Research Shows Effect of Compost on Weed, Seed Germination


July 24, 2015 - Can compost supply crops with nutrients and fight weeds?

Weeds are one of the top challenges for organic crop producers. Researcher Dr. Gladis Zinati set out in her OFRF project to look for a new practical solution for organic weed management: compost extracts. Zinati just completed her project at the Rodale Institute in Kutztown, PA looking at whether compost extracts could reduce weed seed germination.

The study found that different types of compost vary in their ability to suppress weeds, and also in the undesirable effect of suppressing crop seed germination. “The funding I received from OFRF was integral to test the concept of compost extract as a tool for weed suppression,” Zinati said.

Compost is organic material that can be used as a soil amendment, and it provides the nutrients and minerals needed for plants to grow. Compost is made by combining different ratios of plant material, manure, and bulking agents such as wood chips. A compost extract is liquid version of compost, rich with plant nutrients and microorganisms.

The study at the Rodale Institute looked at one potential benefit of compost extract – its ability to snuff out weeds before they sprout.


The study looked at two different types of compost, one with a high ratio of green material (leaf mulch) to brown material (woodchips and straw) and one with a high ratio of brown material to green material. The researchers looked at how these two types of compost extracts affected weeds (pigweed, giant foxtail, and lambsquarter) as well as crops seeds (bell pepper, tomato, cucumber, and basil). To test the impacts on the seeds, the seeds were placed in petri dishes and treated with varying dilutions of the two compost extracts. The compost treated seeds were compared with seeds treated with deionized water as a control to see if the compost had an impact on germination rate.

Impacts on weeds and crops

The only weed affected by the compost extract was lambsquarter. The compost with the high brown to green material ratio at a dilution rate of one part compost extract to four parts water was able to reduce lamsquarter seed germination by 32% without impacting the crop seeds.

The compost extracts had the undesirable impact of reducing pepper and cucumber seed germination, but they had no impact on tomato or basil seeds. This suggests that farmers must be careful about the type of compost extract used to make sure it will not hamper the crop seed germination.

Next steps

Results from Zinati’s research project were shared with growers at the 2015 MOSES conference, and farmers were excited to hear about the potential of compost to fight weeds. This research offers promising indications that certain biological and chemical properties of compost have the potential to suppress seed germination. The next step will be harnessing that potential and assessing the impact of both compost and herb extracts on germination. Understanding the exact compost formulas that will have a desired impact on weeds is a future step for the researchers who plan to look at which phytochemicals suppress or enhance seed germination.

For more information on compost and how to make extracts, visit the Rodale Institute compost extract page

Click here to read Dr. Zinati’s final research report, posted on OFRF’s website.

By Joanna Ory - OFRF