Mixing Grass Seed With Compost

Many homeowners know the importance of keeping plant litter for compost as an investment in the future of their soil’s productivity. Returning these unused materials and nutrients to your lawn’s soil can help grass seeds grow to a healthy, mature coverage.

Using compost to help fertilize your lawn when you lay down grass seed is one part of a larger strategy to keep your soil nourished and your lawn lush and green. When you give attention to the condition of the topsoil, you’re working on the growing environment for your grass.

What Is Compost?

Compost is the process and result of breaking down organic material to use as fertilizer. The resulting end-material is humus: solid matter that remains from decomposed organic material. Compost and humus contain nutrients that can be used as a rich fertilizer.

What Is Topsoil?

Topsoil is the top several inches of the ground’s surface level where most of the soil’s bioactivity occurs. As a result, this is where most nutrients are available for all plants, including your lawn’s grass. In nature, the topsoil is a constant state of decomposition and composting, where leaf litter and other materials fall, broken down, and are incorporated into the ground.

In the lawn, however, we clean up leaves and other yard clippings to keep the grass fresh and exposed. When it comes to the topsoil in our yards, it may or may not have a full nutrient profile since nutrients are pulled from the ground by the grass and other plants but aren’t replaced naturally. This is why fertilizers, like compost, are so important to add to soil.

Compost Vs. Topsoil

Compost Vs. Topsoil

Taking a look at the two side-by-side will show how these balance each other out, where too much of one or the other can be bad for your lawn.
Compost breaks down, and topsoil remains. When applied to your grass, topsoil might smother seeds or roots since it doesn’t decompose.

Compost is more nutrient-dense than topsoil but may not have the nutrient diversity of topsoil.

Compost retains water and holds moisture in the soil. Topsoil holds less water than denser organic material and dries out faster.

Is Topsoil Or Compost Better For Grass Seed?

Seeds might be able to grow in topsoil without compost, but seeds can’t grow in compost without topsoil. While rich in many nutrients, compost doesn’t have all of the vitamins and minerals available in topsoil, and while topsoil might dry out, compost can hold too much water and drown the seeds. Compost also lacks the soil structure and is too compact to retain air and provide space for roots to grow.

You should mix compost with topsoil for grass seeds to have the best growing environment. If you mimic natural conditions as much as possible, you can get the most desirable effects for your lawn. In this case, that means maintaining a nutrient-rich topsoil with fertilizers like compost.

Applying Compost And Grass Seed

You can apply compost and grass seed in a few different ways:

  • Mix compost into the soil, then sow seeds
  • Sow seeds, then lightly apply a layer of compost
  • Mix seeds into compost and apply to the ground

The way you put down compost and grass seed might be conditional on the state of the lawn at the time of planting.

Full-Yard Tillage

If you’re replanting the whole lawn, you’ll loosen and aerate the soil by default during the tilling process. This is an excellent opportunity to mix compost into the topsoil before sowing the seeds across the yard. When you leave in the roots and grass of your previous lawn when you till the yard, it’s known as “green manure,” which will decompose and release its nutrients and be available to your new grass.

Sections And Patches

Sometimes only certain parts of the lawn do poorly, or some sections might be older than others. If you’re patch-replanting, be sure to till the area to loosen the soil as you would for a large planting. You can mix in compost to the soil, and either sow the seeds and apply some compost, or mix the seeds into compost, then apply it.


When the existing lawn could use more grass for density reinforcement or to replace a soon-to-be dormant type of grass at a seasonal change, you can opt to leave the lawn in place and sow the seeds across the grass. Since you won’t disturb the roots in this case, the best strategy is to apply a thin later of compost after sowing the seeds.

Be sure to mow and rake for thatch before spreading seeds and compost so they can reach the soil surface and still receive enough air, light, and water during the first few weeks of germination.

Will Compost Kill Grass?

You can cover grass seed with compost, but using too much can block sunlight and oxygen from reaching the seeds during their critical growth period. Grass seed will germinate on top of compost and soil, but it won’t if the seeds are too covered up.

Using a fair amount of compost won’t kill the grass as long as you don’t bury the seeds or blades. A quarter-inch of coverage is the maximum for air and light to still reach the seeds, and it shouldn’t be applied with any pressure. Make sure the soil isn’t compacted when putting compost over seeds.

Using too much compost or topsoil when overseeding will have those same effects and potentially damage the existing lawn by suffocating the roots and blocking grass blades from performing photosynthesis.

What Is The Best Compost?

The best compost is one that is made of a variety of organic materials, which will result in a fertilizer rich in all of the major elemental nutrients (nitrogen, phosphorus, and potassium) and a variety of other vitamins and minerals like sulfur, iron, copper, and magnesium, among many others. Like a soil test, you can have your compost analyzed for its nutrient content.

Should I Apply A Fertilizer Along With Compost?

Should I Apply A Fertilizer Along With Compost

Compost is a slow-release fertilizer made of organic materials; it’s broken down and released into the soil over several seasons. When you put down compost with grass seeds, it isn’t necessarily to provide immediate nutrients but to provide a healthy soil environment over the long term.

Using a starter fertilizer with your compost and grass seeds will provide even more support to your seeds as they establish themselves in the ground. Starters supply phosphorus and potassium immediately to the germinating seeds and support their growing roots. Be sure to do a soil test to have a good idea of how much fertilizer to use.

Artificial fertilizers are usually liquids in sprays, but organic fertilizers can also apply those necessary nutrients that need to be available when you put the seeds in the ground. Mixing manure with grass seed, like compost and other organic fertilizers, adds a potent fertilizer while also contributing material that will build the soil.

The same strategies apply to manure as they do to compost in terms of mixing into topsoil, application, and coverage.