CHAPTER 7: PHYSICAL AND HYDROLOGICAL PROPERTIES OF URBAN SOILS (by Dr. Klaus Lorenz and Dr. Rattan Lal) ABSTRACT: Soils exhibit typical physical and hydrological properties including texture, structure, water retention and transmission. Soil physical properties and processes depend on the relative magnitude of solid, liquid and gaseous phases in soil. These phases are altered by direct and indirect effects of the conversion of natural soils to urban ecosystems. Alterations include physical disturbance, removal or desurfacing, burial or coverage of soil by fill material or impervious surfaces; and soil, water and vegetation management practices (e.g., fertilization, irrigation, mowing, and drainage). However, changes in the abiotic and biotic environment by urbanization indirectly influence also soil physical and hydrological properties. For example, the climate is modified, the soil hydrophobicity is altered, plants and animals are introduced, and a range of pollutants are introduced. Transformation of natural to urban ecosystems may, thus, alter soil stratification, soil texture, structure, water retention and transmission, and probably result in a higher relative proportion of coarse fragments (> 2mm). Urban soils are characterized by high variability in bulk density and saturated hydraulic conductivity. With increasing anthropogenic perturbation, the air-filled porosity and, thus, the air capacity may increase. The field capacity and available water capacity are, however, often lower due to the high contents of coarse fragments. The ability of urban soils to store plant available water is important since most urban soils are used for garden, lawn, park or forest land uses. This chapter provides an overview of the soil physical and hydrological properties, and on effects of urbanization and human disturbance on this fundamental soil functions, with a focus on soil hydrological properties.