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Composting 101

Here's how it works. Mother Nature has lots of little helpers: bacteria, fungi, actinomycetes, nematodes, earthworms and more. These critters break down the leaves, weeds and moldy broccoli that end up in your compost pile. Bacteria and fungi in particular can reproduce very fast if there is adequate food - carbon for energy and nitrogen to build proteins. Microbes also use the other elements needed by plants - everything from the potassium and phosphorus found in every fertilizer to the trace elements like cobalt, magnesium and zinc that are not found in chemical fertilizers, but are found in plant waste and organic fertilizers.

As they reproduce and grow, the soil organisms give off heat. The hotter your compost pile, the more likely it is that the heat will kill weed seeds. I once tested annual grass seeds for germination after placing them in my compost pile and monitoring the temperature with a soil probe. I placed the seeds in sachets made of panty hose to keep track of them. I found that 130-140 degrees will kill annual grasses, even if kept at that temperature just for a day or two.

The problem is, of course, that it is virtually impossible to get the entire compost pile to the right temperature and keep it that way. The limiting factor seems to be nitrogen. If you add manure or fresh grass clippings, the pile will heat up quickly, particularly if you turn the pile regularly to mix in oxygen. Even so, I try to keep noxious weed seeds such as purple loosestrife out of the compost because I don't want to risk introducing them into my soil later on.

I start a new compost pile every spring, but don't use my compost for 3-5 years, at which point everything is broken down, and it resembles good, rich soil. The compost is rich in minerals, but is not a substitute for fertilizer, as most of the nitrogen - which fuels green growth - has turned into gaseous forms and gone away. Some people cover their compost piles with old carpets to minimize off-gassing of nitrogen, and to keep down weeds, though I do not.

There are many other factors that influence how fast your compost decomposes. The pile should be moist, not dry, but it shouldn't be soggy, either. If you squeeze a handful of the decomposing matter, you shouldn't be able to wring out water. It should be as moist as a damp sponge. Don't start a compost pile in a spot where water settles, as it can wick up water, keeping it too wet. During dry periods, you may have to water down the pile with a hose for maximum efficiency.

The microorganisms that do most of the work need oxygen. That's why some people turn their compost piles, or aerate them with a specialize tool that looks like Ahab's harpoon. That's a lot of work, so I don't bother. I do start a new pile by placing brush as the bottom layer, and will throw sticks or branches in from time to time. I've read about people who place 4 inch diameter PVC pipes in compost piles after drilling hundreds of large holes in them. That will help air penetrate, but it sounds like a lot of work to me.

Just as in the National Basketball Association, size matters. For a compost pile to heat up, it needs to be about 3 feet in each direction. If you have neighborhood skunks, raccoons or loose dogs, you probably need a bin of some sort. Plastic bins are good for that, or you can make an enclosure using pallets, or cedar poles and chicken wire.

Composting organisms need the proper balance of carbon and nitrogen containing materials to grow well. Scientists have determined that a ratio of 30 parts of dry, brown materials like hay or leaves to 1 part green things like grass clippings works best. Manure, though brown, counts as green because it has lots of nitrogen, the key ingredient. I just compost everything, and don't worry about ratios.

What not to put in the compost? Dog and cat manure, as they may carry diseases. Fats, grease. Roots of goutweed or creeping grasses that might survive and start growing in the garden. And don't add limestone. It will react with nitrogen sources and hasten nitrogen losses as ammonia gas.

So why bother with all this? A clean garden produces healthier plants and minimizes the spread of disease. Composting recycles minerals that your plants need and produces humus, a complex organic compound that can serve as a kind of piggy bank for moisture and minerals.

If people who compost could be compared to the three little pigs of storybook fame, I'd be the lazy pig. I don't have time to turn, aerate and water my compost pile. I agree with the bumper sticker I once saw: Compost Happens.

Henry Homeyer is the VT/NH associate editor of "People, Places and Plants" magazine. Write him at gardening.guy@valley.net or P.O. Box 364, Cornish Flat, N.H. 03746.