If you think it's a soil issue, then one possibility is to begin an aggressive campaign of mulching the soil around the trunks to build it. Get a bunch of organic matter, hay from your local hardware or garden store works fine, and spread it around underneath the tree about a foot deep. On top of that spread some composted manure and cover the whole thing with wood mulch (not the treated kind, the kind that is meant to rot). Do that a few years in a row and you'll have much healthier soil and probably a healthier tree.
You can also try companion plantings around the base of the trees. The local community orchard I volunteer at has comfrey and garlic planted in the mulch around each of the fruit trees. The comfrey is a dynamic accumulator, it reaches deep into the soil and pulls nutrients up to the top soil. It grows very aggressively and you can chop it down almost to stubs several times a season and spread the leaves on the ground beneath the tree to add to the mulch. It will add a lot of nutrients to the soil. I've never tried this, this is just what the folks at the orchard tell me. The garlic is just to ward off various pests. It's a deterrent to rabbits and various insects, I believe.
Look up sheet mulching (here's one good article), there are a whole bunch of techniques for composting in place to build the soil.