You mentioned moving the soil around earlier, was that before or after you stopped using the Miracle Gro? I might be inclined to fill in the depression with new medium and see what happens. I don't know that I would change all the soil since the plants are doing well and we don't know for sure what the problem is. I'm assuming it is actually some kind of soilless medium and not actual garden soil (I'm not sure what "plantation soil" is)?
