Calin, it sounds like that mix is too dense. This is a term that means there is very little air space in the soil mix. It can occur in any mix if it has too many small particle size aggregates. Most often it happens in clay soils, but it can easily be a problem in pots of mineral soils, too, if it includes a lot of fine sand (small particle size). Small air spaces in the soil easily fill up with water and displace what little air there is, starving the soil (and plant roots) of much needed oxygen. So you can understand how overwatering can also produce this symptom. Lori is a geologist, and knows a lot about this stuff.
