What is Soil? – Definition, Structure & Types

Soil is the material found on the surface of the earth that is composed of organic and inorganic material. Soil varies due to its structure and composition. Learn about the different types of soil and soil structures in this video lesson.

What Is Soil?

Chances are that you haven’t thought a lot about the soil under your feet, but you may be surprised at the complexity of soil. Soil varies in its composition and the structure of its particles, and these factors are closely examined by farmers, who need appropriate soil for planting crops, as well as engineers who may need to understand how soil is going to hold up under different demands. Soil is also vitally important to the sustainability of an ecosystem because it serves as the natural medium for the growth of vegetation. In this lesson, you will discover just what soil is and which factors are looked at when determining the structure and the types of soil.

So, what exactly is soil? Soil can be defined as the organic and inorganic materials on the surface of the earth that provide the medium for plant growth. Soil develops slowly over time and is composed of many different materials. Inorganic materials, or those materials that are not living, include weathered rocks and minerals. Weathering is the mechanical or chemical process by which rocks are broken down into smaller pieces. As rocks are broken down, they mix with organic materials, which are those materials that originate from living organisms. For example, plants and animals die and decompose, releasing nutrients back into the soil.

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Soil Profile and Soil Horizons

Now, if you look down at the soil under your feet, you cannot tell very much about that soil. So, when you study soil, it’s helpful to grab a shovel and dig a hole that is big enough to reveal a vertical section of soil that ranges from the surface to the underlying rock, referred to as a soil profile. The soil profile is somewhat like the soil’s fingerprint, and it will differ from other soil samples based on factors like its color, texture, structure and thickness, as well as its chemical composition.

Each layer of a soil profile is referred to as a soil horizon. These horizons are identified by letters. Horizon A is the upper layer, closest to the surface. You can think of this horizon as the topsoil. In fact, you can use this as a memory jogger to help remember the order of the horizons. The letter A is at the top of the alphabet and refers to the topsoil layer. As you move deeper into the layers of the soil profile, you have horizons B and C, giving us the three main horizons.

Types of Soil

There are three basic types of soil: sand, silt and clay. But, most soils are composed of a combination of the different types. How they mix will determine the texture of the soil, or, in other words, how the soil looks and feels.

One type of soil is sand. Sand within soil is actually small particles of weathered rock. Sand is fairly coarse and loose so water is able to drain through it easily. While this is good for drainage, it is not good for growing plants because sandy soil will not hold water or nutrients.

Silt is another type of soil. Silt can be thought of as fine sand, and it will hold water better than sand. If you were to hold a handful of dry silt in your hand, it would feel almost like flour. If you were to add water to the silt in your hand, it would do a fair job of holding the water and feels slick and smooth.

Clay is very fine-grained soil. Its particles are even smaller than silt, so there is very little space between the fine grains for air or water to circulate. Therefore, clay does not drain well or provide space for plant roots to flourish. If you were a farmer, you would not want your field to be mostly clay. However, if you were a potter, you would think clay was the best type of soil. When moisture is added to clay, it can be molded into shapes, such as a pottery bowl or a building brick.

Now, we can consider loam as our fourth type of soil, even though it is really a combination of sand, silt and clay. Loam will vary depending on how much of each component is present, but generally if you are a gardener, this is the type of soil you want because it holds moisture, but also allows for good drainage. If you were to hold loam in your hand, you could mold it into a ball, but the ball would easily crumble when disturbed.

Soil Aggregates and Soil Structure

So, we learned about the types of soil. This told us about the texture, which really comes down to the size of the particles and how much of each particle is present. Now we can turn our attention to soil structure. The soil structure is based on the arrangement of individual particles of sand, silt and clay. When we have these individual soil particles bound together, they form soil aggregates. These aggregates form distinct shapes, and we want to know about these shapes because they tell us how easily water will penetrate down through soil.

For example, if the soil structure is granular, then it is made up of small crumbs of soil. Water will easily drain down past these small structures, and therefore, granular structure is a good medium for drainage and aeration. However, if the soil structure is platy, then we see a horizontal plate-like structure to the soil particles. This makes water draining down from the surface take a windy, slow path around the plates, making for poor water and air circulation. Platy structure is common in highly compacted soil, such as forest soil and lake sedimentation.

We also have columnar or prismatic structures, which have long vertical dimensions that look like columns of soil. These structures are very dense, and penetrating water has to work fairly hard to travel through this structure. Blocky structure is, as you might have guessed, a block-like structure. If the blocks are large, then water penetration and movement of water will not be good due to the fact that the water must flow around the blocks as opposed to taking a more direct route downward.

Lesson Summary

Let’s review. Soil is the organic and inorganic material on the surface of the earth that provides the medium for plant growth. A soil profile is a vertical section of soil that ranges from the surface to the underlying rock. The soil profile is somewhat like the soil’s fingerprint and contains distinct layers. A layer of a soil profile is referred to as a soil horizon, and we see that there are three main horizons named using the capital letters A, B and C.

There are different types of soil based on the size of the particles and how much of each particle is present. One type of soil is sand, which is small particles of weathered rock. Sand is fairly coarse and loose, so water is able to drain through it easily. Silt is fine sand, and the particle size is in between sand and the next type of soil called clay. Clay is very fine-grained soil with particles that are even smaller than silt. Clay does not drain well. Loam is a combination of sand, silt and clay. Loam is the best soil for gardening because it holds moisture, but also allows for good drainage.

Soil structure is based on the arrangement of individual particles of sand, silt and clay. Soil aggregates is the term used to describe the individual soil particles bound together. These aggregates form distinctive shapes and tell us how easily water will penetrate down through the soil. A granular structure is made up of small crumbs of soil that are easy for water to penetrate; whereas a platy structure is a horizontal plate-like structure that requires water to take a windy, slow path. Other soil structures include columnar or prismatic structures, which have long vertical dimensions, and blocky, which has a block-like structure. Water has to work fairly hard to travel through these structures.

Learning Outcomes

After you’ve reviewed this video lesson, you should be able to:

  • Define soil, soil profile and soil horizon
  • Identify the three main soil horizons
  • Describe different types of soil, including sand, silt, clay and loam
  • Explain the different types of soil aggregates
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