I've never personally tried it. I believe it has to be applied at the time of planting, but I know little about it. I usually use a root stimulator in liquid form made by Fertilome and have had great results.
Are these indoor or outdoor palms? Assuming they are outdoor, the liquids are easy to apply and do a great job, but I would also recommend a once a season topdressing of compost and working it in as much as you can without damaging the roots. Nature will do the rest and eventually the soil will become more friable.