There are 7 soil horizons beneath the surface of the Earth. Each has a unique mineral content and variation in texture, but all contribute to the health of the soil of an area and how well plants grow overtop.
What Are Soil Horizons?
Soil horizons, according to Britannica, are layers of soil that are underground, which “develop from the combined actions of living organisms and percolating water.” This definition basically means that many living creatures and other forces of nature shape the soil in a way that creates identifiable layers. You can take a vertical piece of soil from the ground and see noticeable changes as you look down the sample – these are the soil horizons.
Many factors in the environment can contribute to the formation of soil horizons, such as the topography (physical features in the land), its parent material, nearby climate, and the length of time the soil’s components have spent in the ground. Anything that moves, compacts, or erodes the soil can cause a soil horizon to form, or it will at least affect the overall mineral content.
After a long time, when these soil horizons begin to form, they will start to have distinctive differences in their characteristics. The factors that played a part in the horizon’s formation are what give it its color and texture. The color of the soil horizons helps identify the different layers when analyzing the soil profile.
Each soil horizon is at a unique range of depth, but the thickness of the horizon can increase or decrease depending on the area. The soil horizons under mountain ranges and hilly slopes are much different than the soil horizons under a flat field or plateau. Whether the horizons are thick does not necessarily depend on the size of the hills.
Soil horizon depths can range from a few centimeters to several meters, but will always stay in the same order – horizon O, A, E, B, C, and R. The soil horizon, O, starts at the topsoil and the other horizons are below it. Horizon R is the deepest horizon, which represents the bedrock at the bottom of the soil, but it is not actually a component of the soil and consists mainly of rock.
The 7 Soil Horizons
There are 7 soil horizons in total. These include horizon Oi, Oa, A, E, B, C, and R. As you may have noticed, horizon O is split into two types – Oi and Oa. We will discuss both, but it is important to recognize that they are much different in their composition and their effect on soil type.
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Horizon Oi and horizon Oa are both located in horizon O.
The Oi horizon is the part of horizon O that contains the uppermost materials in the soil. Slightly decomposed organic matter, such as the remains of plants and animals, are what create this layer, and they will slowly push down into the deeper horizons over time.
The organic material in the shallowest part of this horizon is saturated with moisture due to its exposure to the weather and other environmental forces that do not usually reach the other layers of the soil.
The Oa horizon is just below horizon Oi, and also has decomposed organic matter. However, the organic matter in this section of the O horizon is slightly more decomposed and has been sitting in the soil longer. Typically, the Oa layer of the soil is less saturated than the Oi layer.
Once decomposing organic matter goes through the first horizon, it moves into Horizon A. When this movement happens, the soil is then referred to as eluviated soil.
By this point, the organic matter has turned into humus, which is the dark material that forms when organic matter finishes decomposing. The humus provides vital nutrients to the soil and acts as a natural type of fertilizer for the plants that are still alive in the topsoil. Once organic matter reaches this stage, it looks less like leaves, twigs, and bones, and more like soil.
Although Horizon O is the top horizon in most soils, some have only a horizon A; in which case, it would the surface horizon.
In horizon E, the humus that formed in the previous horizon is now lacking nutrients and various minerals, such as iron and aluminum. When plant roots pull these nutrients out of the soil, it starts to become lighter in color and pushes farther into the soil to become this horizon.
The texture of horizon E is primarily sand and silt particles. Typically, soils have a balance of sand, silt, and clay. However, when the soil reaches this horizon, it no longer contains the same amount of clay. This is because of the loss of organic matter and minerals.
Horizon E is typically light brown and sandy.
Ironically, horizon B tends to have more content in the soil than horizon E. The B horizon is older and also has more structure, which has built up over many cycles of the soil.
The B horizon has a higher concentration of silicate clay compared to the E horizon, and it also contains an increased amount of minerals, such as iron, aluminum, gypsum, and silica.
According to ScienceDirect, horizon B can also show signs of the following:
- Evidence of the removal of carbonates
- Residual concentration of sesquioxides – coatings of these sesquioxides make the horizon lower in value and more colorful
- A granular, prismatic structure
Overall, horizon B acts as a buffer horizon between the upper layers and the deeper horizons that have more rocks and stone.
Horizon C is the bottom layer of the soil, which is also called the substratum. This layer has unconsolidated earth material.
Horizon C is substantially different from the other horizons since it has not undergone the same soil-forming factors that effected the upper layers of the soil.
