Build up sandy soil?
Your in a different part of the country then I am so I don't know that I can be of much help. I would start with a soil test first. Your county extension agent should be able to help with this. A legume would certainly help to provide nitrogen and if incorporated into the soil would help to build up organic matter. An application of manure, if allowed in your area, would be helpful as well to build up organic matter. The manure should be tested before applying and then reference your soil test to determine the amount that should be applied. A nutrient application plan maybe required in your area. You may consider another forage that likes well drained soils and your county extension agent should have suggestions for what grows well in your area.