Gardeners get lesson in composting

Ten days ago, 20 UMaine Extension Master Gardener trainees gathered around a compost bin at the back of Shead High School in Eastport. Each student had brought to this class a useful ingredient in a recipe for compost: dry straw, alpaca manure, seaweed, vegetable and fruit peelings, coffee grounds, eggshells, grass clippings, dried leaves, whole or shredded. One student contributed a half-gallon of old herbal tea, leafy sediment included.

In 30 minutes, the class filled the bin, a closed wire fence measuring 3 feet in all directions, the minimum size for a successful compost pile. I watched in amazement as something that takes me months to build was accomplished in half an hour.

Their teacher advised them to begin by turning over the soil beneath the bin, providing easy access to the pile for earthworms, ground beetles and microbial agents of decomposition. She also pointed out that a compost bin in full sun will easily dry out, slowing down decomposition. The best site is well-drained with partial shade and enough space to maneuver and stockpile ingredients.

The pile started with a 12-inch layer of fluffed straw – not hay, which is full of weed seeds. Dry straw, hollow stems of harvested grain, is used as the initial layer because it is stiff enough to resist compaction and will allow air to enter the pile from the bottom. Like dry leaves, straw is an excellent source of carbon, the primary energy source for the bacteria and fungi that decompose organic matter.

Six inches of fresh grass clippings were added next, providing the compost-manufacturing bacteria with nitrogen essential for reproduction. Grass clippings are not essential, however, and many gardeners prefer to leave them on the lawn. Animal manures (except from dogs and cats) are excellent sources of nitrogen, as are coffee grounds, vegetable and fruit waste, and seaweed collected from above the high-tide line.

Layer by layer the pile grew, shredded leaves followed by garden weeds that had been chopped into small pieces with a digging spade, then dry whole leaves. At this point, with the bin about one-third full, the pile was watered and thoroughly stirred with a garden fork.

Next came layers of chopped seaweed, straw, alpaca manure, shredded leaves, chopped kitchen wastes, more leaves. Last, the pile was thoroughly stirred and watered, then topped off with final layers of seaweed and leaves.

Chopping bulky ingredients such as garden weeds, vegetable and fruit peelings and seaweed increases the surface areas of these ingredients. Bacteria and fungi work at the surfaces of decaying materials.

Decomposition occurs quickly when the pile’s carbon-to-nitrogen ratio, or C:N, is approximately 30:1. The students accomplished this by mixing high-carbon materials (C:N of 50:1), such as straw and dry leaves, in roughly equal amounts with high-nitrogen materials such as manure, seaweed or grass clippings (C:N of 15:1).

Success in composting is measured with a thermometer. A well-constructed compost pile heats up as bacteria do their work, reaching a peak between 90 and 140 degrees F, and then starts to cool. When the pile returns to ambient temperature, turn it so that the least decomposed material on the cooler outside of the pile is moved to the middle. After turning, water the pile and the temperature in the middle should start to rise again.

Two students, Bill and April Mullins of Eastport, volunteered to take the pile’s temperature every other day using a compost thermometer, its long stem inserted deep into the middle of the pile. One week after building the pile, its temperature was a healthy 149 degrees.

The finished compost will be dark brown with a crumbly texture – chocolate cake for the garden. None of the original ingredients will be recognizable. These Master Gardeners will have their compost out of the oven by early October!

Send queries to Gardening Questions, P.O. Box 418, Ellsworth 04605, or to rmanley@ptc-me.net. Include name, address and telephone number.