Our grove of potted dwarf citrus trees are a bit like pets with benefits (a phrase I borrowed from my wife Chris, who says that about chickens). I only have to take them for a walk twice a year (once around Thanksgiving to bring them in the house and again in March/April to take them out in the sun) and they give us fragrant flowers, delicious fruit in winter and evergreen foliage.
I've had a sour orange tree for almost a dozen years, a Meyer lemon for about 7 years, a Clementine tangerine for 3 years and just bought a baby Meiwa sweet kumquat.
Every few years I've done some root pruning and re-potted them. But this year I decided to figure out a way to simplify that job. So instead of spreading out a tarp and pulling these 2'-3' tall little trees from their 14"x14"x14" pots and messing around with the new and old potting soil and dealing with a lot of clean-up, I decided to figure out a way to root prune (to generate newer, more vigorous roots) and fertilize with less time and trouble.
I pulled out the soil sample tube from my days of organic farming (you can get one on Amazon for $30-$60). It's a stainless steel tube of about 20" with a T handle and a sharp tip for cutting through roots and soil. It has an open side meant for examining the soil profile. Farmers and others use these tools to see the changes in soil type below the surface. For my purposes it allowed me to cut a channel in each of the four corners of the pots all the way to the bottom and extract the potting soil and roots of about a one inch wide well.
The soil probe pretty easily allowed me to dump the extracted potting soil through it's open side into another pot for re-use or composting later. Then I poured Espoma Citrus-Tone organic fertilizer (5-2-6) from the bag into the well until it was full. I poked the fertilizer down with my finger and added some more to top it off, replaced the mulch of attractive river rocks and watered them in. The probe worked about equally well in both wet and dry potting soil. The roots will have access to air in the channel and the organic fertilizer.
And I didn't have to make a mess. Instead of a couple of hours of re-potting and clean up, it took me all of 30 minutes and one 4 lb. bag of Citrus Tone (here's the fact sheet) to put 4 wells in the potting soil of each of my four citrus trees and a few other potted tropicals that I knew would like the high nitrogen mix: two night-blooming cereus and a coffee tree.
To save money on this technique, I suppose you could make a soil probe (or call it a potting soil extractor) from pvc, or copper which might be too bendable for this task. Either way, it would benefit from a T handle for twisting and the bottom would need to be sharp enough to cut roots. And you want the tube open on one side to get the potting soil out easily. Let me know if you make one that works and I'll post it.
Meanwhile I'm looking forward to next winter's harvest of lemons, tangerines and sweet kumquats.