Do Substratum Soil and Bedrock Affect My Garden?

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Depending on the depth of your soil, these low-lying strata can impact the success of your garden and lawn.

When you start digging in your garden, you’ll first encounter topsoil — dark and full of organic matter, worms and other organisms. As you go farther down, you might reach a lighter-colored sandy layer called the eluvial or “E” soil horizon. Beneath that, there’s one with a lot of clay and iron called subsoil, or “B” soil horizon.

Below that, things are likely to get rocky and hard. You’ve now entered the domain of substratum and bedrock. Depending on how close they are to the surface, they might affect your garden and lawn. Or not.

“There are several layers, also known as ‘horizons’ to soil scientists, that can be found when we dig deeper and deeper down into the soil,” says Dr. Anthony Fulford, a nutrient management and soil quality advisor with the University of California Cooperative Extension.

“We can imagine all the individual layers of a soil stacked one on top of the other like a layer cake. This is called the soil profile. Bedrock is the bottom layer of the soil profile layer cake.”

What Is the Substratum Soil Level?

The substratum is the material found on top of the bedrock, often made up of loose, rocky particles. Sometimes it’s called unconsolidated bedrock, saprolite, regolith, parent material or soil horizon C.

In soil science terms, it’s material that hasn’t yet been heavily affected by the processes that create soil, such as weathering. But over time, chances are it will eventually break down and create soil.

How Does the Substratum Layer Affect My Garden or Lawn?

If it’s a few feet below the surface, it probably won’t affect your garden. The substratum layer contains limited water and air, and roots usually don’t grow in it. But if it’s closer to the surface, you may be better off with raised beds.

“Once you’ve dug down to this horizon, take note of color and odor,” says Kathy Glassey, director of renewable resources for Monster Tree Service. “If you see a grayish color, it could be indicative of poorly drained soils. And if the odor is sulfur-like or sour, it is a great indicator that you have more anaerobic conditions that need to be addressed if you want healthy plants.”

What Is Bedrock?

Bedrock is solid rock like granite, basalt or sandstone. You can see bedrock on exposed mountaintops and rocky coastlines.

In your yard, it may be under a thin layer of soil, like limestone bedrock is in parts of Florida. If you’re in the Midwest, it might be below hundreds of feet of dirt and sand.

Sometimes bedrock weathers to create the soil above it, but other times soil comes from glacial and river deposits. In parts of the Caribbean, soil comes from wind-blown dust, which crosses the Atlantic from Africa.

How Does Bedrock Affect My Garden or Lawn?

If bedrock is at least several feet below the surface, the roots of your garden will never reach it. Bedrock will not have any effect on your garden or lawn, though it may have created the types of clay minerals found in your soil.

However, if bedrock is near the surface, you will probably need raised beds to ensure your plants can root deeply and get enough water.

“Simply put, bedrock is not a suitable rooting layer and normal gardening practices will be a significant challenge,” says Dr. Tony Provin, a soil chemist and professor at Texas A&M University.

What Grows in Bedrock and Substratum?

Some trees and bushes root in cracks in exposed bedrock. While bedrock and substratum are not great for growing a lawn or garden, scientists are discovering they can play a crucial role in supplying water to forests. That the type of underlying rock affects a forest’s diversity, growth rate and ability to store carbon.

Karuna Eberl
Karuna writes about wildlife, nature, history and travel for magazines, newspapers and websites including National Geographic, National Parks, Discovery Channel, Atlas Obscura and the High Country News. She's also produced a number of independent films and directed the documentary The Guerrero Project, about the search for a sunken slave ship. She and her husband, Steve, wrote an award-winning guidebook to the Florida Keys and are currently completely renovating an abandoned house in a ghost town. She holds a B.A. in journalism and geology from the University of Montana. Member of OWAA, SATW.