Lecture
Unlike last module, you should read through the textbook before completing this Module Lecture. Your textbook does a really good job of explaining the structure of soil into horizons (Section 17.2, pp 590 – 592), Soil Properties (Section 17.3, pp 592 – 594, and Section 17.4 (p 595), Soil Classification (Section 17.6 (pp 596 -598), and the Engineering Properties of Soil (pp 599 – 602). In this Module Lecture, we will learn more about how soils develop as it will reinforce how the different Earth Spheres interact through Environmental Unity. This discussion on soil development is introduced on page 594. We will also expand on section 17.5 (p 595 – 596) as it relates to the some Engineering Properties of soil that are important to Indiana, and finally, we will discuss more issues with soil use and pollution in the Case Study section of this Lecture, expanding on Sections 17.9 – 17.10 (pp 603 – 610).
This picture from a vertical roadcut shows the relationship between rocks,
sediment, and soil. The soil is the top most layer in this picture, followed
by two layers of sediment deposited by past glaciers. Underneath this
sediment is the rock limestone (V. Hernly/IUPUI Earth Sciences).On to the Lecture…
What is Soil?
Soil isn’t dirt. Dirt can be any pile of loosely held together sediments. But soil is very different. Just ask an earthworm. If you ask a soil taxonomist what soil is, she might say:
Soil is a natural body comprised of solids (minerals and organic matter), liquid, and gases that occurs on the land surface, occupies space, and is characterized by one or both of the following: horizons, or layers, that are distinguishable from the initial material as a result of additions, losses, transfers, and transformations of energy and matter or the ability to support rooted plants in a natural environment (Soil Taxonomy, 1975).
This is a bit more specific than your textbook definition, but the important thing to remember is that soil forms layers (or horizons) and soils must support rooted plant life. So, when you buy a bag of “top soil” from Lowes, you aren’t purchasing soil in the geological definition of the word; you’re purchasing a bag of dirt. Until you put it in the ground and it begins to form layers and plants grow in it, it’s not technically soil.
In order to support rooted plant life, soil needs to have minerals and nutrients that plants need. This is provided for by rock sediments and the organic material called humus (broken down or decomposed bits of plants and animals). Plants also need water, but not too much that it drowns them. So, soil must have a good combination of water and air. To do this, soil needs to have spaces between the particles. All-in-all, soil is the perfect mixture of rock sediments, humus, water, and air to allow plants to grow and thrive.
Below is Figure 17.3 in your textbook with a reminder of what each of the layers is composed. Think about the process of weathering as you look at this figure and table. In the next few pages, we’re going to discuss how you get from a rock to these layers of sediments, organic material, water, and air that are necessary to support plants (and us humans).
Table and Figure showing the soil layers or horizons. The figure is Figure 17.3 (p 591), Pearson / Prentice Hall. * Leaching – Leaching is the process by which minerals are dissolved from top layers and deposited into deeper sediment layers.