WHY DOES THIS HAPPEN
If you notice that the soil in one of your houseplants starts to smell unpleasant, there could be a number of different causes.
Overwatering or overfertilizing are two of the most common causes of this symptom, as if there is too much water that ends up staying in the pot this can cause the soil to start to smell. However, note that many soils naturally contain various types of harmless fungi - sometimes this may be what causes the smell instead.
The smell itself isn’t usually harmful to your plants, however, overwatering or overfertilizing can lead to further problems such as root rot, so if you think that your plant is getting too much water, it is a good idea to keep an eye on its overall health too to see if it is coming to harm as the result of unbalanced watering / fertilizing.
An alternative reason why soil starts to smell may be because of the fertilizer itself. Some fertilizers smell stronger than others, and it's essentially down to personal preference! You can either try switching to a different brand or type of fertilizer if the smell bothers you, or you can try repotting your plant into a pot that has very good drainage, as this will help to stop the smell from 'dwelling' in the pot.
In fact, repotting your plant is often a good idea if the soil smells. Adding fresh, new soil can help to remove any smelly fungi or residues and allows the soil to aerate a little - just bear in mind that repotting is stressful for your plant, so shouldn't be done too often.