Can You Reuse Potting Soil From Last Year? How to Refresh It

Can You Reuse Potting Soil From Last Year?

Yes. You can reuse potting soil from last year if the plants that grew in it stayed healthy. University of Illinois Extension answers the question “to a certain extent, yes”: when the plants had no major disease issues, remove the plant material at the end of the season and reuse the mix the next one. North Carolina State Extension draws the line at soilborne disease, saying container media can be reused as long as problems such as Verticillium or Fusarium wilt did not occur in the previous season.

Used mix is never ready to go straight back into service, though. It has lost its fertilizer and some of its structure, so the standard advice from University of Maryland Extension is to mix last year’s growing media 50:50 with fresh growing media, compost, or both.

Sort your pots before doing anything else. Mix from containers of annual flowers, herbs or salad greens that grew well until frost is the easy case. A pot where a tomato wilted while the soil was still moist, or where roots came out brown and soft, belongs in a different pile. If you cannot remember what grew in a pot, treat the mix as doubtful and heat it before reuse.

Why Old Potting Soil Needs Refreshing

Most bagged potting soil contains no soil. It is a soilless blend of sphagnum peat moss, perlite, vermiculite, composted bark, compost and coconut coir, with small amounts of lime and fertilizer. Each of those parts changes over a season.

The fertilizer goes first. According to NC State, the nutrients premixed into commercial media last only a couple of weeks and do not cover long-term growth. A pot that carried petunias or peppers from May to October has been running on added fertilizer for most of that time, and the mix itself holds almost nothing in reserve.

Structure goes more slowly. As the organic matter decomposes, particle size and pore space shrink, and drainage and aeration fall. That is why second-year mix looks darker, sits lower in the pot and stays wet longer after watering. Roots need the air in those pores as much as they need the water, and plants in tired mix often show it by wilting in wet soil.

Salts are the third change. Fertilizer residues accumulate, and NC State notes that the sign is a white crust on top of the potting mix.

How to Refresh Used Potting Soil

The method below combines the Maryland and NC State guidance. Do it in fall when you clear the pots, or in early spring before planting.

1. Empty the pot. Tip the mix onto a tarp or into a tub. Remove all plant residues, old root masses and plant tags.
2. Break it up. Crumble the clumps and any remaining old roots by hand. NC State points out that emptying, breaking up and re-blending the mix keeps it from becoming too compacted.
3. Blend 50:50. Combine the old mix with an equal volume of fresh potting mix, compost, or a combination of the two. Compost brings major and minor nutrients, and fresh mix brings back coarse particles.
4. Add fertilizer. Work in a time-release fertilizer at the label rate. Compost does not replace it: Maryland notes that the nutrients in compost are released slowly and are usually not sufficient for an entire season.
5. Store or plant. Refill the containers, or store the blend until spring.

If the surface carried a white crust, leach the mix before you blend it. Run water slowly through the media for several minutes and let it drain freely. Leaching also suits pots you fed heavily all summer, since its purpose is to remove excess nutrients.

During the season, feed the refreshed mix as you would a new one. NC State’s schedule for liquid fertilizer is once a week at half strength or every two weeks at full strength. Vegetables generally grow best at a soil pH of 5.5 to 7.0, and fresh mix contains a little lime, which is one more reason to include it in the blend. For feeding schedules by crop, see fertilizing container plants.

When to Throw Old Potting Soil Out

Disease is the firm limit. Maryland’s rule is short: do not save the media if root diseases were a problem. Verticillium and Fusarium wilts, the two that NC State names, live in the soil and infect the next plant through its roots, so a clean pot and a new plant will not solve the problem if the mix goes back in.

Signs that a pot belongs in this group include a plant that wilted while the mix was moist, and roots that were brown and mushy when you pulled the plant. Fresh mix is the safe choice for those containers. Scrub the pot as well, since NC State recommends washing previously used pots in a non-bleach household disinfectant and rinsing them thoroughly before use.

Age is the second limit. Illinois Extension advises replacing the mix when its organic materials have decomposed to the point where drainage and aeration are lost. Mix that stays soggy for days, has turned fine and muddy, or has sunk well below the rim after only one summer has reached that point. It still has value outside a pot. Illinois suggests sending it to the compost pile or the garden, where it acts as organic matter, and refilling the container with fresh media.

Heat-Treating Doubtful Mix

Heat is the option for mix you are unsure about, such as pots with no record or a season of fungus gnats. It is also worth doing for any reused mix that will start seeds. Oregon State University Extension gives an oven method for small batches:

1. Moisten the mix slightly.
2. Place it in a heat-resistant pan and cover it with a lid or foil.
3. Put it in a 250°F oven and check the temperature with a candy or meat thermometer.
4. When the mix reaches 180°F, keep it there for 30 minutes.

Oregon State warns against overheating, which can damage soil structure, so pull the pan once the half hour is up.

For larger volumes in a hot summer, solarizing uses the sun instead. The University of California’s pest program describes two setups. In the first, bags, pots or buckets of moist soil sit on an elevated surface such as a pallet under a double tent of clear plastic. In the second, moist soil goes into closed black trash bags placed on pallets. The targets are temperatures, not days: 158°F or higher for 30 continuous minutes, or 140°F or higher for 60 continuous minutes, the treatment standard quoted in a University of California study of the double-tent method. In warm parts of California, soil in black bags can pass 160°F. A soil thermometer pushed into the middle of the bag tells you whether your climate gets there. In a cool or cloudy summer it may not, and the oven is the more dependable route.

Storing Used Potting Soil Over Winter

Maryland’s storage advice is to keep emptied media in a trash can or heavy-duty trash bags. A lid keeps out rain, which would leach whatever nutrients remain, and blocks weed seeds. Label each container with what grew in the mix so you can put a different plant family in it next year. That simple rotation lowers the odds that a problem you did not notice carries over.

Mix left in the pots through winter is also reusable in spring. Empty it then, break it up, and blend it 50:50 the same way. If the pots themselves are staying outdoors, how to overwinter potted plants covers protecting both the container and anything still growing in it. When it is time to buy the fresh half of the blend, the best potting mix for containers compares the common types, and perlite vs. vermiculite explains the two white ingredients you will see on the label.