2-1 Soils
Soils are formed from weathered rocks, and so it’s important to understand some of the processes of rock weathering summarized in the textbook, including mechanical weathering (as discussed in section 10.1) and chemical weathering (section 10.2).
The common products of mechanical weathering are rock fragments, sand grains, and clay mineral fragments. The common products of chemical weathering are clay minerals, iron oxide minerals, and ions in solution.
Exercise 2-1 Processes of Chemical Weathering
Please fill in the blanks in the statements below.
The formation and characteristics of soils formed under different conditions and from different parent materials are summarized in section 10.3 in the textbook. Figure 10.3.1 shows the range of variation in the composition of soils, but its critical to remember that this is just the mineral component. As described, soil also has organic matter and spaces that are filled with air or water.
Make sure that you understand the various factors that play a role in soil development. The two most important of these are the parent material and the climate, as the former will determine what raw materials are available for the soil to develop from, and the latter will determine the types of weathering and soil-forming processes that can take place and how much plant matter will be available.
An example of how soil might develop over time is provided on Figure 10.3.3. The process starts with the exposure of bare rock—and that might have resulted from glacial retreat, or a slope failure or a high-energy flood that eroded away the existing soil. The starting point might also have been an event—like a flood—that resulted in the deposition of a layer of loose sediment.
If the starting point was bare rock, then both mechanical and chemical weathering would have been necessary for soil development to begin, providing an environment for early plant growth. Over time, weathering would have resulted in the conversion of solid rock into smaller clasts and mineral grains (including clay minerals), and plant growth would have contributed organic matter.
As described in the textbook, the development of soil horizons is facilitated by upward (by capillary action) and downward (by percolation) transportation of ions in water solution. These processes are highly variable depending on the climate (warm, cold, wet, dry), and so different types of soils develop in different regions.
The various types of soil in Canada are described in section 10.4 of the textbook.
If you live in Canada, use Figure 10.4.1 in the textbook to figure out what type of soil is likely to be present in your area, and try to understand why this is the case.
If you live in a different country, you might be able to find an applicable soil map for your region, although bear in mind that soil classifications differ from country to country.
