I've been playing with {upwelling} card for a while now I just want to make sure I'm using it right. So let's say I have it in play then 4 forest now that mean that I have 4 green mana available ? Then on my next turn again I cast nothing tap my mana and now I have 8 green mana available? Next turn I cast a spell for 4 mana I can use 4 floating mana from {upwelling} and then I'll have 4 left is this correct?
I believe "steps and phases" and "turns" are different. I believe the mana empties at the end of your turn still, but I could be mistaken, I'm no judge :p
From the ruling on the card:
10/1/2009 Players can keep mana in their pool indefinitely while this is on the battlefield, but once it leaves the battlefield, they have until the end of the current step (or phase) to use the mana before it disappears.
In other words, it's there until you use it.