I'm new to bonsai and have 1 tree, a chinese elm. I have read that it should be fertilized during the spring and summer with a high nitrogen fertilizer. Most of my plants are african violets and orchids, which require a urea-free fertilizer. Is urea ok for use on bonsai? I have some slow-release pellets that I use on the garden. Are these ok or should I use one of my water soluble urea-free ferts?