), nitrate-N, Bray-phosphorus (P), exchangeable potassium (K), and pH. Bark and compost mulch depths were determined three times, and plant growth measured annually. Half the depth of surface-applied compost and 26 to 41% of the initial soil C increase from incorporated compost remained five years after application, and significant changes in bulk density, compaction, infiltration, and nutrients were apparent. Compost incorporation had a greater effect than surface application on soil C, N, and bulk density. Infiltration, soil moisture tension, and aggregate stability were similar in incorporated and mulched treatments, and nutrient availability was similar except for N. Bark had similar effects as surface-applied compost on bulk density and infiltration. The greatest plant growth occurred when both bark and compost were present. Surface application of compost could provide significant benefits where incorporation is not feasible.