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College of Food, Agricultural, and Environmental Sciences


Benefits of Organic Mulching Extend Below the Soil Surface

January 15, 2002

WOOSTER, Ohio - The impact of organic mulches on soil nutrient content and plant health may extend deeper than just the topsoil.

Ohio State University researchers have found that certain types of organic mulches, with the aid of soil microbes that break down the organic matter, have a profound effect on soil fertility and overall plant health below the soil surface - fighting off diseases and insects and increasing plant growth.

"Whether or not organic mulches applied to the soil surface could impact fertility of the soil below ground has been a topic of debate," said Dan Herms, an Ohio State entomologist. " However, we found that as soil microbes decompose organic mulch, they have a dramatic effect on soil fertility and plant health, including warding off insect pests and diseases." Herms, a researcher with the Ohio Agricultural Research and Development Center (OARDC) in Wooster, Ohio, will discuss the results of the three-year study at the 73rd Annual Ohio State University Nursery Short Course, Jan. 21-23 at the Greater Columbus Convention Center in Columbus, Ohio.

The program, in conjunction with the Central Environmental Nursery Trade Show (CENTS), is the third-largest event of its kind in the country that provides nursery, landscape and greenhouse professionals plant selection and maintenance information, pest and disease management tips, economic advice and research results.

The Ohio State research focused on composted yard trimmings and ground wood pallets, two products that are increasingly being recycled for use as mulch in ornamental landscapes in an effort to divert them from landfills. According to the study results, the two mulches have very different effects on the soil. "Organic mulches can have a dramatic impact on soil fertility and plant nutrition, but the effects depend on the carbon-nitrogen ratio of the mulch," said Herms.

Composted yard trimmings, for example, have a low carbon-nitrogen ratio, hence a higher nitrogen concentration. As a result, as the organic matter is decomposed, enough nitrogen is released to support the growth of both plants and microbes.

"We found that when trees and shrubs were mulched with composted yard trimmings, they grew faster and produced more flowers," said Herms. "Furthermore, when we fertilized the plants that were composted with yard trimmings, we found that the fertilizer didn't have any effect on plant growth, which means that plants are getting all they need from the yard trimmings alone." He speculates that composted yard trimmings provide an excellent source of organic fertilizer for plants in ornamental landscapes by helping to restore natural nutrient cycles much like those found on a forest floor that is covered with leaves. In addition, they improve infertile soils that are low in organic matter, such as those often found around newly constructed homes.

Wood pallets, on the other hand, with a high carbon-nitrogen ratio, do not contain enough nitrogen to support the microbes that are decomposing the wood. The microbes must scavenge additional nitrogen from the soil, which decreases the amount of nitrogen available for plants. As a result, plant growth is reduced. However, Herms found that mulching with ground wood pallets did increase the resistance of mulched plants against various insect pests.

"Homeowners need to be aware that there are a number of products available on the market that can be used as mulches, and that they will have different effects on their plants and soil. The choice of the mulch should be dependent on the objective that homeowners have for their landscape," said Herms. "Faster growing plants have lower concentrations of defense chemicals because they are using more of their energy to support quicker growth, whereas slower-growing plants are more resistant to diseases." Some mulches, like pine nuggets, are resistant to decomposition, and therefore have little effect on soil microbes. As a result, they also have little effect on soil fertility or plant nutrition. However, because they are so stable, they do not to be replenished as often as mulches that decompose more rapidly, such as composted yard trimmings, and they are better at suppressing weeds.

"The different properties of the various mulches available for use in landscapes creates the opportunity for prescription mulching based on the objectives of the homeowner or the landscape professional," said Herms. For more information on CENTS/Ohio State Nursery Short Course call (614) 895-1356, or (800) 825-5062 or log on to or CENTS is managed by the Ohio Nursery and Landscape Association. The Ohio State Nursery Short Course is sponsored by Ohio State Department of Horticulture and Crop Science, Ohio Nursery and Landscape Association, and the Ohio State Extension Nursery, Landscape and Turf Team.

Candace Pollock
Dan Herms