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The Formation and Composition of Soil: Definition and Factors

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  • 0:05 What Is Soil?
  • 0:56 How Soil Forms
  • 3:39 The Layers of Soil
  • 5:27 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

In this video lesson you will learn how soil is formed and what it is made of. You will also learn about how soil is classified and distributed in layers underground.

What is Soil?

Remember playing in the dirt as a kid? You probably didn't put too much thought into how that dirt was formed or even what it was made of. Dirt is dirt, right? Well, actually, that dirt is called soil, and soil is a mixture of mineral and organic material that sits just below Earth's surface. Soil is far from the simple dirt you may be picturing - there are over 20,000 different kinds of soil just in the United States!

How do scientists tell the difference between them? They can look at the soil's physical characteristics such as pH, color, particle size, permeability and water-holding capacity. Soil is very important because it provides a place for plants to grow and contains a lot of living and non-living material, like rocks, minerals, bacteria, animals and nutrients.

How Soil Forms

It takes a very long time to make soil, sometimes a thousand years or more. Basically, when surface rocks break down, they mix with decaying organic material, like plants and animals. As this organic material breaks down, it returns nutrients back to the ground, which provides food for plants. Specifically, there are five main factors that create soil: parent material, climate, biology, topography and time.

Let's start with parent material. This is the material that new soil forms from. Parent material includes both mineral and organic material. The organic material is usually very dark and spongy, and this is also known as humus. The mineral material comes from sediments and weathered rocks, and the type of mineral material present helps determine which type of soil will form and how long it will take to form.

Climate affects soil formation because temperature, moisture, rainfall and wind influence mineral material weathering and the production of organic matter. In climates that are warm and moist like rainforests, plants grow much faster and more consistently throughout the year. This creates a soil with more organic matter than a climate that is dry and cool, but this organic matter also gets broken down faster, so there is less accumulation in the soil.

Soil formation is also impacted by biological influences such as plants, animals, bacteria and fungi. As we saw before, plants help recycle nutrients by decaying as well as by taking up nutrients. Plants also put down roots into the soil, which helps anchor the soil in place and prevent erosion. There are millions of tiny organisms in the soil that you can't even see, and they help mix the soil and recycle nutrients.

Topography is the shape of the land, including the steepness and features like mountains, depressions and floodplains. Remember that soil is affected by water and the sediments and rocks that are present? Well, if the land is very steep, there will be more runoff from rainfall, which will transport more rocks and minerals. This increased level of erosion also means that there will likely be less organic material, and, as we know, this also influences the soil.

Finally, time plays a critical role in soil formation because the interaction of all the previous factors is a slow and continuous process. It takes a long time for sediments to be transported and weathered, and organic material needs time to decay.

The Layers of Soil

As soil forms, it becomes sorted. Horizons develop, which are the layers of soil with distinct characteristics that are parallel to the surface of the ground. To determine the types of soil present, scientists will use a soil profile, which is a vertical section of soil from the surface. There is no limit to the number of horizons a soil profile can have, but there are six major horizons that are typically used to describe the soils in a profile.

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