What Is Subsoil and How Does It Help a Garden?

Between the fertile topsoil and the solid bedrock lies the often-overlooked subsoil, the unsung hero of the soil profile. This vital layer, found just beneath the topsoil, plays a crucial role in supporting plant life by acting as a reservoir for deep water reserves during droughts and a drainage space for excess water during wet periods.

While subsoil is denser and less fertile than the topsoil, it is far from barren. It is home to an array of hardworking creatures such as earthworms, beetles, and grubs, whose tunneling helps break up the compacted soil, improving water filtration and allowing plant roots to reach deeper moisture. Additionally, roots tap into essential nutrients like iron, calcium, and magnesium, which are stored deep within the subsoil.

Despite its humble nature, the subsoil is a key player in maintaining the balance and health of the soil ecosystem. Ahead, learn all about subsoil and how it can help your garden this growing season with help from three gardening experts.

What Is Subsoil?

Subsoil is the layer of soil below topsoil where minerals accumulate as they work their way down from the surface. It is also called “undersoil” or “B soil horizon.”

You might find it a few inches under the topsoil or more than a foot down, depending on your garden and how much your soil has been disturbed in the past.

“It serves as an important storehouse for moisture due to its thicker composition and presence of clay, which has a high water-holding capacity,” says Carmen Eldridge, gardening expert and manager of Arden Farm. “This rougher layer acts as a supporting base providing additional strength to the topsoil above it.”

What Does Subsoil Contain?

Subsoil typically contains more small rocks and clay minerals than topsoil but less organic matter and fewer nutrients, insects and microbes.

The three major components of subsoil are sand, silt and clay.

“The subsoil is indirectly influenced by the activities that occur at or near the soil surface,” says Dr. Anthony Fulford, nutrient management and soil quality advisor at the University of California Cooperative Extension. That means excess fertilizer, herbicides, pesticides and other dissolved salts accumulate here, especially in areas with heavy rainfall or irrigation.

What Does Subsoil Look Like?

Sometimes subsoil can resemble topsoil, but typically it’s lighter in color and more compact. It can be a little sticky due to its high clay content. Because it has fewer roots and animal activity, it also has less structure.

“Therefore, as you dig deeper in the soil profile, if you see the soil color begin to fade from a dark brown or black to lighter shades of gray or brown, you notice it becomes harder to dig deeper into the soil,” says Fulford.

“When you notice the soil is no longer loose and crumbly but becomes denser and lacks plant roots and worm channels, it is very likely you have reached the subsoil layer.”

What is Subsoil Used For?

Subsoil on its own is not that good for growing plants, but it provides a critical layer in the overall health of plants and soil. It is also used as fill soil, a source of clay for building materials, and for absorbing on-site wastewater disposal.

Benefits for gardens and lawns include:

• Supporting the topsoil;
• Absorbing and draining water for the plants growing above;
• Providing minerals to promote plant growth;
• Anchoring taller plants and trees.

How to Maintain Healthy Subsoil

“Maintaining a healthy soil profile isn’t about addressing one horizon, but rather viewing our soils as a viable ecosystem beneath our feet,” says Kathy Glassey, director of renewable resources for Monster Tree Service. “We should never forget the biology that makes all of this remarkable growth occur.”

So if you’re treating your soil as an ecosystem, remember that what goes into the topsoil eventually ends up in the subsoil. It’s important to apply the proper soil care even for subsoil.

To maintain healthy subsoil:

• Regularly mix organic matter and other soil amendments into the topsoil.
• Don’t over-fertilize.
• Don’t compact any soil. Avoid heavy loads, especially when it’s wet.
• Encourage good surface growing conditions.
• Plant deep-rooted plants to improve soil structure.
• Also, plant plants with various root depths to encourage rooting channels and microbe activity.
• Keep earthworms happy. They make the channels deep-rooted plants follow down into the subsoil.
• Resist the urge to till your soil.

“Disrupting soil biology can alter our plant health,” says Glassey. “Tilling destroys much of the soil biology and interrupts soil biodiversity. While this has been a common practice for hundreds of years, we now know how damaging that practice really can be.”

While knowing the difference between subsoil, topsoil and other soil horizons is helpful for a healthy lawn or garden, Glassey encourages people to focus less on specific layers and more on what happens in the soil.

“Decades of information focus solely on the horizons, and so much more is being learned and proven about soil microbes — like bacteria, fungi, nematodes, and micro-arthropods — and how they work together to create healthy conditions and keep soils functioning as they should,” she says.

FAQ

How do I determine the quality of my subsoil?

Collect samples of subsoil from several areas of your yard, combine them, and then either use a soil-test home kit or send it away to a lab for testing. These tests check for soil pH, nutrient levels and organic compounds. To determine the composition of your subsoil, fill a mason jar halfway with subsoil, add water, shake and let settle overnight. You’ll be able to see the different levels of sand, clay and silt in your subsoil. Sand will settle to the bottom, silt next and clay will be the top layer.

What are cover crops, and how do they benefit subsoil?

Cover crops are fast-growing plants grown during fallow seasons to protect and preserve the soil. They also improve soil structure, protect the soil from wind and rain erosion and increase water filtration. In turn, this benefits the subsoil as well by reducing compaction, increasing water retention and improving structure and aeration.

About the Expert

• Carmen Eldridge is a gardening expert and manager of Arden Farm, part of the Arden HOA community in Palm Beach County, Florida.
• Dr. Anthony Fulford is nutrient management and soil quality advisor at the University of California Cooperative Extension.
• Kathy Glassey is director of renewable resources for Monster Tree Service.