My soil is sand, should I fertilize more than usual? No significant fruit production for last 3 years. Tree appears healthy but doesn't really product. It's a Valencia Pride that's about 10 years old. I know mango don't need much but after recent trench digging in yard, once I went down a foot it was basically sand.

Yes. fertililze. This is a good time to do it. Use a low nitrogen fertilizer. Also, is the tree in full sun? Or, after ten years is it shading itself? It's recommended to cut out the center and let sun hit around the tree.