Well congrats to taking on the adventure of growing trees from seedlings. Osmocote is a great plant food for young trees because it is slow release and contains all the micro and macro nutrients growing plants need. I would suggest the Indoor Outdoor Osmocote as it is a general purpose formula. As for how much, follow the instructions on the label using one tablespoon scoop per gallon sized container or per young tree. Apply in late spring as the weather is warming up. Keep growing, Marianne Binetti