Stacey, I’ll take a go. The best kind of soil amendment would be compost or, next, well-rotted manure. Failing that, leaf mould (decomposing leaves). So, do you have a mulching lawnmower that you can use to run over leaves when they fall? You can dump all of these in the sandy area to help amend the soil and over time, help hold in moisture. So anything and everything you can get your hands on would help. It might take a couple of years before the worms have really done their job and made the soil moisture-retentive enough to start planting. Good luck!