You may like the smell of burning leaves, but did you know you were sending an excellent soil conditioner up in smoke? Instead of burning leaves or stuffing them in garbage bags for the trash haulers to take away, compost them.
Compost improves the garden soil by increasing its organic matter. This, in turn, improves soil drainage. Organic matter is especially beneficial in heavy clay or light, sandy soils. Organic matter reduces soil crusting and helps soil hold water and nutrients. Decomposing leaves in your compost or garden feed earthworms and beneficial microbes. Leaves also supply a small number of nutrients, including those trace elements and minerals that trees have mined from deep within the soil. Between 50 and 80 percent of the nutrients that trees extract from the soil end up in leaves, ready to be recycled when the leaves fall (IF left on the ground or put into compost).
Microorganisms are what decompose materials to make compost. To do their work they need carbon sources for food and nitrogen for proteins. They are most effective when the ratio of carbon to nitrogen (C:N) is an average of 30 to one, by weight. You don’t need to weigh what you add to the compost pile, just be aware of approximate amounts that you’re adding. Generally, two to three parts (by volume) of brown to one part green materials work well. Some gardeners add an “activator” to help the microorganisms, which provide a source of protein and nitrogen. You can buy these commercially, or use alfalfa meal from garden or feed stores.
In general, coarse woody material (sawdust, leaves) is high in carbon. Moist, dense material (manure, grass clippings, non-meat kitchen scraps) is high in nitrogen. Too much carbon materials and the compost pile will decompose slowly. Too much nitrogen and you may smell ammonia gas.
To compost leaves, alternate leaves with layers of soil or manure. Make layers of leaves six to 12 inches thick, layers of soil or manure about one inch thick. To hasten decomposition, shred leaves first with a rotary lawn mower or shredder. Moisten each layer. Finish the compost pile by slightly rounding the top to help the pile hold water. Cover with an inch of soil. Some also alternate layers with a sprinkling of lime and fertilizer. Some leaves such as sugar maple may be more acidic with a pH of 4.3 and so need lime added, while other leaves such as of ash have a more neutral pH of around 6.8.
Next, cover the compost pile with plastic. Hold the sides in place with wire, concrete blocks, or boards. Turn the pile every few weeks throughout the fall, adding moisture during prolonged dry periods. Both the plastic (heat) and turning (aeration) will help speed decomposition and make the final product more uniform. Unless the pile is already moist, uncover when rain is predicted.
Compost piles are simple to make, but it does take time for the process to work. If you start a compost pile this fall, don’t expect to use it in the spring. However, it should be ready to spread next fall.
Keep in mind that you are not limited to leaves for composting. You can use any plant material that’s not diseased, doesn’t contain mature weed seeds, and hasn’t been treated with pesticides. In addition, non-meat kitchen scraps can be composted.
Plant materials and products that are easy to compost, and which generally decompose most rapidly, include egg shells, coffee grounds, pine needles, fruit peels and rinds, paper, sawdust, straw (not hay, as hay often contains weed seeds), vegetables, tea bags, wood ash, and wood shavings. Materials that are slow to decompose and may take two years to break down include coarse wood chips, branches, corncobs and corn stalks, and nut shells. Breaking these materials into smaller pieces, and adding high nitrogen materials will speed up their composting.
If you don’t have room or time to compost all those fall leaves, you can put a pile aside to add to a compost pile when you need brown carbon material this coming season. Or simply rake the shredded leaves to use to mulch around perennials and shrubs. Mulch helps conserve moisture in summer, keeps soils warmer in fall and spring, and reduces frost heaving in winter. Just don’t use too much of this organic mulch (a couple inches a year is good) or you’ll smother your perennials, and provide a habitat for mice during winter which can chew bark off of trees and shrubs.
For vegetable gardens and flower beds, once they’re cleaned out in fall, you can cover the soil with a couple inches of shredded leaves. Then sprinkle on some organic fertilizer, top with a half-inch or so layer of compost, and roughly mix all this into the soil. A shovel or spading fork works well. Your beds will then be rich with this simple compost, and ready for spring planting.
Dr. Leonard Perry, Horticulture Professor Emeritus
University of Vermont