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Lecture 1 - Soil Formation

Narrator:Listen to part of a lecture in the geology class.

Professor:So, we all know soil. It’s important to plant growth, right? And we know that there’re different types of soil in different places and that some soils are more fertile than others. But what is soil? And how’s it formed? Well, we’re going to go into this in some depth, but for now let’s just lay down the basics.

Soil is composed of two kinds of material: inorganic material, basically small pieces of rock, and organic material, which is animal and plant matter.

OK. So, what do you think? If I mix bits of rock with composted vegetables, will I get soil? The answer is no, because the formation of soil is a dynamic process. It involves not only the initial inputs, the raw materials, but also the transformation of those materials, and the movement of some of the materials and the loss of others.

So, the inputs are bits of rock and organic matter.

Now, the bits of rock, the inorganic input to soil, uh…they come from the breakdown of rocks on Earth’s surface through a process called weathering.

Weathering can be either physical or chemical. Physical weathering, uh…that’s when exposure to the elements over time causes a rock to break up and eventually disintegrate. Uh… of course, some rocks are more resistant to physical weathering than others. If you think of the sand particles in soil, those are the result of physical weathering, and they have the same chemical composition as the original rock.

Now, chemical weathering, uh… that’s the chemical breakup of rocks. It differs from physical weathering in that the chemical properties of the minerals are actually changed. The clay minerals you find in soil are the result of chemical weathering. Clay minerals are called secondary minerals, because their composition has been altered.

Okay. So we have weathered rock, which needs to be combined with organic matter.

So what does the organic input consist of?

It’s the remains of plants and animals, but mostly plants. Now, just as rocks are broken down by weathering, the animal and plant residues are broken down, too.

They’re reduced to simple chemicals by microorganisms in a process called mineralization. And just as some rocks are more resistant than others to weathering, the compounds found in the soil’s organic input resist mineralization at different rates. The compound cellulose is the major constituent of most plant tissue. It mineralizes relatively quickly. But there are woody substances in certain plants that strengthen the cell walls. They are found in smaller concentrations and their mineralization can take several years.

Weathering and mineralization transform the inorganic and organic inputs in a number of ways. And it’s partly from these transformations that soil gets its unique properties.

How does it work?

Uh… take the dark brown color of soil. After microorganisms have broken down the cellulose, we’re left with two things: the microbe’s waste and the more resistant plant material that microorganisms can’t break down easily. These materials ultimately get transformed into a new material called humus. And when humus is combined with the clay minerals in soil, that’s what gives soil its dark brown color.

So now, if we’ve got clay and humus, these transformed materials, and we mix them together, we’ve got something very close to soil.

But soil isn’t static, and there’re still other processes that go into the formation of soil: the movement and loss of materials. The soil in any location isn’t a uniform mixture.Its composition varies with depth. You see, mineral and organic materials move through soil vertically.Some materials move more easily than others. Water carries the more mobile materials from the upper level of soil to the lower levels.So the upper levels of the soil eventually get depleted of these materials, while the lower levels get enriched with them. And that creates distinct layers of soil as far down as the rock underlying the soil. And the materials that dissolve easily in water can get lost completely if the water carries them horizontally out of the soil and into rivers.

Now, of course, new mineral and organic material will be deposited at the surface and become incorporated into the soil, but you see how the processes of movement and loss contribute to the formation of soil.