I have a pepper plant in a 2 cubic ft container on my deck,
and something is not right about it because it's leaves are
starting to turn yellow. It used to be dark green, but it's
now yellowish green, so I assume it needs some fertilizer,
but I'm not sure if it needs mostly N, K, P or Fe and/or
some other trace element.

It was originally planted outside of my home in '99, but it
was dug up after the first frost, and it's been in the same
container ever since, either inside or outside, depending on
the weather.