30 Minute Presentation
Compost and Compaction Effects on Soil Hydraulic Properties
Urbanization can degrade the natural function of soil through vegetation removal, stripping of topsoil, and compaction by equipment. Compacted soils have limited infiltration and are susceptible to erosion, yet infiltration is a critical part of urban runoff design. There has been a widespread interest in using compost to amend urban soils post-construction in order to improve hydrologic functions. Previous research in field plots suggested that combinations of vigorous vegetative growth and compost incorporation could reduce the impact of compaction from mower traffic. The relative contribution of the vegetation and the compost was not clear, but it appeared both were important. The amount of compost needed to have a measurable impact on infiltration has not been determined, and recommendations for compost application rates vary widely and appear to have no basis in research. Because compost is a relatively expensive soil amendment, it is important to determine the minimum rate necessary to achieve the soil physical property goals. The objective of this study was to determine the effects of compost amendment rate on saturated hydraulic conductivity (Ks) and water retention in order to identify target compost rates for enhanced soil infiltration. Soil cores were prepared in the laboratory using three different textured soils amended with certified yard waste compost at 0, 10, 20, 30, 40, and 50% by volume. All soils had a consistent porosity of 0.5 m3 m-3, and the sandy loam soil was further divided into high (0.55 m3 m-3) and low (0.4 m3 m-3) porosity sets. Saturated hydraulic conductivity was measured using a combination of constant and falling head methods. Water retention was measured by pressure plate extraction at 25, 50, 100, 200, 333, 1020, 5099, and 15296 cm of water. Results indicate that porosity level was a more important determinant of Ks than was rate of compost addition in the sandy loam soil. Additionally, with increasing rate of compost amendment, the Ks and the water retention of the soils largely increased. The results suggest if a soil is amended with compost but becomes compacted, benefits of the compost for infiltration may be limited even at a high rate of the amendment. However, if the soil is protected from compaction, the water holding capacity and infiltration rate increased with increasing rates of compost. This suggests that areas intended for high infiltration and possible runoff treatment need to be managed to avoid compaction. Other questions we plan to address include compost type or source, and the interactions between soil texture and compost rate.