How Much Topsoil Do You Need Over Fill Dirt?

If you have just moved into a newly-constructed home, you may find that your new property is covered with what is known as “fill dirt”. You can’t just start spreading grass seed or rolling out sod on top of this sub-soil; first, you need to add a good layer of good quality topsoil to ensure that what you plant will thrive.

Once you have a good base of fill dirt, you should add 4 to 6 inches (10-15 cm) of topsoil across your entire yard. That will give you sufficient depth for planting a lawn or starting a garden bed. If you are planting trees, you will need to add additional topsoil in the holes.

What’s the Difference Between Topsoil and Fill Dirt?

Fill dirt is soil that is composed mostly of small rocks, clay, and sand, depending on the region that you live in. It has very little organic matter and not much in the way of nutrients for growing plants. It may be mostly what was dug up when the foundations of your house were put in, or could have been trucked in from other construction sites. It is ideal for building up the grade or elevation of a piece of land. Because of its dense structure, it provides a sturdy underpinning for topsoil.

Topsoil has lots of organic matter, making it a better medium in which to grow plants. It’s dark and will hold moisture well, with few visible stones and debris. In nature, topsoil is formed when plants and animals die and rot into the top layer of the ground. If you dig a shallow hole in the forest floor, you will find spongy, black soil created by years of leaves and animals that have been turned into rich, black earth. Good topsoil also contains essential minerals for healthy plant growth.

Take a look at the comparison image I created to show the difference between fill dirt and topsoil:

fill dirt vs topsoil

The topsoil is darker because it contains more organic material and nutrients to help grass grow, while the fill dirt is lighter, rocky, and dry containing almost no nutrients for plants to thrive from. 

Will Grass Grow on Top of Fill Dirt?

Grass may grow on top of fill dirt, but it won’t thrive. For instance, the previous owners of our home claimed to have trucked in many tonnes of topsoil. However, wherever they built up the elevation, for instance over the septic field, the grass dies as soon as things get dry in summer, because their “topsoil” was basically pure sand. That makes great fill, but is terrible for growing a healthy lawn.

Read our related post on the Minimum Soil Depth for Growing Grass Over Concrete here.

Will Plants and Flowers Grow in Fill Dirt?

Plants and flowers will not grow properly in fill dirt. Their roots will struggle to penetrate the heavy, rocky fill, and there will not be enough nutrients to support healthy growth, while water will drain right through instead of staying available for the plants to use.

Does the Thickness of the Topsoil Make a Difference?

Does the Thickness of the Topsoil Make a Difference

You need to have a minimum of 3 inches (7.5 cm) and preferably 6 inches (15 cm) to give grass roots something to spread down into as it develops into a thick, healthy lawn. 6 inches (15cm) is the bare minimum for a flower or vegetable bed, as the roots of even annuals usually go at least that deep.

What Happens if You Use Too Little Topsoil?

If you don’t add a thick enough layer of topsoil on top of the fill dirt, you will find out when the plants suddenly stop thriving, when their roots have gone below the layer of good topsoil, and hit the inhospitable environment of the fill dirt.

Can You Use Too Much Topsoil?

There’s not really such a thing as too much topsoil, but there’s a point at which any extra depth will become unnecessary. Because good topsoil can get quite pricey, you need to strike a balance between your budget and your gardening needs. Make your landscaping plans before you start spreading topsoil so that you know where you will need the extra depth for flower or vegetable beds as opposed to lawns.

Are there Any Mistakes You Can Make When Adding Topsoil Over Fill Dirt?

The biggest mistake you can make when adding a layer of topsoil over fill dirt is if you do just lay down a thick layer over the fill dirt, smooth it out, and plant. Eventually, the roots of your plants will hit the top of the fill layer, and they will just stop right there. When there’s too sharp a line between the two soil types, it will end up stunting the root growth of your plants, which will of course impact their overall health and appearance.

You need to create a soil structure that gradually shifts from pure topsoil to pure fill dirt. You do this by tilling some of the topsoil into the top few inches of the fill layer, creating a layer that has some of both. Then, add the final layer of topsoil over top. That way, the plant roots will not get shocked when they reach the fill dirt.

Final Thoughts

While you can turn fill dirt into dark, humus-rich soil given enough organic matter and time to let it break down, it’s not a practical approach when you need to landscape a new property. Instead, buying good quality topsoil to spread across your yard will get things off to a great start!