Commercial composting operations have a fairly precise definition of what constitutes finished compost. When ready to use, the compost has a carbon-nitrogen ratio of approximately 15 to l, has undergone a volume reduction of about 50 percent, and shows a weight reduction of about 50 percent.
For homeowners, the decision to start using the compost they’ve made is more subjective. Knowing when backyard compost is ready to use is as precise as knowing when homemade chili is ready to eat. It’s done when you believe it’s done. Finished compost is usually dark brown and resembles commercial potting soil, although it may be much coarser. Even when the finished compost is lumpy, all the ingredients have lost their individual identities.
If you prefer quantitative measures of doneness, you may want to conduct a temperature test, although it involves some effort. First, turn the compost pile. Then record the temperature inside the pile after it’s had a chance to heat up. The test is effective only if you take the temperature after you introduce oxygen by turning the pile. A temperature that doesn’t exceed 110 degrees is a sign that microbial decomposition has finished.
The outside layers of the pile don’t usually decay completely, but compost doesn’t have to be fully decomposed to be useful around the yard. For example, coarse compost that’s only 50 percent decomposed can be used as a mulch or soil amendment anywhere in the garden. It’ll continue to break down slowly. However, if you need compost for a potting mix, make sure it’s completely decomposed.
To produce a uniformly textured, fluffy product, run the finished compost through a shredder. You can also sift the compost through a coarse or fine sifter. Put it through a coarse sifter, with holes about 1 inch across, for ordinary garden use. Put it through a riddle (a fine garden sifter) with a mesh of about 1/4 inch if you need very fine material for starting seeds or to mix with peat moss for container plants. Recycle the material that doesn’t go through the sifter into the next compost pile. It makes a good source of starter microbes.
Store extra compost in a protected spot or under a tarp. Compost exposed to rain or snow will lose many of its nutrients, although it will still be a valuable soil amendment.