compost vs topsoil

Compost vs Topsoil for a New Lawn – What’s Better?

In Lawn & Garden by Jamie

So you’re looking to establish a new lawn, but you’re not sure what’s better to use: topsoil or compost. What are they and what do you need to grow a vibrant lawn?

Topsoil and compost are both necessary for starting a new lawn. Topsoil is the layer that roots grow through and compost provides nutrients. But you only need to add topsoil to your lawn to fill in holes, level out the lawn, or replace eroded topsoil on a budget. Otherwise, topdressing existing topsoil with compost will give your lawn the head start it needs.

Are Topsoil and Compost The Same Thing?

Topsoil and compost are not the same thing. Compost is decaying plant and animal matter that is broken down by beneficial microbes into nutrients that plants can then absorb. Topsoil describes the top 5-10” layer of soil and contains organic matter, minerals, air, and water. This layer is where roots grow and beneficial microbes live. Compost is a component of topsoil.

Should I Use Compost or Topsoil Over My New Lawn?

You need both topsoil and compost to start a new lawn. You may already have enough topsoil (at least 6”) where you want to grow your lawn and all you need to do is improve it by sheet mulching with compost.

If you need to level out the lawn, fill in holes, or you need to replace more than 2” of topsoil, then you may need to buy topsoil. Topsoil is usually cheaper than compost (unless you make it yourself or can get it free from a municipal program) so if you need a lot of soil, buying topsoil instead of compost could save your budget. You can then mix in compost to provide added nutrients.

The downside to topsoil is that it may include weeds seeds. Good compost made through hot composting should have no weed seeds remaining.

When to buy topsoil for a new lawn: When to just use compost on a new lawn:
●      You need to replace the existing topsoil.

●      The topsoil has eroded away.

●      The topsoil is heavy clay and you need to establish a large area.

●      You need to level out the lawn and/or fill holes.

●      You need to use a LOT and are on a budget.

●      You want to improve the quality of the existing topsoil over the long-term.

●      You have clay soil.

●      You have a few inches of topsoil already in the spot.

●      You don’t need to level out or change the grade of the lawn.

What about clay soil?

Do I need to add topsoil to improve drainage? You don’t need to use topsoil to replace clay soil. 1-2” of compost can also improve clay soil over time. Compost adds organic matter to the soil which adds aeration and absorbs water which eases flooding. If you have a large area that needs rehabilitation, using topsoil may save you money in the short term.

Do I have to use compost?

Adding at least ¼” of compost adds nutrients and has other benefits, but if you’re short on cash and the topsoil you’re working with is fertile enough to grow grass, then skip the compost.

Can I Use Topsoil And Compost On My New Lawn?

You can topdress your topsoil with compost! Use the topsoil as the bulk of what you add, then add a layer of compost 2” deep to further nurture your lawn. In the short term, the compost will also help keep any weed seeds in the topsoil from germinating.

Can I Put Topsoil On Top Of Grass Seeds?

Add topsoil and compost before you put down the grass seeds or grass seedlings. If you cover the seeds or seedlings, you’ll kill them before they even get a chance to grow. Grass seeds need to be surface-sowed as they need light and new grass blades aren’t strong enough to push up through topsoil.

Should I Replace The Topsoil For My New Lawn?

If you don’t have to replace topsoil, don’t do it. The existing topsoil will already be home to beneficial microbes, insects and worms, and it’s easier and cheaper to improve it with compost than to replace it.

However, if the topsoil has eroded to expose the subsoil or clay, or heavy construction equipment has compressed it, then it may be worth investing in topsoil for the short-term. (Compost can help rebuild topsoil and aerate clay and compressed soil over the long-term.)

How Many Inches Of Topsoil Do I Need For A Lawn?

topsoil amount on lawn

Grass needs at least 4 – 6” (or 15 cm) of topsoil for roots to grow, and the deeper the roots can grow, the more resilient grass is and the less watering it needs. This includes any existing topsoil that you start out with or any compost you wish to sheet mulch on top.

Letting the grass grow long in between mowings will also encourage grass to grow more roots.

How Many Inches Of Compost Do I Need For A Lawn?

Before you start a new lawn, you can add as little as ½” or between 2-3” for long-lasting nutrients and (while the compost is breaking down) weed suppression. This is also known as sheet mulching.

After grass has started to grow, only add a ¼” layer, or less. The grass blades should be able to come through the layer. Otherwise, you’ll smother the grass.

Spread it like you’d spread topsoil.

What Should I Look For In A Quality Topsoil?

Outside of a soil sample test, seeing or touching the topsoil yourself is the best way to determine the quality. Quality topsoil should be dark brown and crumbly and smell like loamy earth. Earthworms are a fantastic sign. (This is the same with compost.)

Bagged or bulk topsoil comes in different grades at different price points:

  • Screened topsoil has been screened to remove additional materials like small pebbles. Screening costs more.
  • Premium grade topsoil is the most expensive option and best for garden beds and seedlings, and is unnecessary for growing a lawn. There’s also topsoils especially designed for growing fruit trees or vegetables.
  • General-purpose topsoil is best for landscaping and establishing new lawns, and maybe natural topsoil, manufactured topsoil, or premium topsoil that was handled poorly.
  • Economy topsoil has not been screened and may contain weed seeds, but is a good option if you’re on a budget or you need to really build up an area. If you cover with compost or a layer of general-purpose topsoil, weed seeds shouldn’t be a problem.
  • Triple Mix is a mixture of topsoil, peat moss, and compost.

And ask for or look for recommendations for topsoil companies in your area. Not all topsoil companies are equal and topsoil is not subject to regulations or standardization.

Is Bagged Topsoil Any Good?

Just like bulk topsoil can vary wildly in quality, so do bagged topsoils. Read the bag as some topsoil might be recommended for filling in holes or building up beds and not actually good for growing plants, while others will be specifically for growing plants.

If you see a ripped bag or topsoil coming out a corner, take advantage and feel the topsoil. If it’s dark brown, crumbly, and smells good, then it’s quality topsoil.

If neither of those options work, then buy one bag to test it out.

How Do You Spread Topsoil For New Grass?

Depending on the tools you have and how big your new lawn is, you can spread topsoil by:

  • Dumping wheelbarrows of topsoil throughout the space and use a rake to even it out.
  • Using a spreader to deposit layers of topsoil while running it over the space in different directions to ensure an even spread.

Whichever you do, if the topsoil isn’t screened, make sure you break up clumps or it won’t spread evenly.

Final Thoughts

Topsoil and compost are both necessary to growing new lawns. Topsoil fills out the layer that grass needs to grow. Compost adds organic matter and nutrients to the soil. Both can help you grow the lawn of your dreams.