Regarding the comment about "A potting soil made from pine bark, peat, perlite ... works OK for a year and then shows strong nutritional (disease?) problems (general chlorosis) ... I'm still baffled."

What is the pH and composition of mineral salts in the irrigation water. This type of problem is common in soilless mixes after they are watered for a while with groundwater that is relatively high in pH and high in disolved lime (Calcium carbonate). It's a very common problem in my potted, acid-loving plants like citrus, camellias, gardenias, etc. My groundwater is very hard and very high in lime -- it crusts over shower doors, coffee pots, etc in just a few days. Aside from using a water softener, 'Lime-Away' brand lime remover is a constant in the household, because the water softener doesn't remove it all no matter where it's set. The untreated groundwater is even worse -- even a single, hour or two session of watering with a sprinkler can leave everything coated with lime scale.

For the potted plants, I find it necessary to remove them from their pots about every other year, wash all of the potting mix from their roots, and repot in fresh soilless mix. I also use acidifying fertilizers and trace element supplements in between.

I wonder if you have a similiar problem. Initially, the orginic compounds in the soilless mix can absorb and neutralize the calcium, but eventually, the amount of lime from the water just overwhelms the ability of the soilless mix to act as a calcium sink, and the pH rises rapidly, resulting in the chlorosis symptoms you describe. There is realistically next to nothing you can do to purge all of this lime out of the soil at this point that wouldn't also fry the plant, so it's time to replace the soil.

My thought would be to only use rainwater, if you can get it. I've only water my blueberries with rainwater, never with the hose and they do fine...

I'm guessing PawPaws prefer an acidic soil?

Thanks both.

I wish it were that simple. I was more concerned by an insufficiency of lime (thus the dolomite) in both potting soil and acidic yard sands. The water is relatively soft, a city supply from the same river whose flooding seasonally waters the native bottomland paw paws. I do wonder a bit about the fluoride added for tooth health, but the fact is, native seedlings in mineral rich compost grow realatively well with the same water, so a potting-soil or a regional genotype suceptibility has to remain suspected (late this coming winter I'll do some soil swapping to try to test). HOWEVER, native seedlings from one prized (and now dead) wild tree show strong problems in a mineral-poor soil (the pine bark soil later repotted adding a great deal of mushroom compost, again low in mineral content). It would probably have been easy to determine had I known the problem existed but I was just trying to grow them, not experiment with all scenarios. Too many variables as they grow now (or don't grow). It has been pretty disappointing to see prized crosses or native seedings turn yellowish and wane. (I additionally lost some Peterson selections and some crosses to an unknown wasting disease: die back with a blue-black infection in the bark. Oddly, sometimes the roots resprout from ungrafted trees, as blighted chestnut can.) I have hauled home a lot of bottomland silt to experiment with amending. Once plants are in 10-25-45 gallon pots the soil changing becomes pretty difficult though. BTW: circumstantial evidence would suggest paw paws prefer a circumneutral to mildly acidic soil, not notably acid as for blueberries (which BTW grow fine with the same soft water).