Sifting Compost - Is It Necessary?

Finished compost is nothing short of a miracle. At my house, the compost waste stream combines garden plants and weeds, kitchen food waste and small amounts of chicken manure, which slowly morphs into a beautiful bio-active amendment for my garden beds. I also sift or screen some of my compost to use in containers or seed-starting mixtures. This is a slow but enjoyable task that saves me a ton of money not spent on specialty soils. It’s a great activity for the first warm days of spring.

When NOT To Sift Compost

First, a good reason not to sift compost. For compost that is destined for garden beds, as long as it no longer contains live plant tissues you’re good to go. Simply leave it in piles atop your beds, or bury it in holes or trenches in the garden. Bits of sticks or bark or chunks of eggshell-encrusted soil are welcome in your soil’s secret society of microbes because they provide physical habitat and shelter. In heavy soils, they may help improve drainage.

Coarse compost is also best when you have a lot of compost to move, or plan to pile it more than 2 inches (5 cm) deep. As a rule of thumb, the finer the compost, the less you need. Vermicompost that is heavily worked by worms is naturally sticky and may contain high levels of salt, so it is best left in scattered chunks. Weather will do the rest.

Screening Compost for Containers

Beyond using my finished compost in garden beds, I screen quite a bit to use in containers. Used potting soil can be rejuvenated with the addition of screened compost and a handful of organic fertilizer, but you don’t want rotting bits of organic matter moldering in your pots. Hence the need to screen or sift the compost.

Step one is often to dry out the material until it is light and crumbly. The easiest way is to wait for a sunny day and throw several spadefuls onto a tarp spread on the ground. Stir every few hours, and you should have a nice batch of lightly moist compost ready for sifting.

Over the years I have tried a number of compost screens, but they tended to get so clogged with organic matter that I spent more time clearing the screens than sifting compost. A few years ago, after being inspired by a friend who sifted compost using a plastic milk crate, I discovered using a plastic bulb crate as a compost sieve. Set inside a wheelbarrow, it’s sturdy enough to work the compost by hand, with a trowel, or you can pick up the crate and shake it about. Either way you get fine crumbles, just right for mixing into potting soils for containers.

Sifting Compost for Seed Starting

Can you start seeds in finely sifted compost? You bet! In the days before packaged soils became a commercial commodity, vegetables were given a head start in compost mixed with the best garden soil, and the method still works. You don’t need peat moss or coconut coir to make seed starting mix if you have good compost to work with.

But wait. Won’t the seedlings get attacked by diseases lurking in the compost and rot off? Shouldn’t you sterilize the compost first by heating it to 140°F (60°C) for an hour? Yes and no. Heat treatment kills most weed seeds and all microbes, good and bad, which many people think is a travesty. In this view, preserving the life forms in compost is more important for plant health than a small risk of disease. Here I tend to agree, though it means plucking out tiny weed seedlings that appear alongside my veggies. Since switching from commercial mixes to finely sifted compost, I have yet to lose a seedling to disease.

To make seed starting mix, you will need a fine sieve such as a food colander, or you can use a sturdy flowerpot with several holes in the bottom. Place a few handfuls of dry, cured compost in the colander or pot, rub it around with a gloved hand, and capture the sifted compost in a tray. Store the wonderfully fluffy sifted compost in a dry place, and throw the chunkier debris into a pot, flowerbed, or back into the compost pile. Composting is continuous.