Good morning!

I would not suggest adding sand as a soil amendment. It may create more pore space but it could also make your soil harder in the long run to work with. The best type of soil amendment is compost - this can increase the porosity of your soil but also add some nutrients!

Here is a great resource on different types of soil amendments you can add into your soil.

Here is another resource on how to properly plant a tree. You do not want to dig a hole too big for the tree - this resource has photos as well!

Thanks for reaching out!
Rachel