You should root prune and repot every 4 years. Either soil mix idea will work. If you do decide to mix 2 mixes together make sure you thoroughly mix both as any layers will stop water until the upper layer is completely saturated before water can pass through the lower layer.

If it is root bound (likely) you should remove all circular roots, dead roots and broken roots (to above the break) before repotting. Be as gentle as possible in extracting, manipulating and repotting to break as few of the micro-roots as possible to minimize "transplant" shock. A garbage can and an adjustable stream hose attachment can clean the old container soil off to inspect the roots. Cut roots to half the diameter of the pot and above a bud at a perpendicular (90 degree) angle to minimize surface area of the cut. Repot immediately (don't let roots dry out) in new soil and if desired a bigger pot (larger is better for citrus and rootstock dependent as to how frequently to repot 3 or 4 years).

I do not like the idea of pulling a rootball out of a container and then putting it into a larger container with more soil UNLESS the new container soil is identical to the old container soil. Too often water will pass through the old soil without wetting the new. Also old soil may be high in salts from the evaporated waterings over the years even with periodic flushes.

Here is a link that might be useful: citrus in containers

This post was edited by Fascist_Nation on Sat, Apr 19, 14 at 17:49