Porosity of roadside soil as indicator of edaphic quality for tree planting

Abstract

Roadside tree pits commonly suffer from small size, poor soil, and heavy compaction. Their three soil types with different properties, respectively site soil, backfill and rootball, could constrain tree establishment and long-term growth. Sixty-nine soil samples were taken from 19 tree pits, with multiple artificial layers sampled separately, at roadside sites in Hong Kong. They were analyzed for profile characteristics, pH, bulk density and particle-size distributions. Pores were divided by into three classes: (1) unavailable moisture UM at <0.2 μm (also known as micro-pores); (2) available water AW at 0.2–60 μm (meso-pores); and (3) and air capacity AC at >60 μm (macro-pores). Critical pore-volume thresholds, namely extreme, marginal and optimal, assessed soil-porosity quality. Site soils were heavily compacted with <40% and <30% total porosity, denoting respectively marginal and extreme thresholds, equivalent to bulk density exceeding 1.6 and 1.9 Mg/m3. The upper soil zone was more compacted than middle and lower zones to generate undesirable surface sealing. Backfill and rootball soils had less stressful porosity and bulk-density limitations. Long-term root growth into site soil would be hampered to suppress tree health and stability. The excessively sandy texture, upon compaction to a certain degree, generated a continuous coarse matrix. It established inter-granular contacts and high load-bearing capacity to arrest further compaction. Some AW pores could be sustained for available-water storage to support tree growth. The findings could inform porosity specification in urban soil management to foster roadside tree performance.