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College of Food, Agricultural, and Environmental Sciences


Organic Mulches May Restore Fertility of Degraded Soils

March 26, 2002

WOOSTER, Ohio - Organic mulches may offer hope in restoring fertility of degraded soils in urban areas.

Ohio State University researchers are studying the effects of composed yard trimmings, as well as a mixture of hardwood bark and composted manure on the health of disturbed soils in ornamental landscapes.

"Topsoil is often removed when new homes are built, and the subsoil that is exposed when digging the basement is often spread over the surface," said Dan Herms, an entomologist with the Ohio Agricultural Research and Development Center (OARDC). "Subsoil is virtually devoid of organic matter and nutrients, which makes landscaping and gardening nearly impossible without using pesticides and fertilizers."

In previous studies, Herms and his associates found that certain organic mulches stimulated the growth of beneficial microbes, increased soil fertility, boosted plant growth, and fought off insects and diseases. The researchers are now taking this research one step further by trying to determine if organic mulches can create the same cascade of effects in subsoil.

"We think certain mulches have the potential to function as organic fertilizers and pesticides in highly disturbed soils in urban landscapes," said Herms.

The researchers are focusing on mulches with a low carbon-nitrogen ratio, meaning that enough nutrients exist in the mulch to support the needs of both plants and soil microbes. The composted yard trimmings, which consist of wood chips, leaves, and grass clippings, have a carbon-nitrogen ratio of about 18:1. Hardwood bark has a carbon-nitrogen ratio of 70:1, but blending it with composted manure reduces that ratio to about 35:1.

By comparison, mulches with high carbon-nitrogen ratios, like recycled wood pallets, do not provide enough nitrogen to support microbial and plant growth. As a result, the microbes out-compete plants for nutrients, thereby reducing plant growth. Applying such mulches to soils that are already low in nutrients would severely stress plants.

"Applying mulches like composted yard trimmings with a low carbon-nitrogen ratio may not only increase plant growth, but also improve the health of degraded soils by increasing organic matter and beneficial soil microbes," said Herms. "At the same time, a valuable natural resource would be recycled that might otherwise be destined for a landfill."

The researchers are studying the effects of these mulches on the health and pest resistance of paper birch and rhododendron, common plants used in Ohio landscapes. They have just begun the second year of a five-year study.

Candace Pollock
Dan Herms