Think of fill dirt and topsoil as a foundation and the house that’s built on it—the two go hand-in-hand. Both perform important functions and are needed, regardless of how big or small your yard is.
Learn about the differences between fill dirt and topsoil and how and when to use it in your landscaping.
What Is Fill Dirt?
Fill dirt is rocky, sandy, and shaly material sourced from building and construction projects. Before the excavation of a foundation begins, the top layer of soil is peeled off and set aside to be reused for landscaping on site. Then everything underneath is removed to dig the foundation. Some of that dirt is used as fill on site, the rest is hauled away and made available as inexpensive fill dirt.
When to Use Fill Dirt
Just as the name implies, fill dirt is used for filling in gaps and holes, or for reshaping a landscape, such as smoothing out a steep slope. Even if you had no budget constraints and unlimited access to high-quality topsoil, you would still use fill dirt because it drains much quicker than topsoil that has a high amount of loam and organic matter. If you had a 3-foot ravine in your yard and you only used topsoil to fill it, a topsoil layer of that thickness would act as a sponge, resulting in poor drainage, which is detrimental to plant health.
Fill dirt is always used as the bottom layer, followed by a layer of topsoil. In the example of the 3-foot ravine, you would use about 2.5 feet of fill dirt and cover it with 6 inches of topsoil.
What Is Topsoil?
Topsoil is a valuable substrate that is used to cover fill dirt. The fact that it is sold in 20- or 40-pound bags in garden centers and not by the truckload like fill dirt says it all.
Unlike fill dirt, quality matters a great deal for top toil. To make most plants—lawn, vegetables and fruits, ornamentals—thrive, you want topsoil that provides good drainage, is rich in organic matter, beneficial microorganisms, and nutrients.
Topsoil is a rare and expensive commodity because it takes hundreds of years for one inch of topsoil to form. That’s why it is much more expensive than fill dirt.
Not all topsoil is created equal; its texture and composition vary greatly. That’s why it is so important to match your landscape plants to your soil type (the main soil types are clay, sand, silt, and loam).
If you want to add topsoil to your yard, look for sandy loam, which is ideal for most plants. Check the label of bagged topsoil to make sure that it contains a maximum of 15 precent clay and at least 5 percent organic matter.
When to Use Topsoil
Topsoil is used as the layer on top of fill dirt in which you plant; it is the soil depth at which plant roots absorb nutrients. Just like the natural occurrence of topsoil varies—its depth varies from location to location—there is no hard and fast rule how much topsoil you need to add. For healthy grass to grow, you need a topsoil layer of 4 to 6 inches. Flower beds, vegetable gardens, and foundation plantings should have at least 6 to 8 inches of topsoil.
When you dig a hole for a tree or a shrub, remove the top 6 to 8 inches of soil and set it aside. After setting the tree in the hole, first use the fill and then the top soil to recreate the natural soil layering. As larger shrubs and trees get established, their root systems will find their way into the fill layer.
Especially for vegetable gardens, it is important to add 1 or 2 inches of fresh topsoil every year, as crop production depletes the soil.
Adding topsoil can also become necessary when it has been washed away from heavy rain.
Fill Dirt vs. Topsoil at a Glance
|Low cost||Higher cost|
|Higher availability||Limited availability|
|Limited to no organic matter||Higher organic matter|
|Low loam content||High loam content|
|Fast drainage and low water retention||Slower drainage and higher water retention depending on soil texture and organic matter content|
|High number of rocks and stones||Few or no stones, no rocks|
|Used as is, no amendments required||Work in progress, requires ongoing soil amendments to maintain good drainage and nutrient content|
Soil Basics. Soil Science Society of America.