It took about 10 years to get our Lake Michigan dune garden going. You are doing the right thing by adding compostable organic material, keep doing it. Two other things we did: grow bushes and thick undergrowth (we grow flowers) to protect the soil from sun and wind, and use drip irrigation all summer. There are still many challenges, some trees make it, some don't, beach grass tries to take over, other invasive such as Chinese yam came in with compost brought in, animals and insects love our garden and treat it like a restaurant. We love our garden, it's a little paradise near Lake Michigan.