Physical and chemical properties of a Hong Kong roadside soil in relation to urban tree growth

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Tree planting in urban areas faces many limitations that suppress performance and increase management burdens. Soil problems are increasingly recognized as an important cause of establishment failures and declines. Recent city-greening programs in Hong Kong necessitate a better understanding of soil difficulties, but relevant information is lacking. A detailed study of physical and chemical properties of a representative roadside soil provides baseline data on edaphic restrictions and hints for correction and amelioration. The soil depth is inadequate for normal root expansion of trees, is excessively stony and sandy, and is beset by poor structure and heavy compaction. Root growth is likely to be restricted by the low levels of porosity and high shear strength. The densely packed surface layer reduces aeration and water movement into the lower layers. The nutrient stock is meager, and the ability to hold available moisture and nutrients is very limited. Soil reaction comparing with local natural soil is uncharacteristically alkaline with pH reaching 8.5 which may induce iron and manganese deficiency. Heavy metals register elevated concentrations especially for lead and zinc, but do not attain phytotoxic levels. The common multiple substrate problems need to be addressed properly before trees are planted. Evaluation of selected soil attributes is advocated for all planting sites.

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