Calusa,
In addition to the proper nutrition, I wonder if the way you are staking and supporting the tree contributes to the trunk remaining week. I have found that loosely staking my plants so they are able to move is best. When a plant is not able to support its own weight I will space three stakes away from the trunk and tie with a strong nursery tape that will not bind or scratch the branches, allow movement but at the same time keeps the plant upright. I would also thin out some of the branches to relieve some of the weight while the trunk strengthens. Tight staking over a long period of time may contribute to weak trunk and branches. When I start seedlings in an indoor area, I will brush my hands over the plants regulary, use a fan, or blow on the seedlings when young to strengthen them.
Janet