Potting soil smells bad: what a sour or rotten-egg smell means

A sour, swampy or rotten-egg smell from a pot means the mix has run out of air, most often because it is holding too much water. Stop watering, empty the saucer, and slide the plant out to look at the roots: that tells you whether the mix just needs to dry and be replaced, or whether the roots are already rotting. Not sure the plant is overwatered at all? Overwatered vs underwatered lays out the other signs.

What the smell is telling you

A healthy pot smells faintly earthy, or of nothing much. A bad smell is a signal about oxygen.

University of Maryland Extension says excess water reduces oxygen in the soil, which damages fine roots and leaves the plant unable to take up water. UConn Extension describes the same chain in a pot: water that stays in the saucer or cachepot keeps the potting medium saturated and displaces necessary oxygen, so roots die or deteriorate. The RHS says the same thing about garden soil, where oxygen is used up by plant roots and soil microorganisms in prolonged saturated soil.

The smell itself comes from what survives without air. The RHS says a root sample from waterlogged soil will show blue-black roots, a typical sign that may be accompanied by a sour, rotting smell. R. W. Sheard’s turfgrass soil paper, archived by Michigan State University Libraries, names a lack of oxygen in the soil as the basic cause of black-layer conditions and links the “rotten egg” odour to hydrogen sulphide. A rotten-egg note in a pot is the same kind of signal, only on a smaller scale.

Because University of Maryland Extension notes that overwatered plants show the same symptoms as plants with root rots, the smell alone cannot tell you which one you have.

Rule out the likely causes

Work through these in order.

Cause	What you see	Test
Too much water, too often	Mix wet below the surface, pot heavy	Finger test and lifting the pot
Water standing in the saucer	Water in the saucer or cachepot, wet base	Look under the pot
No drainage or packed mix	Water pools on top or drains slowly	Pour in water and watch
Rotting roots	Soft, dark roots, wilting despite wet mix	Unpot the plant
Unfinished compost in the bag	Sharp smell from a pot you just filled	Smell a handful of fresh mix

Check how wet it really is

University of Maryland Extension suggests testing the soil with your finger to a depth of about two inches, and says that if the soil is dry it probably needs water. Lifting the pot works too: it says a pot with dry medium weighs much less than one with ample water still in it. A pot that smells bad and still feels heavy and wet has your answer.

Look under the pot

UConn Extension says to empty saucers or cachepots within an hour or so of watering. University of Maryland Extension adds that pouring off water that collects in saucers helps prevent problems with fungus gnats and other insects, so a smelly pot with gnats around it is usually the same overwatering story. See fungus gnats in herb pots for that side.

Check drainage

A pot without a drainage hole holds water at the bottom even when the surface looks dry. If your pot has no hole, or the hole is blocked, the drainage holes guide covers the fix. A layer of gravel in the bottom does not stand in for a hole; the gravel-in-the-pot myth explains why.

Unpot it and check the roots

Slide the plant out and look. Chicago Botanic Garden says healthy roots are firm and white; infected or rotten roots are soft, darkened and often produce an unpleasant odor.

• Mostly firm, pale roots: Chicago Botanic Garden says to trim out any damaged ones and remove yellow or dead foliage, then repot the plant using a fresh soilless mix and resume normal care. Do the repot in a clean pot with a drainage hole.
• Mostly soft, dark roots: this is root rot, and how to save a potted plant with root rot walks through the rescue. Chicago Botanic Garden says that if more than 50 percent of the roots are damaged, you might consider discarding the plant.

Either way, the plant goes into fresh mix in a clean pot, which gives it air back.

Can the soil be used again?

If the roots were healthy and the mix was only waterlogged, you can spread the mix out to dry and then judge it for reuse; can you reuse potting soil answers whether it is safe for the next planting. If the mix is merely tired and compacted rather than waterlogged, how to refresh old potting soil covers what to work back in.

Mix that held a rotting root system is a different case from tired mix. If the roots were rotting, discard that mix rather than reuse it; can you reuse potting soil treats root rot as a reason to discard.

When the smell is in a brand-new bag

Mix that contains compost can smell off on day one. Penn State Extension says finished compost has a sweet smell, and that anaerobic, sour or putrid smells are suspicious. It also says unfinished compost may release volatile organic acids that can negatively affect seedling growth and development. A sharp smell from a fresh bag is worth taking seriously for that reason, especially for seedlings. Utah State University Extension says finished compost should look and smell earthy, even when wet, so a mild, earthy smell is fine. Compost choice for edible pots is covered in choosing compost that’s safe for edible containers.

Keep it from coming back

Water when the top two inches are dry, rather than on a schedule. Empty the saucer within an hour or so of watering. Keep drainage holes open. And repot with fresh mix if the plant has sat wet for long enough to smell.

Sources

• RHS, Waterlogging and flooding
• University of Maryland Extension, Symptoms of overwatering indoor plants
• University of Maryland Extension, Watering Indoor Plants
• UConn Extension, Watering Houseplants
• Chicago Botanic Garden, Overwatered Houseplants
• Penn State Extension, Potting Media and Plant Propagation
• Utah State University Extension, Using Compost in Utah Gardens
• R. W. Sheard, Black Layer Formation in Turf (Michigan State University Libraries, Turfgrass Information Center)