By Barbara Pleasant, February 22, 2024
Finished compost is an incredible resource, transforming garden scraps, kitchen waste, and small amounts of chicken manure into rich, bio-active material that benefits my garden. At my home, I sift some of my compost to use in containers or seed-starting mixtures. While it’s a slow but satisfying process, it saves me money on specialty soils and is a great way to spend the first warm days of spring.
When Not to Sift Compost
There are instances when sifting compost isn’t necessary. If you’re using compost to enrich garden beds, you don’t need to sift it as long as it no longer contains undecomposed plant material. You can simply spread it in piles or bury it in holes within your garden. Larger, coarser materials like sticks, bark, or eggshells offer physical habitat for soil microbes and even improve soil drainage in heavier soils. In fact, the chunks are often welcome by the soil’s microbial community.
Coarse compost is also ideal if you have a large amount to move or if you plan to pile it deeper than two inches (5 cm). The finer the compost, the less you need to use. Vermicompost, rich in worm activity, is naturally sticky and can be left in larger chunks to decompose further with the weather.
Screening Compost for Containers
I frequently sift compost to use in containers, particularly when rejuvenating used potting soil. Adding screened compost and organic fertilizer can refresh spent potting mixes, but the rotting matter in un-sifted compost can lead to issues in pots. For this reason, sifting is essential to avoid problems like mold or decay in your containers.
To begin, it’s important to dry the compost until it’s light and crumbly. A sunny day works best for this, as spreading compost on a tarp allows it to dry out quickly. Stir it every few hours, and soon you’ll have a batch of compost that’s ready for sifting.
I’ve experimented with different compost screens, but many of them would clog with organic matter, making sifting more laborious than it was worth. Then, inspired by a friend, I tried using a plastic bulb crate as a sieve, set inside a wheelbarrow. This method works great, as it allows you to shake or work the compost with a trowel to separate fine crumbles. These fine particles are perfect for mixing into potting soils for containers.
Sifting Compost for Seed Starting
Finely sifted compost also makes an excellent medium for starting seeds. In the past, before commercial seed-starting mixes became popular, gardeners used a mixture of compost and garden soil to give plants an early start. This method still works well today, and with the right compost, there’s no need for peat moss or coconut coir.
While some might worry about diseases in compost, it’s not necessary to sterilize compost by heating it to 140°F (60°C) for an hour. Heat treatment kills beneficial microbes along with harmful ones, which can defeat the purpose of using compost in the first place. In my experience, I haven’t lost a seedling to disease since switching from commercial mixes to finely sifted compost—though you may occasionally need to remove a weed seedling.
To create a seed-starting mix, use a fine sieve, like a food colander or a flowerpot with holes in the bottom. Rub dry, cured compost through the sieve and collect the sifted compost in a tray. Store it in a dry place for later use. The coarser bits can go into your garden or back into the compost pile, as composting is an ongoing process.
Conclusion
Sifting compost is not always necessary but can be incredibly beneficial for specific tasks, like container gardening or seed starting. By understanding when and how to sieve your compost, you can maximize its potential, save money on store-bought soils, and support a thriving garden ecosystem.