To sift or not to sift?
That is the compost question.
I really wish that just once I could answer a question about compost with something other than “it depends,” but on the topic of sifting it just, simply, depends. I’m sorry. The reason you see conflicting opinions about this on the internet is because there are different answers for different situations. To understand why, a quick primer on the underlying chemistry can be helpful:
Microbes in your compost depend on a balance of carbon and nitrogen in order to process either element. Specifically, they utilize nitrogen to break down carbon, disappearing both in the process. (This is called “decay.”) When you maintain a compost, you provide the nitrogen your microbes need each time you add high-nitrogen materials, like food scraps or manure. However, if your finished compost retains visible pieces of high-carbon product, like wood chips or twigs, and you use that compost on your garden, your microbes will harvest what nitrogen they need to finish decomposition from your soil—possibly at the expense of your plants. If your overall soil health is very good, this may not matter. There’s plenty of nitrogen to go around. If your soil conditions are degraded, though, the competition for nitrogen becomes more meaningful. Your plants may suffer.
That’s why many compost professionals will recommend that you sift your compost before using it in your garden. It’s a safe way to ensure maximum nutrient availability in variable soil conditions. Sifting also has other benefits, like screening out stickers, rubber bands, and other small-scale garbage that can accidentally make its way into a heap. However, unsifted compost can also double as a high-quality mulch and can be used effectively as such. That’s why some compost professionals tell you not to sift your compost.
Isn’t that annoying?
If you think this all sounds too complicated and is difficult to follow, I don’t blame you. Even I consulted some friends on this question (thank you Tom Gilbert at Black Dirt Farm and Cory Groshek at Greener Bay Compost for your helpful conversation, plus regenerative soil scientist and all-around superhero Andie Marsh for your additional review). Ultimately, my advice is “just try it out and see what happens,” but for those who endeavor to be more discerning in their compost practice, I put together a short-hand guide.
Should you sift your compost? A Guide
Sift:
When mixing finished compost into the soil of your garden or house plants.
When generally unsure of soil health.
When starting seedlings.
When you have pets who might consume partially-decomposed compost, at risk to their health.
Don’t sift:
When preparing a garden bed at least a season in advance of planting.
When top-dressing healthy garden soil around mature plants.
When top-dressing mature house plants.
When repairing bare soils.
In situations where plant growth is not a top concern.
Today’s question brought to you by loyal reader Molly Hodson and loyal-er friend Cowboy, pictured here with some unsifted compost. It’s also been brought to my attention that some folks think I will only feature a compost question if a dog is involved. Well, yes. The rumors are true. Also, hello from New York City. I’m here for a few more days before I return to the West Coast and get back into my normal writing groove.
I can’t wait.
I screen my compost through 1/2" hardware cloth before I give it to people in big bags. They're always amazed at how uniform and spongy it is, and at all the little worms. The coarse stuff that gets screened out is ideal for reincorporating in the compost pile because its irregular shape creates air space, and while it has little sticks and things that haven't broken down, it's coated with all the same good microbes that are all through the pile, so it will inoculate the new material you mix it with. It's a win-win.