Choose Compost Carefully When Topdressing the Lawn

Turf experts often recommend "topdressing" lawns with a thin layer of compost. The material is spread one-quarter to one-half inch thick in spring or fall, depending on local climate and soil.* 

Compost improves the moisture-holding capacity of the soil, adds nutrients, and feeds soil microbes. These microbes are key to the complex process that makes food available to grass plants. A healthy microbe population encourages a dense green lawn. 

Compost spread on top of thatch can also speed thatch decomposition and eliminate the labor of mechanical removal.

Complications of Topdressing

But there is a problem: Compost quality varies. Whether you buy it by the bag, pick it up at the municipal leaf dump or have it delivered by the yard, how can you know that it is “good” compost? Consider these questions:

• Is it at the finished stage?
• Does it contain unwanted ingredients such as dyes or building materials?
• Are there viable weed seeds in the material?
• Could it have pesticide residues?

The parent materials and the composting process both affect the quality of the final product.

“There is only one way to know what compost contains,” says Al Rattie, director of market development at US Composting Council (USCC) in Bethesda, MD. “That is to use compost that has undergone compost-specific testing.”

Recognizing the problem this creates for consumers and the land care industry alike, the USCC started the Seal of Testing Assurance (STA) in 2000. STA is a testing, labeling, and disclosure program designed to bring visibility into the world of compost sales.

According to Rattie, STA is the only nationally recognized test.

STA looks at 14 compost characteristics including organic matter, salts, pH, major nutrients, pathogens, metals, stability, and maturity.

Rattie says one of the biggest concerns is the presence of pesticide residuals. Well-made compost, however, carries little of this risk. 

“A proper composting system destroys the overwhelming majority of pesticides and herbicides," says Rattie. 

Unfortunately, consumers are not likely to find STA compost in big box stores. “The majority of STA participants sell in bulk through local landscape supply yards, garden centers, and through direct sales,” says Rattie.

USCC offers several resources to find STA compost and a calculator to help determine the amount:

• Find a list of 200+ STA participants
• Visit BuyCompost.com and use the USCC’s compost calculator on the upper right of the homepage. 

Rattie strongly encourages anyone who is topdressing a lawn to use bulk compost.

“Bagged compost supplies only one cubic foot of material,” he says. “It takes 27 bags to get one cubic yard. Do the math!” 

When we used the compost calculator offered by the USCC at BuyCompost.com, we found that a one-quarter inch layer on a quarter-acre lawn (10,000 square feet) requires 7.7 cubic yards or 5.9 cubic meters of compost. 

Rattie also encourages core aeration after the compost has been applied.

"Golf course and ball field superintendents having been using this technique successfully for many years," he says.

Note: Timing Compost Applications

Compost is spread in spring or fall, but there are important nuances. In cool-season areas, the spring application is typically lighter and the fall application heavier. In the south, topdress warm-season grasses in early spring.  Consult with the local extension service or your soil test provider for recommended depth and dates.

Keep in mind, too, that compost has some fertilizer value and may partially offset fertilizer application rates.