There could be some variations depending on the soil type and species you are growing but any 'complete' fertilizer will help your trees.
Trees in the ground are not as affected by leaching as the ones in pots so you won't need to fertilize as often. A couple of times each year should be sufficient.
Liquid fert is available quicker but also quicker to leach away. Compounded and organic ferts are slower to start but then release nutrients over time so keep feeding for weeks or months after application.
Too many possibilities. Your question is too vague. Kind of trees? Seedlings or larger? Type of soil. Has it been analyzed? How many trees and how large an area? Local climate. What is the purpose of the trees? Bonsai I assume but I would rather not.
Usually, fertilization is not the limiting factor for field grown trees because contrary to agricultural production there is very little exportation every year. Irrigation when soil is dry and improving soil properties are very important.