Unit 9
Water and Soil Resources

Overview

Water and soil are two essential resources for life on Earth. Water and soil are closely linked. Not only does water play a key role in biological processes, but it also controls a broad range of geological processes, particularly those that occur close to the Earth’s surface, such as weathering and soil development.

Water is a major aspect that sets Earth apart from the other planets in terms of Earth’s capacity to sustain biological life. Of all our natural resources, water is the most abundant, covering approximately seventy-one per cent of the Earth’s surface. However, this figure is misleading, as ninety-seven per cent of all the water on Earth is in the oceans and seas. This water is too mineralized for human consumption, for agricultural use, for consumption by livestock, or for many industrial manufacturing uses. Desalinization is costly and time-consuming, and has not proven viable on a large scale. The remaining three per cent of Earth’s water is fresh water, but three-fourths of this amount is frozen in glaciers and polar ice-caps. Consequently, usable water is a much scarcer resource than is water as a whole.

Earth’s other vital resource, soil, occurs abundantly at the interface between the atmosphere and the lithosphere. Soil forms at the Earth’s surface through the progressive breakdown of rock into smaller clasts and through the alteration of mineral species that are ordinarily stable under high pressures and temperatures inside the earth’s crust. These processes, collectively called weathering, can be physical or chemical in nature. Physical weathering is the disintegration of rocks by mechanical action into smaller lithic fragments. It occurs through processes such as frost shattering, wetting and drying, and salt weathering. All of these processes involve water in some way. Chemical weathering, on the other hand, involves the decomposition of rocks by chemical reactions that may alter the mineralogy of the original rock. Such reactions always occur in solution, with water as the solvent. Chemical weathering also frees up nutrients, making them available for plants. Water acts as a transport agent for the products of weathering. For a true soil to form, organic material must be added to the products of rock weathering, which again depends on the availability of water.

The first part of this unit explores the roles of water as a natural resource and examines problems associated with its accessibility and exploitation. The second part of the unit looks at soil, emphasizing soil formation, soil distribution, and land use. It also considers soil erosion and conservation methods.

Objectives

After completing this unit, you should be able to

1. describe the hydrologic cycle and discuss its components.
2. discuss the advantages and disadvantages of dams and channelization for flood control.
3. list and describe the various uses of water.
4. state the causes and discuss possible solutions for the major water problems presented.
5. describe soil formation processes.
6. outline the important properties and characteristics used in classifying soils.
7. describe the factors that control the spatial distribution of soils.
8. explain the role of soil type on land use.
9. outline the various forms of soil erosion and their effect on soil quality.
10. describe basic techniques used in soil conservation.