If your plants are "rootbound" then you are basically growing like a hydro grower would, ferts inputs in, plant mass out.
Its "probably" not a soil issue as much as a ferts issue........almost all soils run out of their "own" fertilizer in a season or so, and they must be top-dressed with ferts, watered continually with ferts, or repotted with new soils.
If roots are too dense and packed, then plants can't be watered either, and pots have to be soaked in a bucket to get water through them.....
Some slow growers, like succulents, can go for a few years without additional ferts, but most fast growing tropicals, like ferns, flowers shrubs etc etc. run out in a few months during a warm growing season. ( ferts like Nbreakdown as a function of warmth, and of course plants use N like crazy during growing season)
Many cycads,cactii and palms are grow rootbound, plants roots take up 90% of pot, need cutters and prybars to get pots off...hardly any soil left, just crumbly leftover sand and duff..... its all gone up into the plant.....these are repotted in 2-4 inch larger size increments, so they can't be overwatered, free up roots abit for new soil, and put 1 inch of time release ferts onto new surface, then water it in.
If ferts inputs are stopped, plants will die. Organic soil can't save them, I've switched over to time release pellets for small and large pots,(1-100 gallons sizes) because as compared to liquid, it makes life a lot easier, (no mixing) and is less wastefull of both ferts and money....most liquid ferts goes right out the bottom of a pot anyways...(Nitrogen wastage is a big issue in commercial operations, as its not cheap, and price of natural gas determines price of nitrogen ferts) anyways.... 1 lb per inch of trunk, turned into the top 1 inch of soil is good for a year of so, unless weather gets hot...as it breaks down quickly in heat.
Plants which drop lower leaves after they turn yellow are nutrient (N) starved, plant pushes nutrients to top, which is more important than lower leaves.
I used some great "mine reclamation" Nitrogen 38-0-0 this year, non-leaching, non burning, you could leave it out in the sun and it wouldn't go off!!! Toss it right onto plants....pretty easy.... costs like 70 bucks for a 50 lb bag though (as compared to 35 for regular 20-20-20...but after burning down 8 square feet of patch with too strong ferts, I'm paying a little closer attention.....duh ..my bad.
