Smell your soil to check for biological activity

Dig up a handful of soil from 4-6 inches deep and smell it. Healthy, biologically active soil has a pleasant, earthy aroma. Sour, rotten-egg, or sewage smells indicate waterlogged, anaerobic conditions. No smell at all suggests very low microbial activity.

Why It Works

The earthy smell of healthy soil is caused by geosmin, a compound produced by beneficial Actinomycetes bacteria (particularly Streptomyces). Per the Microbiology Society, the human nose can detect geosmin at concentrations as low as parts per trillion — making this an extraordinarily sensitive biological assay. Hydrogen sulfide (rotten egg smell) is produced by anaerobic bacteria in oxygen-deprived, waterlogged soil.

Tips

• A strong earthy smell after watering (petrichor) indicates an active microbial community
• Sulfur/sewage smell means your soil needs better drainage before anything else will help
• This test is entirely qualitative — it cannot tell you nutrient levels or pH
• Useful as a quick "is this soil alive?" check when evaluating a new garden site