Soil Profile: Definition, Development & Types

Soil Profile: Definition, Development & Types
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  • 0:00 What is a Soil Profile?
  • 0:44 How Does the Soil…
  • 3:25 The Horizons
  • 5:26 Different Kinds of…
  • 7:17 Lesson Summary
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Lesson Transcript
Instructor: Jordana LaFantasie

Jordge teaches college Biology, Ecology and Environmental Science. She has a Doctorate degree in Agronomy.

Explore the characteristics of soil profiles, their horizons and how they develop. Then you can use this knowledge to help you determine the conditions under which the soil developed and understand the history of the surrounding landscape.

What is a Soil Profile?

On the surface, soils look pretty much the same… dirt. The soil profile is where the secrets of the soil and landscape around it are hidden. The soil profile is defined as a vertical section of the soil that is exposed by a soil pit. A soil pit is a hole that is dug from the surface of the soil to the underlying bedrock.

Because of the way soils develop, most soil profiles are composed of a series of horizons, or layers of soil stacked on top of one another like layers of a cake. These horizons can tell us a lot about how the soil formed and what was going on around the soil in the past, much like a diary of the landscape.

How Does the Soil Profile Develop?

Let's pretend we're looking at a brand-new landscape surface. It could be a bed of lava that just cooled, some rocks and minerals exposed by a receding glacier, debris laid down by a flooding river, or many other things. We call that brand-new landscape surface parent material, and at that point, it is new and not technically soil!

Will it remain that way forever? No… it won't take long before it can be called 'soil,' which just means that the parent material has been altered, or weathered, in some way by the five soil forming factors.

While we discuss the soil forming factors, think about how they might influence a parent material differently in different locations or situations.

An easy way to remember the five soil forming factors is to remember the word ClORPT, where Cl stands for climate, O for organisms, R for relief (another way of saying topography), P for parent material, and T for time.

The climate in which a soil is developed determines many things, most importantly, the amount of water that will flow, or leach, through the profile. It might seem odd that water dissolves rocks or minerals, but the more water a parent material is exposed to, the quicker it will be weathered. Freeze/thaw cycles and other climatic factors also weather parent material.

Organisms, especially plants and soil microorganisms, do a lot to weather parent material by producing acids and other organic matter. Different groups of organisms have different effects on parent materials. For example, trees in a forest have different root systems and produce different organic matter than grasses in a grassland.

Relief, or topography, influences where water and other materials accumulate on or leave the landscape. For example, the bottom of a hill will receive more water than the top because water runs down the hill. So, the parent material at the bottom will have more water leaching through it than the parent material on the top, so the soils will eventually look different.

Parent material, which we've been discussing all along, is what is being altered into soil. While it will change over time because of weathering, a soil with a parent material like basalt lava will be different than a soil with a parent material like beach sand, because the parent materials are so different chemically and physically.

Time is the last soil forming factor. Soils take a very long time to develop; new soils do not have distinguishing profiles or horizons. But given enough time and the other four soil-forming factors, soils develop interesting and storytelling horizons.

The Horizons

Now that we know the different factors that drive soil development, we can move on to the features the development creates, giving each soil its unique profile. Again, most developed soils have horizons that can tell us about the type of environment in which the soil was produced. The most commonly referred-to horizons are easy to remember: they are the O, A, E, B, and C horizons.

The O horizon, when it is present, is the uppermost horizon. The O stands for 'organ.' You can think of dead plants and animal parts falling on the surface of the soil. Sometimes it's very recognizable and sometimes, the organic material is decomposed enough that it just looks like muck.

The A horizon is usually the first mineral horizon. Often referred to as the topsoil, the A horizon is rich in organic matter, which gives it its characteristic brown color. In fact, many soils don't have a true A horizon because there isn't enough organic matter to make the surface horizon brown enough. Typically, tilled crop agriculture takes advantage of the nutrient richness of the A horizon.

The E horizon follows the A horizon if it is present. The E stands for 'elluvial,' or very leached, and has been very weathered by water and organic acids and, depending on the parent material, may be white. In soils with very low productivity, such as desert soils, the E can be the uppermost horizon because there isn't enough organic matter production to develop an A horizon.

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