Answer: Your goal is to leave sufficient space between each tree and any structure so that the mature branches are not interfering with each other or with structures. So, you need to obtain each tree's mature size, both height and width, from the grower or plant tag and then space accordingly. Citrus is much healthier if low branches are allowed to remain on the tree (rather than pruning them up) to protect the trunk bark from both sunburn and freeze.
Do not add any amendments to the soil. Although this was once a common practice, it is no longer recommended for a variety of reasons. In particular, tree roots quickly spread beyond the planting hole, and it is better if they continue to grow in the same soil profile. Good luck with your fruit trees!
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