Your Potting Soil Might Be Killing Your Plants—Here’s How to Tell

Before you reach for that half used bag of potting soil that’s been sitting on that shelf in your shed or garage for who knows how long, check it first. Many gardeners don’t realize that potting soil has a shelf life. While it may not spoil like a gallon of milk, it does go bad. Over time, the peat moss, bark, and compost that make up potting soil begins to break down, becoming compact, and in some cases moldy or even infested with bugs. Using bad potting soil can attract diseases, stunt plant growth, and even introduce pests that will munch on your plants.  

Related: This Is the Only Grass Seed Worth Planting Right Now (Most People Get This Wrong)

Signs That Potting Soil Has Spoiled

There are several telltale signs to look for that will let you know when potting soil has gone bad: 

• Swampy smell: When potting soil remains compacted and moist in a bag for an extended period, it creates the perfect environment for anaerobic bacteria growth. You’ll know if this is happening if you open the bag and catch a whiff of swampy rotten egg smell.

• Compaction: Soil that has a dense sludge-like consistency and won’t fluff up when you move it around won't allow nutrients, water, and air to reach plant roots or allow for good drainage. 

• Visible mold: When potting soil goes bad it often develops a white, fuzzy mold. This is a clear sign of high moisture content that has led to mold growth. Mold will cause root disease to spread in plants. 

• Pests: If you dump out that bag of potting soil and you find insects or insect larvae in the mix, then it’s no longer safe to use for your plants.

How long Potting Soil Lasts

How long potting soil lasts depends on how you store it. An unopened bag of potting soil will typically last from 1 to 2 years before the peat moss begins to decompose and degrade. 

Opened bags should be used within 6 to 12 months before they begin to break down and lose their structure and nutrient content. 

Properly storing your potting soil can extend its life. 

• Store indoors: If possible, store bags of potting soil indoors in a cool dry place to prevent it from getting saturated by rain.
• Seal the bags: Transfer potting soil from open bags to an air tight container to protect it from pests and excess moisture.

Related: Why You’re Seeing More Bugs Right Now—Blame This Winter’s Wild Weather

Can I salvage old potting soil?

If your potting soil is mold and pest free, you can often revive it. Here’s how:

• Dry it out. Empty the bag and spread it over a tarp evenly in direct sunlight to dry it off and kill any bacteria. Leave the soil for a couple of days to allow it to dry out.

• Aerate the soil.  Rake the soil to loosen it up so air and water can circulate through the soil. 
• Add nutrients. Mix in some fresh compost and perlite to improve aeration in the soil. You can also mix the old potting soil 50/50 with new potting soil to help refresh it. 
• Add water. Since you dried out the potting soil, you’ll need to reintroduce moisture by lightly spraying it with water. 

FAQs

What is potting soil?

Although it’s called potting soil, potting soil isn’t really soil at all. It’s actually a mix of peat moss, coconut coir, compost, and perlite or vermiculite. The mix is designed to be sterile while balancing water retention, nutrients, and oxygen flow. 

Can I reuse potting soil?

You can resume potting soil, but you’ll need to freshen it first by adding nutrients and restoring its structure. Remove any old roots from the soil then mix in 25% new potting mix or compost and add a slow-release fertilizer. Never reuse potting soil from diseased plants.