Subsoil

Underneath the cover of growing plants, leaf litter, and crawling insects is a layer of dark, nutrient-rich topsoil. Below that is something else entirely.

Flooding exposes lighter colored subsoil (USDAgov) CC BY 2.0

We are all familiar with the importance of topsoil and how it helps our plants grow, but the subsoil layer is also very important to plant health. Do you know what is in yours?

What is in subsoil?

Subsoil is mainly weathered rocks and clay. It contains little if any organic matter, so it is often lighter in color than the soil above it. But this doesn’t mean it has nothing to offer your plants. This is where gypsum, silica, and clay particles filter down and collect. The clay found in subsoil has been used throughout human history to make adobe. It is also the material of choice in wattle and daub fencing. But clay isn’t the only thing that collects in the subsoil.

Aluminum, calcium, iron, magnesium, and other nutrients accumulate in the subsoil, as well. If there is a lot of iron present, the subsoil layer will have a more brown or reddish tint. Because these minerals are often moved by percolating or illuviated water, it is also called the illuvial horizon. The subsoil layer often has a distinct soil structure from the layers above and below.

Subsoil layer (David Brinicombe) CC BY-SA 2.0

How do plants use subsoil?

All of a plant’s early growth takes place in the topsoil layer, where nutrients, helpful microorganisms, and water are in abundance in most cases. After that initial growth, many roots move into the subsoil. The subsoil provides anchorage, food, and water. Tucked into the spaces between the minerals and tiny rocks that make up subsoil are pockets of water and mineral nutrients. Water held in the subsoil is protected from evaporation. When topsoil is dry, water can still be found in subsoil.

In the photos below, each block represents ​one square foot of soil.

Cabbage root system 55 days after transplanting (Soil and Health Library) Public Domain

Identical cabbage 20 days later (Soil and Health Library) Public Domain

Subsoil, erosion, and compaction

Rototilling, construction, and heavy traffic can strip away topsoil, exposing the subsoil. Because subsoil does not contain the same organic matter, microorganisms, and root systems that hold topsoil in place, erosion occurs at a much higher rate. Those actions can also lead to the creation of a hardpan layer that blocks air, water, and roots almost completely.

Compacted soil interferes with plant growth, drainage, and overall soil health. It is often corrected by aeration and deep-rooted cover crops. Compacted subsoil can be detrimental to plant health for many years. One study found that a compacted subsoil layer affected plant growth for nearly 20 years after the compaction occurred. Correcting subsoil compaction is expensive and difficult. Unfortunately, it is also very common in gardens due to home construction. This is why we are all required to conduct perc tests when buying a home. If water can’t percolate down and away, your home might find itself down the river after a particularly bad rain. [In the past, contaminated soil was used to create an artificial subsoil layer for home construction. Luckily, those days are behind us.]

You can protect your soil and plants by being judicious about rototilling, creating paths, and avoiding walking on wet soil. Subsoil can be converted to topsoil by adding substantial amounts of aged manure, compost, green manure, and time.

What’s in your subsoil?

If you have a soil sampling tool, you can use that to collect a sample of both the topsoil and subsoil layers. Or, you can use a trowel or shovel. You will need to dig down a foot or two. Look for changes in both color and texture. You can take a sample of subsoil and send it out to a lab for testing, or you can test its texture for yourself. You can also test your subsoil’s permeability at home. These home tests won’t tell you which nutrients are present, however.

Take a look at your subsoil and tell us what you find in the comments. Extra points for fossils and treasures!