Soil being constantly wet is exactly the conditions anaerobic bacteria need to rot everything thats submerged.
The solution is pretty obvious: don't put soil direct under water.
There are many possible ways to achieve that:
- Add a drainage layer made out of LECA, pumice, or whatever inert material you can get. Then, add a polyester cloth or whatever synthetic fabric you can get, and sandwitch it between, and then add your soil. This prevents it from washing out. LECA and similar substrates can also wick water, but don't get soaked and can't rot.
- Or, you can use a polyester cloth or a proper self watering wick and put them in the holes, or fill them completely with it. They can wick way stronger, but shouldn't be submerged completely, because then the soil gets too wet again. Or,
- Ignore the self watering capabilities and just water regularly, but use it sparingly, e.g. when you go out of town for a week, and then let it dry again
- Or, don't use soil, and visit [email protected], where we use different substrates and techniques and also make use of those self watering pots. I use something very similar, but with mineralic substrates (I personally love LECA), where self watering works exactly like that, and the media can be reused indefinitely, without soil borne pests and much more!