Black earth into water

[Guy]

Last summer (2005) a flower pot accidently felt into the pool.
All the black earth got mixed up with the pool water making it very dark.
This was in september 2005.I emptied the pool and replaced the water.
The pool was closed for the winter using a kit for winter closing.
In may ,i opened up the pool still a black flow of water comes out after
water is filtered. I have done numerous backwashes.
Waiting for a good suggestion