Based on my experience the stuff off the bottom can't be used for anything useful as is until it's been spread out fairly thinly over a large area and allowed to sun bake dry for an extended period. Even then it usually has to be mixed with regular dirt at that point.

Others more knowledgeable will respond as to the hows and whys.

Dale

[Linked Image from i90.photobucket.com]

"When tempted to fight fire with fire, remember that the Fire Department usually uses water." - anonymous