At this time of year, many gardeners are repotting their plants before bringing them indoors or into the greenhouse for the winter. As a result, they may have a growing pile of used-up compost on their hands. Can it be reused to save time, money, and resources?
Of course it can! Many people turn to gardening to help them save money and reduce waste in the first place. After all, if you grow your own fruits and vegetables, you’re already cutting down on packaging and other waste. If you purchase compost from your local garden center, the costs can add up. Even if you have your own compost bins at home, you may still want to reuse the compost you’ve worked into your containers to reduce overall waste.
Around this time of year, you’re likely emptying your pots of compost and putting any dead plant material in your bins. Be sure to pull out all the weeds and roots, too. You don’t want them making an appearance during the next planting season.
Did you grow tomatoes in your pots this year? You'll want to set out two bins or trash cans to store your used compost over the winter — one for the compost you used to fill the tomato pots, and one for the compost you used on all your other crops. Place the bins in your garage or greenhouse until spring. Although some plants, like salad greens, kale, herbs, beans, peas, and root vegetables aren’t picky about growing in old compost, others are. Tomatoes (and peppers) fall into the latter category. Since tomatoes tend to be nutrient-intensive and more susceptible to pests and diseases than other crops, you’ll want to make sure you’re not planting your new tomatoes in soil or compost that nourished last year’s bunch. When you’re ready to plant in the spring, use the compost from the second bin on your tomatoes and the compost from the first bin on all of your other plants and vegetables.
An important part of reusing compost is replacing the nutrients in it, which may have been used up during the previous growing season. To do so, you'll want to mix it with fresh compost, finished (i.e. well-rotted) manure, worm compost, or an organic general-purpose fertilizer, which you can purchase from your local plant nursery. How much you augment the compost depends on what was planted in it, as some crops use more nutrients than others. Additionally, keep in mind what you plan to plant in the reused compost. If you’re planting a nutrient-intensive crop, you’ll want to make sure the reused compost will meet its needs. For example, if you’re growing leafy vegetables, you’ll want to put more nitrogen in your compost. If you’re planting crops and want large fruit, on the other hand, you may want to put more potassium in it.
Add the amendments when you’re ready to plant, not before. It’s better to add a few enhancements at a time and adjust when the compost is in the container than it is to just pump it full of enhancements now. If you’ve combined all of your spent compost in one big container (like a trash bin), you may not have to add as many amendments, assuming you planted a variety of crops the previous year. If you're going to be mixing it with a store-bought fertilizer, be sure to follow the instructions on the label to help maintain the health of the soil and the plants themselves. Your plants will let you know what other nutrients they need by way of yellowing leaves, slow growth, pest problems, and more. Keep an eye on what you’ve planted in the reused compost and add nutrients when needed to address these concerns. Common additions include more compost and nettle tea.
If you’re concerned about weeds thriving in the used compost, just add it to the bottom of each pot and dress the top half with fresh compost, well-rotted manure, worm castings, or another planting medium. Once you’ve planted your crops, add mulch to the top of the soil to help retain moisture. By relegating the old compost to the bottom of each container, you’ll reduce the chances of last year’s weeds pooping back up and smothering your plants.
Other Things to Consider
Although you may be able to reuse your compost indefinitely, you'll eventually notice (after a few years or so) that you’ll have to rebuild the structure of your soil. This is easily done by working medium-grade vermiculite or perlite into it. You can also use biochar, or ground-up charcoal, to do this, and it's sold at many garden shops. Another option is green waste compost. Check with your city to see if they collect green waste and sell it to the public, or make your own using grass clippings, leaves, etc.
If you tend to refill your pots with fresh soil and compost each year but still want to reuse your old stuff somewhere, use it in the garden. Your garden crops will appreciate the new dressing, and it’ll still pump nutrients back into your soil.