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Potting soil doesn’t go bad the same way food goes bad. It can’t go bad to the point where it’s dangerous. It can become less fertile, compacted, or moldy, but using it can’t harm you.
However, potting soil can become contaminated, and that is a potential health hazard.
Potting soil has gone bad if it gives off an unpleasant odor, is growing mold, has become crushed and compacted, or has become waterlogged. Contaminated potting soil is rare but harder to detect, and may have discolored bags, chemical smells, or pests.
Potting soil that has gone bad can still be salvaged and used. Contaminated soil should be disposed of immediately.
If you’re concerned that your potting soil may be unusable, start by diagnosing the problem.
3 Ways Potting Soil Can Go Bad
As potting soil sits, it can lose nutrients, lose structure, and begin to grow mold. This is most common with bagged soils that have been left outside in the elements for several months.
You can easily revive bagged potting soil once you’ve identified the problem.
Potting soil that has been left in the rain or in a wet environment will leach nutrients and may eventually grow mold and begin to give off a foul odor. Exposure to sunlight can also break down some nutrients, so if your bag shows evidence of sun bleaching, you can assume it has lost some nutrition. Even if bags are kept in a dry, climate-controlled environment, nutrients will degrade with age.
If old potting soil appears to have a loose, fluffy structure and earthy smell, you can simply amend it with fertilizer and use it as normal.
Potting soil that has been left outside for months on end, or stacked into large, heavy piles, will eventually become compacted. However, potting soil doesn’t compact the same way garden soil compacts, because there is no clay.
Potting soil should be loose and spongy, but as the structure degrades, it will have a fine, sandy texture that compacts and shrinks drastically when it gets wet.
If potting soil doesn’t bounce back when you squeeze a handful, it has become compacted. Use materials like perlite, vermiculite, coconut coir, compost, or peat moss to lighten the soil structure.
Moldy, Waterlogged, or Smelly Soil
Potting soils contain organic materials, which are meant to slowly break down in containers. However, inside plastic bags, anaerobic bacteria will build up as the materials begin to decompose.
This can result in an ammonia-like odor, especially if the bag is constantly wet. Mold and other fungi that thrive in dark, damp conditions may begin to grow.
If you want to use moldy or waterlogged soil, open the bag and pour the soil into a shallow container. Leave the soil in a bright, sunny spot until the soil is completely dry. Then, add perlite or coconut coir to adjust the structure.
Waterlogged soil has most likely lost nutrients to leaching. Amend dried soil with fertilizer to support container-grown plants.
2 Ways Potting Soil Can Become Contaminated
Contaminated soil is unusable.
While old potting soil lacks nutrients and a healthy soil structure, contaminated potting soil will cause direct harm to the plant.
The most common type of chemical contamination is herbicide and insecticide residue. Potting soil that has been stored on palettes in nurseries or garden centers can become contaminated if chemicals spill or have been sprayed near the bags.
Unfortunately, it’s difficult to tell if potting soil has come in contact with pesticides or other chemicals. Some signs include:
- Discolored bags (especially if bags on the palette show a spill pattern)
- Chemical smells
- Discolored perlite or vermiculite in the potting soil (although this can happen in waterlogged soils as well)
Many chemicals will leave no odor or visible sign of contamination, and you may only notice a problem if your plants begin to show signs of distress. However, stress has many causes, and soil contamination is extremely rare.
If you have reason to believe your potting soil has been contaminated by a chemical, dispose of it away from your landscape and garden areas.
Although rare, potting soil can become contaminated with insects and/or weed seeds.
The most obvious signs of insect infestation are:
Some pests, like nematodes, are invisible to the naked eye, and you may never know they have invaded your potting soil.
Weed seeds are rare, but it’s possible that your potting soil became contaminated if the soil was stored outdoors near open, weedy areas.
If you suspect your potting soil is contaminated with weed seeds, prepare a sample as if you were going to start seeds, and keep it moist for a few weeks. If plants sprout, you can assume the bag was contaminated.
Dispose of potting soil contaminated with weed seeds or insects far away from landscapes and gardens. Do not add potting soil to your compost pile.
How to Tell if Seed Starting Mix is Bad
Seed-starting mix is a special type of potting soil. This mix is light, fluffy, and usually sterile.
Seeds are at a higher risk of dying from poor soil conditions than mature plants. Pathogens like root rot, damping off, and other rots and molds will attack young seedlings and result in poor germination rates.
In order to prevent these pathogens from attacking seeds, these mixes are sterilized. However, if the mix becomes waterlogged or sits open outside, pathogens, pests, and weed seeds can invade.
Seed-starting mixes should remain sterile, so if the bag begins to smell, grows mold, or shows any other sign of life, you should discard the entire mix.
If, however, seed-starting mix has stayed sealed and dry, it is perfectly fine for seeds. Don’t worry about nutrients leaching or the mix becoming compacted. Once the plants are old enough to need nutrients and a spongy soil structure, they will be transplanted into more suitable potting mix.
For more information on this topic, read our comprehensive article Does Soil Go Bad?