Urban soil characteristics and limitations for landscape planting in Hong Kong

Soils in urban areas have not received adequate attention in relation to landscape planting. Recent urban tree surveys in Hong Kong identified many arboricultural problems related to stressful habitat conditions including edaphic ones. Soil constraints in the crowded city are acute and yet relevant data are lacking. This project evaluated in the field and laboratory 100 soil samples obtained from sites of varied natural and disturbance history at difficult roadside tree-pit sites in urban Hong Kong. An extensive range of physical and chemical properties were assessed to establish baseline information on soil limitations to amenity-vegetation growth. Most soils have lost natural soil horizons with morphological features of fill materials such as poor structure and artificial layering. They are excessively stony and coarse-textured, with abundance of building rubble and other foreign substances. The extensive soil compaction, associated with structural degradation and loss of porosity, is inimical to aeration, drainage, storage of plant-available moisture, and root growth. Release of carbonate from the calcareous construction waste has induced alkaline pH, with consequences on micronutrient and phosphorus deficiency. Organic matter contents, and the related supply of essential nitrogen and phosphorus, are all meager. The capacity to hold nutrient bases and the amount held in available forms are both inadequate for vigorous plant performance. There are signs of pollution by heavy metals, especially lead, which are derived from air-borne sources related to vehicular traffic. The results help to ascertain the need for soil tests in landscaping endeavors, to reinforce a weak link in landscaping programs, to establish a package of recommendations to improve urban soil management, to enhance the cost-effectiveness of greening programs, and to augment urban-tree performance so as to reduce recurrent management liability and burden.