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 sieve 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 speciality bagged compost mixes. It’s a great activity for the first warm days of spring.

When NOT To Sieve Compost

First, a good reason not to sieve 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.

Sieving Compost for Containers

Beyond using my finished compost in garden beds, I sieve quite a bit to use in containers. Used potting mix can be rejuvenated with the addition of sieved compost and a handful of organic fertiliser, but you don’t want rotting bits of organic matter mouldering in your pots. Hence the need to sieve 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 sieving.

Over the years I have tried a number of compost sieves, but they tended to get so clogged with organic matter that I spent more time clearing the screens than sieving compost. A few years ago, after being inspired by a friend who sieved 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 blending into potting mixes for containers.

Sieving Compost for Seed Starting

Can you start seeds in finely sieved compost? You bet! In the days before packaged potting mixes 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 sterilise 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 sieved 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 sieved compost in a tray. Store the wonderfully fluffy sieved compost in a dry place, and throw the chunkier debris into a pot, flowerbed, or back into the compost pile. Composting is continuous.