I've had mine at some point for 7 years. It was a bit too much, after about 5 you could see the decline. When I changed it, towards the bottom you could find a sort of soft rock formations, some quite big, like a nut. I think it was calcium carbonate or phosphate (from reaction with the fertilizers) due to my extremely hard water, and that was what made the production go down (constant problems with micronutrients).
As for foliar diseases, it's really not the soil, but the humidity, sun, aeration that make the big difference.
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