What Is Subsoil and How Does It Help a Garden?

Updated: Sep. 15, 2023

Learn about subsoil's role in your garden's success and how to make it healthier.

Below the glitz and glamour of topsoil, subsoil modestly plods along, supporting the main act with minerals and helping it compensate for overwatering and drought.

The subsoil is a land where earthworms, beetles and grubs dwell. Their tunneling helps roots grow deep to find water and key soil nutrients like iron, calcium and magnesium. Fun fact: Earthworms actually eat soil, and a single one can digest 26 tons of it a year!

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.