Citrus have come a long way in the past 30 years and there are now rootstocks for most soils.
These rootstocks have also been virus indexed meaning most of the dangerous viruses have been cleared.
It's very important to do some homework before buying a grafted tree because rootstocks vary greatly in their suitability. A reputable nursery will say what rootstock has been used.
In the past most trees in sandy soils were grafted to the Rough Lemon or Rangapur lime that is vell suited to deep sandy soil.
I expect there are modern rootstocks available so a search of available rootstock/ scion combinations is worth the trouble.
Your mandarin is struggling going by the photo. I will make some suggestions later.
