The question of whether grass can be grown in compost has a nuanced answer. Compost, the product of controlled organic matter decomposition, is highly beneficial for turfgrass, but it is rarely used as a standalone growing medium. It functions better as a soil amendment incorporated into native soil or as a top dressing to improve existing turf structure. When used correctly, compost introduces organic matter, beneficial microorganisms, and slow-release nutrients important for establishing and maintaining a healthy lawn. Success requires understanding the material’s properties and applying it in appropriate ratios.
Why Pure Compost Poses Challenges for Turf
Using 100% compost as a base for a new lawn is discouraged because its composition creates an unstable and potentially toxic environment for young grass. Compost is naturally rich in nitrogen and soluble salts, which, at high concentrations, can cause phytotoxicity, burning the tender roots of new seedlings. This nutrient overload stresses the grass before it establishes a robust root system.
Pure compost is structurally unstable over the long term. As the organic material continues its final stages of decomposition, it undergoes significant volume reduction. This leads to uneven settling of the lawn surface, creating an unreliable base that is difficult to walk on and challenging to mow evenly.
While compost is excellent at retaining moisture, an all-compost medium can hold too much water, leading to anaerobic conditions that suffocate grass roots. Conversely, if a pure compost layer dries out, it can become hydrophobic, repelling water. A balanced soil mixture is necessary to ensure proper water infiltration and gas exchange for sustained turf health.
Amending the Soil Base for New Grass
When establishing a new lawn, compost should be incorporated as an amendment rather than a full replacement. Incorporating compost enhances the soil’s physical, chemical, and biological properties, creating a more hospitable environment for turf roots. The addition of organic matter helps break up dense clay soils, improving aeration and drainage, while also increasing the water-holding capacity of sandy soils.
To successfully amend the soil, mix the compost into the top four to six inches of the native soil profile. Apply a layer of one to two inches of compost across the area, then till or rake it thoroughly into the topsoil. This incorporation results in a ratio where the compost makes up no more than 25 to 33 percent of the final growing medium, preventing issues associated with nutrient overload and excessive settling. Proper mixing promotes deeper and healthier root development before seeding or sodding.
Top Dressing Existing Lawns
The most common method for using compost on established turf is top dressing. This technique involves spreading a very thin layer of finished compost directly over the existing grass canopy. The purpose is to slowly introduce organic matter and beneficial microbes into the thatch layer and soil surface without smothering the grass blades.
Before application, the lawn should be mowed short. It can also be beneficial to core aerate to create small holes for the compost to filter into. The maximum recommended application depth is typically between one-quarter and one-half inch per application. Applying too thick a layer will suffocate the grass, blocking sunlight and gas exchange, which leads to patches of dead turf.
After spreading, the compost must be worked down into the turf canopy using a rake, broom, or drag mat until the grass blades are visible through the material. This ensures good soil contact and prevents the compost from forming a dense mat on the surface. Top dressing is often paired with overseeding, as the fine compost layer helps cover the seed, retains moisture for germination, and provides initial nutrients for the new seedlings.
Selecting the Best Compost for Turf
The quality and type of compost are factors in turf success, especially when top dressing. The material must be fully mature, meaning the decomposition process is complete. Maturity eliminates the risk of nitrogen tie-up that occurs when microbes continue to break down unfinished compost. A mature compost will have an earthy smell and a dark, crumbly texture, with no odors like ammonia.
For both amending and top dressing, the compost should be screened to a fine texture, ideally passing through a three-eighths-inch screen to ensure even spreading. It is important to select compost with low soluble salt content, preferably below 4 dS/m, as high salt levels can desiccate turf roots. Manure-based composts can sometimes have high salt levels and require careful testing before widespread use.
A good compost for turf should also have a Carbon-to-Nitrogen (C:N) ratio below 30:1. A higher ratio can lead to temporary nitrogen depletion in the soil as microbes utilize available nitrogen for their own growth. Finally, using certified compost, such as one tested by the Seal of Testing Assurance (STA) program, ensures the product has been analyzed for stability, pH, heavy metals, and weed seeds.