When it’s time to overseed your lawn to fill in bare or thin spots, you need to make sure your grass seed can establish in a hospitable growing environment. An important step is adding a top dressing to improve the soil before seeding. But should you use topsoil or compost?

Both amendments have pros and cons for overseeding In this guide, I’ll compare topsoil vs compost for lawn overseeding projects so you can decide which is right for your needs.

An Overview of Topdressing for Overseeding

Before jumping into the details, let’s do a quick rundown on why topdressing is an important part of overseeding:

• It improves soil structure fertility and moisture retention. This gives grass seeds what they need to germinate and establish.

• It smooths and levels the soil surface to ensure good seed-to-soil contact for better germination rates.

• It protects seeds from drying out, being washed away, or getting buried too deep.

• It shields seeds and seedlings from predation by birds and other wildlife.

• It visually marks seeded areas so you know where you already spread seed.

Now that you understand the benefits of topdressing, let’s compare some pros and cons of topsoil vs. compost.

Topsoil Benefits

Topsoil offers a few advantages that make it a good choice for overseeding:

It Contains Mineral Particles That Improve Soil Structure

The small mineral particles like sand, silt, and clay in topsoil can help improve soil structure. This creates pores for air and water circulation that grass roots need.

It Raises Soil Height to Bury Seeds at the Optimal Depth

Spreading a thin layer of topsoil can raise the soil surface enough to achieve the ideal 1/4 to 1/2 inch depth for burying grass seed.

It Has Fewer Weed Seeds Than Garden Soil or Compost

Provided you get clean topsoil from a reputable source, there will be far fewer weed seeds than in recycled garden soil or compost. Less weeding is always a good thing!

It’s Readily Available and Affordable

Good quality topsoil can be purchased in bulk from most landscape supply stores or hardware stores at a reasonable price. Having an accessible and budget-friendly resource makes projects easier.

Potential Drawbacks of Using Topsoil

The main downsides to consider when using topsoil for overseeding include:

• It lacks nutrients: Straight topsoil doesn’t contain much nutritional value for growing grass plants. Be prepared to supplement with fertilizer.

• It may compact over time: As topsoil settles, it can become dense and compacted. This reduces the air spaces that grass roots and soil microbes need.

• Weed seeds: No topsoil is 100% weed-free, so there is always a risk of introducing weed seeds. Try to source clean topsoil and consider solarization or fumigation to reduce this risk.

• Erosion: Topsoil is prone to washing or blowing away, especially on slopes or bare ground. Use tackifiers, cover crops, or erosion control blankets to help hold it in place.

Compost Benefits

Compost offers its own set of advantages for overseeding:

It Provides a nutritional Boost

Compost contains a balanced array of macro and micronutrients that grass plants need, right from the start. This gives young grass the fertility it needs to establish.

It May Improve Soil Structure

Compost can serve as a soil conditioner to help bind tiny soil particles into clumps, improving soil structure. The organic matter may also slightly raise soil height.

It Holds Moisture Well

Compost retains moisture far better than mineral soils. This gives seeds and young grass a consistent supply of water they need for growth.

It Encourages Beneficial Soil Microbes

All the microorganisms in compost migrate into the surrounding soil and boost biological activity. Healthy soil means stronger grass plants.

Potential Drawbacks of Compost

Some limitations to keep in mind when using compost for overseeding include:

• Weed seeds: Compost may contain viable weed seeds that can germinate along with grass seed. Use properly aged or hot compost to reduce this risk.

• Rapid decomposition: Fresh compost might decompose too quickly, leaving depressions in the topdressing layer. Focus on more stable, mature compost.

• Nutrient release: The nutrients in compost become plant-available slowly over time. This delay may limit their benefit for establishing young grass plants.

• Limited availability: Not everyone has ready access to quality, affordable compost. Purchasing bags from retailers can get expensive.

How to Choose Between Topsoil and Compost

Now that you know the characteristics of each amendment, how do you decide what’s right for your lawn?

Here are some factors to consider:

For new lawns – Use a mix of topsoil and compost. The topsoil provides bulk and structure for root development, while compost offers nutrition. Blend them approximately 2 parts topsoil to 1 part compost.

For poor, compacted soil – Go heavier on compost, at a 1:1 ratio with topsoil. The compost will help break up compacted soil and add organic matter.

For sloped sites – Topsoil may wash away more readily. Compost can help improve water retention and infiltration to reduce runoff and erosion.

In arid regions – Lean more on compost for its ability to retain moisture in dry climates. Or look for polymer-enhanced topsoil products.

On a tight budget – Topsoil is the more economical option. Just factor treatments like fertilizer and aeration into your plans.

For quick green-up – Compost provides accessible nutrients right away for faster establishment.

If you’re also fertilizing – No need to emphasize compost. Use mostly topsoil and apply starter fertilizer to supplement nutrition.

Overseeding Tips and Techniques

Whichever amendment you choose, proper application is critical for success overseeding. Here are some key tips:

Add up to 1/2 inch depth. More than that risks burying seeds too deeply. Start with 1/4 inch and add more if needed.

Break up large clods and particles. This ensures consistent depth and good seed-to-soil contact.

Water thoroughly after spreading. Aim to moisten the top 2-3 inches where seed is sown. Mist regularly to keep surface damp.

Consider using tackifiers or cover crops like peat moss or straw to hold the topdressing in place on windy sites or slopes.

Overhead irrigate lightly to prevent erosion and crusting while seeds germinate. Adjust sprinklers to avoid washing away topdressing.

Allow time for the amendment to settle before reassessing if more needs to be added. Don’t add too much at once.

Remember to fertilize if using straight topsoil. Young grass needs nutrients to grow strong roots and blades.

Overseed when day temps are 65-75°F. Cool season grasses won’t germinate well when it’s overly hot.

Improving Germination Rates

Taking steps to enhance moisture retention can also boost germination rates when overseeding:

Water infrequently but deeply to encourage roots to grow deeper seeking moisture. Dense, shallow roots won’t establish well.

Allow the soil surface to dry out slightly between watering to avoid fungal disease issues.

Mow existing grass a little shorter prior to overseeding for better seed-to-soil contact through the canopy.

Use a lawn roller to press topdressing into the soil surface for better retention.

Consider shading devices like burlap to protect seeds and tender new growth on hot, sunny sites. Remove once established.

Time overseeding for fall rather than spring, when cool weather offers better growing conditions for cool season grasses.

The Bottom Line

While compost and topsoil both have a place when overseeding lawns, the needs of your particular site should determine which way to go. For pure overseeding projects, quality topsoil may provide the most benefits. For renovating poor, thin lawns, a compost/topsoil blend can improve growing conditions for stronger grass growth.

If you’re unsure which is better for your needs, start with a mix of both. You really can’t go wrong blending compost into topsoil. The organic matter and nutrients will only enhance structure, fertility, and moisture retention.