A riddle of anagrams

I must get to the bottom of this
But it is so far away
It couldn't be further

So I swap round to be a gardener

I hold up my plants
Holding the nutrients they need

But I change again, my choice
From a range of possibilities
But I am the select

What was I and what do I change into?

Answer

Is it:

Top, pot, opt.
Top is the farthest away from bottom.
Pot hold a plant and its nutrition.
Opt is (thanks to @Will) to make a choice or decision.