Urban soil characteristics and limitations for landscape planting in Hong Kong

Abstract

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.

Introduction

Trees in cities can be considered an anathema, for as natural objects they have to live in an overwhelmingly artificial habitat. They are beset by many subaerial and subterranean stresses (Grey and Deneke, 1986; Miller, 1988; Bradshaw et al., 1995). Urban tree management, an applied aspect of urban ecology, is a continual attempt to mediate between the two interacting if not conflicting components. Most city trees have a life-span much shorter than their biological potentials (Gilbertson and Bradshaw, 1985; Insley and Buckley, 1986) with growth in a constant state of turmoil. Urban trees in Hong Kong, with an exceptionally high-density development, poor environmental quality, and frequent construction damages, have to struggle to survive (Jim, 1986a, Jim, 1987a). Recent city-wide surveys of street trees verified widespread poor performance (Jim, 1994, Jim, 1996a).

Poor site condition is the main cause of unhealthy urban trees (Jim, 1992, Jim, 1997a, Jim, 1997b). Urban soil researches conducted elsewhere confirm the severe constraints imposed by inadequacies and obstacles in the subsurface environment (summarized in Bullock and Gregory, 1991; Craul, 1992; Watson and Neely, 1994). Unfortunately, the soil is out of sight and difficult as well as expensive to assess using conventional means (Watson, 1994). It is too often taken for granted if not neglected altogether (Roberts and Roberts, 1986; Patterson, 1990). Urban arboriculture and horticulture practice has meager objective and empirical data to make sound judgements or recommendations. As a surrogate, the vast quantity of information and experience in agriculture and forestry has had to be borrowed, but the extrapolation can become far-fetched if not untenable.

Field evaluation of urban soils in an extensive range of sites in Hong Kong attest a notoriously difficult medium for vegetation growth. They are commonly limited by abundant underground utility lines (pipes and cables). Stones, boulders, construction rubbles, mortar, bricks, old paving, old and extant foundations, and other rather immutable obstructions, often frustrate planting efforts and render many sites unusable. Compacted surface and subsurface soil layers, frequently encountered, are restrictive to root spread and compromise the free movement of air and water. Less tangible chemical restrictions are widespread. Local tree planting practice, however, pays only lip service to the distressful state of urban soils, with little attempt to ameliorate the maladies or to replace them with better materials.

The municipal authority is implementing an ambitious planting programme. The last tree survey (Jim, 1994) found some 12,000 sites for new street trees in Hong Kong. Further urban expansion and urban renewal will provide more planting niches. If these new sites are treated in the same old way, similar mistakes of poor soil will be repeated. In time, the young trees will follow the same path as their predecessors and endure a life of hardship. Of the different urban soils (Blume, 1989; Hollis, 1991), the roadside ones are the most restrictive for plant growth (Barlett Tree Expert, 1968; Evans et al., 1990), yet they are expected to support linear tree belts with significant cityscape impacts. The lack of scientific information on urban soils is a serious impediment to the bold landscaping effort to enliven the otherwise drab city milieu. A better understanding of this fundamental resource base can enhance the cost-effectiveness of urban-tree programmes and reduce tree-management burdens.

This study evaluates the physical and chemical properties of urban soils in Hong Kong, with an emphasis on tree growth, through field assessment and laboratory analysis. The major soil limitations could be identified and remedied. As urban soil research has been largely ignored by soil scientists, empirical data have to be acquired at first hand. This study therefore attempts to encompass a wide range of soil properties, with a view to contributing objective-empirical information. It is hoped that the baseline findings can provide practical hints on soil management, namely to prescribe ameliorative measures in the form of physical manipulation, to add soil amendments, and to determine the need to replace site soils in extreme situations. Due to a lack of relevant systematic information, the study encompasses a large number of samples to cover a broad spectrum of site conditions.