Horizon R is not part of the soil, but it is important to recognize because it acts as the foundation of all the other horizons in the ground. Horizon R is the bedrock, which consists of hard, consolidated rocks and stone that are practically impenetrable.
Importance of Soil Horizons Profile
Soil scientists (pedologists), agricultural experts, gardeners, archeologists, and anyone else who researches and handles soil must use a soil profile to find out vital information about its contents.
Extracted sections of soil (the soil profile) show the soil horizons and how they compare to each other. Anyone analyzing the soil layers and the materials within can learn about the origin of the soil, including its parent material, and well as any other useful information about the mineral contents.
In agriculture, a farmer can use this type of information to adjust aspects of the soil like the pH of the soil or its nutrient content.
Certain crops need specific minerals and nutrients to produce a fully mature harvest. Without this information, the person taking care of the crops could miss out on an entire harvest season due to stunted plant growth.
For example, when it comes to planting sweet corn, they will need a soil pH between 5.8 and 6.5, 70-80% moisture when planting, and a constant supply of nitrogen and phosphorus throughout the growing season. There are many more requirements for planting and growing sweet corn, but these specific requirements are elements that can be checked through an analysis of the soil profile.
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If the topsoil (or horizon at the planting depth) has the nutrients and minerals a plant crop needs, then there will be less maintenance throughout the season. However, if the soil is lacking but has other concentrations of minerals and nutrients, it may be a good idea to rotate the crops or combine nutrient-rich additives to the soil.
Farmers are more likely than gardeners to need a thorough soil analysis. Typically, for home gardening soil, you do not have to view the lower soil horizons because of the fact that most garden plants remain in the topsoil. Soil nutrients and pH are common factors that gardeners check and adjust before planting their seeds.
How to Extract A Soil Profile
A soil profile, if extracted correctly, should show multiple of the soil horizons in one piece, or adjoining pieces.
Soil profiles often come from one area of the ground and will not give accurate results if pulled from more than one location. The goal of extracting a soil profile is to find out the mineral content of a specific area of the soil.
To create a soil profile, you must dig a hole. Find a spot in the soil where it will be the least difficult to dig up a decent-sized hole. Pay attention to nearby plants to avoid breaking any roots and dig a hole that is at least a foot in diameter.
Gardeners may only need a small piece of soil, but the larger the soil profile is, the more it will tell you in the analysis.
Quickly dig a hole that is a few feet deep – 2 to 3 feet will do.
Try to look for a noticeable difference in the soil at the bottom.
Once the hole is deep enough, take a shovel or a gardening tool and scrape one side of the hole to make one long, flat piece. This method will help with comparing the different soil horizons and measuring their depths, but you can also pull a small amount of soil from each horizon as you dig the hole and place them in separate contains for examination.
Soil Test Kits
Some soil test kits allow for a quick analysis of certain aspects of the soil. For example, there are numerous test kits for testing soil pH, but not as many for testing nutrients.
Here are a few soil test kits that you should consider using for your soil profile analysis. Be sure to write down your observations before using the soil in a test kit, so that you do not have to extract a second soil profile.
Frequently Asked Questions About Soil Horizons
Q: What soil horizons are impermeable?
A: Horizon R, which is at the very bottom of soil (the bedrock) is impermeable because of the compact rock that forms the horizon. However, other soil horizons can be impermeable as well, if there is a dense structure of soil, rock, and no cracks or gaps through which water could seep.
Q: What soil horizon is subsoil?
A: Horizon B is the subsoil. It is rich in minerals due to contents that have moved further down into the soil from the upper layers. Horizon B can contain high levels of iron, aluminum, gypsum, and silica clay.
Q: Why do soil horizons form?
A: Soil horizons form because of the effects of nature. If all the water, wind, and animals remained completely still on Earth, all the time, soil horizons would not form because there would be no means for movement of the soil. Plants and animals of all sizes are equally important to the formation of the soil.
The elements in soil horizons can provide knowledge to anyone who is looking to learn more about their soil. Gardeners can use the information they gain from horizons to grow taller plants, and farmers can use it to grow healthier crops.
The analysis of soil is easy, and it is possible to extract and examine a soil profile right at home. However, the deeper layers of soil are harder to reach and are not necessary for the soil analysis of small areas of land. The most useful information comes from the top soil layers, such as horizon O and horizon A.
Soil horizons are informative because they are a collection of everything that has gathered in a specific area of soil since its original formation. Of course, minerals that were present in the original, parent material may not exist so much now, but if they do, you could find them at the deepest point in the soil. Soil formation takes hundreds of years – it is an untapped database until someone digs it up.