a1 Senior Lecturer in Biogeography & Soil Science, Department of Geography and Geology, University of Hong Kong, Pokfulam Road, Hong Kong.
Of the many forms of above- and below-ground stresses facing urban trees, physical soil limitations are generally among the most persistent and least amenable to amelioration. Soil compaction is a common soil malady that afflicts many planting-sites and causes tree decline. The concept of soil structure in terms of porosity and moisture-suction is reviewed, to provide a basis for the interpretation of compaction as structural degradation. The reorganization of solid and interstitial void constituents, which can result in increased dentity of packing, is related to some fundamental physical and physico-chemical soil properties.
Compaction brings about changes in a wide spectrum of related geometric and other soil attributes, the consequences of which are often inimical to plant growth. Many deliberate and inadvertent actions in urban areas increase packing density, which can be diagnosed through some field and laboratory tests. The harmful effects of compaction is exerted via mechanical impediment and other indirect impacts on root growth. Excessively loose soils and soil crusts are discussed in the light of compaction concepts and in relation to tree growth.
A wide range of measures could be adopted to alleviate the problems of soil compaction. Preventive actions taken before tree planting are preferred to post-planting remedies which are cumbersome, and injurious to roots. Soils at planting sites could be partially improved by various mechanical means to reduce packing density and enhance aggregation. Coarse-textured materials with inherently high strength and resistance to dense packing could provide a stable medium for tree growth. The installation of subsurface aeration and drainage pipes could alleviate the ill-effects of compaction. Care should be taken to avoid structural degradation in stockpiling and construction activities.