ABSTRACT

The physical properties of soils are usually discussed as fundamental properties in distinguishing soils with respect to their behavior in agricultural and engineering operations. They are used in soil taxonomy for placement of soils in different categories, but very little is present in the various soils textbooks on the role of physical properties in the environment. The physical characteristics of soils are the products of environmental factors. They are affected by environmental conditions but at the same time they also affect many of the environmental factors in the soil ecosystem. For example, soil texture is an important physical property that is formed by products derived from the disintegration of rocks and minerals as conditioned by environmental factors, such as precipitation and temperature. Rocks and minerals will produce large amounts of clays only under intense weathering conditions, as a result of high humidity and high temperatures. In arid regions, where water is a limiting factor, normally less clay is formed and the texture is generally coarse. The types of clays formed are also synthesized in nature under the influence of environmental factors. Kaolinite, for example, is formed when

conditions favor the loss of Si by leaching, whereas smectite can be produced only when environmental conditions are suitable for accumulation of Si. The texture formed, coarse or fine, affects plasticity, stickiness, and dispersion of soils, factors of importance in erosion and denudation. These are natural forces that form the landscape on the surface of the earth. Together with soil structure and several other physical properties, soil texture affects pore spaces, aeration, water flow, percolation and leaching, hence drainage conditions, topics that have been addressed in Chapters 4 and 5. Rate of water intake, waterholding capacity and water-supplying power are all influenced by soil texture and soil structure. Bulk density, another soil physical property, is a good measure of the density and porosity of soils. However, the concept of bulk density takes into account both the mass of the solid particles and the volume of pore spaces. Consequently, soil texture and soil structure are indirectly connected to bulk density. In fact many of the physical properties are interrelated, and in particular in their functions it is often difficult to say that they are the result of the action of a single property. For example, percolation is affected not only by soil texture, but soil structure and soil density have equally important roles in this flow of water. Several different physical properties usually complement each other in their effect on the flow of soil water. This chapter will attempt to address the major physical properties of soils in relation to the environment. Names such as environmental soil physics and physical edaphology have been suggested for such a topic. The most important of the physical properties will be addressed below, such as soil texture, structure, density and porosity, consistence and temperature. The soil texture will be discussed under the heading of particle size distribution, and soil temperature under thermal properties for reasons explained later in the respective sections. The basics of these properties are presented as background material to enhance comprehension of the ensuing discussions related to some environmental aspect conforming to the title of the book. For a more detailed discussion on the physical properties of soils related to soil survey and soil taxonomy, reference is made to Miller and Gardiner (1998), Brady and Weil (1996), Foth (1990), Hillel (1980), and Soil Survey Staff (1962).