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The carbon content of fallen leaves is a perfect pairing for the nitrogen content of annual landscape plants in the compost bin.
Credit: University of Missouri Extension
Published: Tuesday, Oct. 25, 2016
David H. Trinklein, 573-882-9631
COLUMBIA, Mo. – Tree leaves can be a gardener’s best friend when used correctly. Use leaves to start a compost pile or add nutrients to lawns, said University of Missouri Extension horticulturist David Trinklein.
Some gardeners take a hands-off approach to leaves, Trinklein said. However, leaves left on lawns can pack down into a tight mat. This prevents needed sunlight from reaching grass. Leaves also trap and hold moisture and increase the potential for disease.
He recommends two ways of using leaves effectively: Either mulch the leaves by mowing or make a compost pile.
Clear the lawn by mowing damp leaves. Adjust your lawn mower to its highest setting and mow in a crisscross pattern. Mow twice to cut leaves to the size of confetti. These small pieces of leaves will filter into the lawn, decompose and release nutrients for the grass.
Composting offers another option. Compost is partially decomposed organic matter created when soil-inhabiting microbes break down plant tissue. Added to soil, compost binds small clay particles together to make larger particles. This improves soil aeration, root penetration and water flow in clay-type soils for which Missouri is notorious.
Soil microbes need two elements to form compost: carbon and nitrogen. Most leaves are high in carbon but low in nitrogen. Therefore, add nitrogen to the compost pile to hasten decomposition. Apply a general-purpose fertilizer such as 12-12-12 at a rate of about 2 pounds per bushel of leaves, or add 1-2 inches of animal manure or green plant material such as grass clippings.
Construct the compost pile in layers by placing about 4-6 inches of well-shredded leaves followed by a layer of the nitrogen source. Repeat the process until the pile is about 36-48 inches deep and covers an area of about 25 square feet. If you want to enclose the compost material, specialized composting bins are available, but wire mesh, concrete block or treated lumber enclosures work well, Trinklein said.
Water each layer as you add fertilizer. “A properly constructed compost pile should feel damp to the touch, but not wet,” Trinklein said. If using manure as a nitrogen source, make certain of its history. Some herbicides applied to pastures and hayfields are very persistent. They pass through the digestive systems of animals and into the manure in concentrations strong enough to damage sensitive species like tomato.
When soil microbes encounter abundant carbon and adequate nitrogen, they “go bananas,” said Trinklein. The heat of respiration from the microbial activity raises the temperature of the compost pile. When composting is done correctly, the temperature at the center of the pile should reach about 135-140 F. The heat kills weed seeds, most pathogens and insects.
The time required to achieve this temperature depends on variables such as carbon-to-nitrogen ratio, moisture, aeration and outside air temperature.
As decomposition wanes, the compost pile begins to settle and cool. This indicates the microbes have “run out of food” and the pile is ready to be turned. Use a rake or shovel to move partially decomposed leaves from the outer edges to the center of the pile. This renews microbial activity and causes the pile to heat again. Once the pile fails to heat after being turned, decomposition is complete.
Leaves may not decompose completely before cold weather sets in, but the process will begin again when air temperatures rise the following spring.
Some leaves also may be used as mulch for tender plants such as azaleas and rhododendrons. Trinklein said stiff leaves such as those from oak trees do not mat and work best for mulch. Put leaves in a wire cylinder around the plant to keep them in place.
To download a free MU Extension guide on composting, go to extension.missouri.edu/p/g6956.
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