What To Do With Old Potting Soil

Potting soil loses its potency over time, but that doesn’t mean when it stops effectively helping your plants, you should throw it away. Instead, you may want to know other ways to use old potting soil.

If you’re a composter, you can mix old potting soil with newly completed compost to turn it into fertile soil. You can also put it at the bottom of a pot, then fill the rest with compost, worm castings, and fresh soil. Otherwise, you can lay a cardboard weed barrier down, add old potting soil, then layer the rest with compost and new soil.

You can create a new garden bed without digging and regrading your yard or spending a fortune. Old potting soil isn’t useless unless you’re unwilling to get creative. Follow along as we highlight five ways to reuse old potting soil.

How To Reuse Potting Soil

1. As a Filler

While old potting soil loses some of its best nutritional roperties, it’s not entirely without merit. That’s especially true if you have a large garden and plan to fill other large pots with fresh soil. These containers require a lot of soil, which can get quite expensive if you use fresh soil. You can keep some of your old potting soil and use it as a filler in your large pots.

That way, you don’t have to use as much fresh soil. Instead, put some bulky fillers, like twigs, bark, or compost, at the bottom of the pot. Next, add some used potting soil, and then cover that with some nutritious compost. From there, you can alternate between fresh potting soil and compost. The little bit of old potting soil won’t hurt anything or hold your plants back, but it also won’t directly support the plants.

2. Replenish It

You can use old potting soil as a filler in your pots, but it won't be reinvigorated. However, you can restore old potting soil to its former state by evenly mixing it with compost. The nutrients in the compost, mixed with the stability that the soil offers, can work to help your plants thrive. Old potting soil is only useless if you don't replenish its lost nutrients. Compost contains key nutrients, such as potassium, nitrogen, and phosphorus and helps enrich old soil.

This is a great way to reduce your carbon footprint, reuse potting soil, and put your compost to great use. The key is to maintain an equal mixture between potting soil and compost for the best results. If you want to lean toward one more than the other, favor the compost.

3. Prepare Seeds

Many people plant seeds indoors to prepare them before bringing them outside. This is because seeds are vulnerable, and starting them indoors helps protect them from the elements. What better way to reuse your old potting soil than to start your seeds? If needed, add some perlite and peat moss to your old potting soil to turn it into a great starter mix. Otherwise, you can mix your old potting soil with worm castings or compost.

While this is a good idea, only start some of your seeds using old potting soil, and start others in fresh, nutrient-dense soil. Otherwise, you may notice a poor germination rate if you start each seed in old potting soil. Old potting soil will work, but not as effectivley as fresh soil.

4. Put It In Your Terrarium

Do you have a terrarium? If so, then you may not realize that it is the perfect place for old potting soil. That’s especially true if you reinvigorate your old potting soil with compost or slow-release organic fertilizer. This ensures the old potting soil will at least have many of the same benefits as fresh earth. The key difference is that your old potting soil may seem more brittle, but you can fix that.

Wet the soil and let it sit for a day in a lukewarm environment before adding compost. Otherwise, you can bake the old potting soil at 200 degrees Fahrenheit to kill weed seeds and bacteria. Mix a little bit of peat moss in with your recharged potting soil to maximize its benefits.

5. Sheet Composting

Sheet composting is a tried-and-true gardening tactic centered around sprucing up your garden without heavy digging. Also known as lasagna gardening, it involves laying cardboard and newspaper, then adding organic materials and soil. This lets you build upon existing ground and soil while keeping weeds at bay. You can cover the weed barrier with old potting soil, and then add “green” composting materials.

That includes grass clippings, plant trimmings, straw, and shredded leaves, which add nitrogen. Then, you can add your “brown” layer to introduce carbon using wood chips, dried leaves, and cardboard. Add fresh soil, or mix it with old potting soil. There’s no right or wrong way to do this, so long as you start with a weed barrier. 

How Long Does Potting Soil Last?

You can expect an unopened bag of potting soil to last for up to 2 years. An open bag can go stale and become ineffective in under a year. That’s why it’s a good idea to only buy as much potting soil as you need and will use at that time. This ensures the soil won’t sit and go bad before you need it next. You can tell your open bag of potting soil is going bad when the soil clumps together.

Open bags are also prone to mold and pest infestations, especially when stored incorrectly. In some cases, pests, such as fungus gnats and mosquitoes, lay eggs in open potting soil bags. You must throw it away if you discover an abundance of eggs in the soil. Otherwise, you’re in for a rough time when those eggs hatch, as there are usually more than what you first see.

How Often Should You Change The Soil In Potted Plants?

Ideally, you should replace the soil in your potted plants every two years. However, it typically lasts much longer in slow-growing plants, in which case you must replace it every three to five years. The soil’s color and texture are important identifiers. For example, old potting soil often appears grey compared to when you first opened the bag.

Old soil is also prone to compaction and doesn’t properly absorb water. You can tell that’s the case if your plants appear wilted despite watering them shortly beforehand. Keep in mind that you don’t always need to replace all the soil in a pot. You can often get away with replacing the top 3” to 5” of soil after a year or two. However, a full soil replacement is almost always necessary within the 3 to 5 year range.

What Happens If You Don’t Replace Soil?

Your plants will develop nutrient deficiencies if you don’t replace the soil. This happens because plants absorb the nutrients in soil, and there’s only a finite amount of them. Once they are gone, your plant can’t get the nourishment it needs. That doesn’t mean your plants will die immediately, but they will slowly lose their luster and become less appealing.

The leaves will become discolored, and the plants will appear wilted even after watering them. Eventually, the soil will become so compacted that the roots will be smothered. That’s why it’s worth replacing the soil in your pots and garden beds.

Summing It Up

You can use potting soil as a filler substance in your potted plants, sandwiched between compost and fresh soil. Some people mix old potting soil with compost to reintroduce nutrients, like nitrogen and potassium. Otherwise, you can start your new seeds in old soil mixed with peat moss and perlite before bringing them outdoors when they germinate.

Related Guides:

• Does Soil Acidity Affect Plant Growth?
• How To Tell If Your Garden Soil Is Compacted