Soil Horizons

Soil is fundamental to the health and productivity of your plants. Healthy soil supports healthy plants. But there’s more to soil than meets the eye

Soil layers tend to get progressively lighter in color (Chris Yeates) CC BY-SA 2.0

You’ve heard a lot about topsoil and maybe some about bedrock, but what about the layers in between?

The layers of soil under your feet have developed over eons of glaciation, erosion, flooding, and thousands of other circumstances of pedogenesis. Underneath it all is your soil’s foundation, or bedrock. Of course, if you go deep enough you’ll reach the molten core, but that only affects gardeners in places like Hawaii. For most of us, it is only the top few feet that dictate the health of our plants. There are several different layers and those layers are called horizons.

There are different classes of thought on soil horizons. Some groups define these layers by the soil texture, while others prefer sorting them by use. Keep in mind that not all soils have all these layers. And some soils have astounding numbers of layers.

Soil horizons (Wilsonbiggs) CC BY-SA 4.0

Topsoil (O, A, and E horizons)

Topsoil refers to the uppermost 5-10” of soil. It has the highest concentration of organic matter and microorganisms. The majority of your plants’ roots can be found in topsoil. It is divided into three different layers. The organic surface layer, or O horizon, is where we find plant litter. In areas with waterlogged peat areas, we have the P horizon. Under that, we have the surface soil or A horizon. In some cases, leaching (or eluviation) can cause certain minerals to create a separate, lighter-colored layer between the topsoil and subsoil. This lighter layer is called the E horizon and it often contains a lot of silica.

Subsoil (B horizon)

While not as rich in organic matter as topsoil, the subsoil layer is where many minerals accumulate. This is especially true of aluminum, clay minerals, iron oxides, and some organic bits. The subsoil layer is commonly reddish-brown due to its iron content and it often has a distinctly different soil structure than the horizons above it. The subsoil layer is often overlooked, which is unfortunate since all garden plant roots make their way to this layer. When the topsoil and subsoil have gone through the same soil-forming conditions, the combined areas are called the solum.

Substratum (C horizon)

The substratum layer consists of partially weathered rocks that have not been affected by the conditions that created the layers above. It contains a lot of calcium carbonate and other soluble minerals and tends to be a lighter color than the layers above. This layer is often created by flooding and landslides.

Bedrock (R horizon)

Unlike horizon C’s rocks and boulders, which you can dig up, the bedrock layer is massive layers of rock that make up more of what we consider the earth’s crust.

Other horizons

Limnic soil (L horizon) occurs as a result of aquatic life. It contains the diatomaceous earth, sedimentary peat, and marl left behind when lakes and oceans dry up. You can also have a fluid or frozen water layer (W horizon) or man-made layers that block roots (M horizon).

The next time you pass a construction site or look at an image of the Grand Canyon, take a closer look at all those layers and think about how they might affect plant roots. The closer you look, the more there is to see.