Products of Weathering - Soils
(Click for larger image) Soil characteristics change with observed changes in vegetation and precipitation along a line from Wyoming to Wisconsin. Precipitation decreases from east to west, so vegetation changes from forest in Wisconsin, to grassland in the plains states, to desert scrub in northeastern Wyoming. Organic-rich A horizons are thickest below the tall-grass prairie of Minnesota. Calcite accumulates in the B horizon only in the western, dryer region. The depth to the calcite accumulation increases as precipitation increases, because the dissolved calcite is carried deeper into the soil where there is more water moving through the soil. (Image source: How Earth Works, Pearson)
Soil is a class of sediments with unique properties - soil is defined as Earth material near the surface that has been altered as a result of weathering, includes air, water and organic matter, and is capable of supporting plant life. Soils are susceptible to erosion, just like sediments. The erosion of soil is problematic for growing crops (think, Dust Bowl) and can contribute to pollution issues in other areas. Please review pages 140-144 in your text for a review of the causes and issues of soil erosion.
Soils form as a function of the following factors—which are also factors in the weathering process:
- Slope or topography
- Parent Material
- Biologic Processes
Review the details of these factors in Chapter 5 of your text. These factors can be interrelated. For example, topography affects vegetation and climates. Topography, parent material, and vegetation can impact soil drainage. If you change any one of these factors—even just a little bit, a different soil results. In Indiana, Marion County alone has 32 different types of soils. The United States has tens of thousands of different types of soil, all dependent on the five factors listed above. Review these in Table 5.2 and Figure 5.19 of your text. Can you see how differences in the soil forming factors might create different soils on Earth?
Soils are characterized by their soil profile. Soils develop into horizons or layers, where each horizon contains unique properties. The type of horizons present, their thickness, and their unique characteristics all depend on the soil forming factors. A typical soil profile is shown below; please take a few minutes to review the layers and their differences. You will see that the layers are a result of chemical and mechanical weathering and transport processes. Not all soils have all of these horizons prominently displayed. Younger soils may not have the E and B horizon, and only a small O and A horizon. Compare this to figure 5.18 in your text to understand the differences in soil horizons.