Is it really possible to significantly alter soil in any large scale way? I am trying to garden in sandy soil, on a south facing embankment leading directly to a lake. There are several huge old oak trees adjacent to the area which undoubtedly have large, moisture-sucking root systems. To make matters worse the past few summers have been hot and dry. Over the past ten years I have added huge amounts of topsoil (black earth, three-in-one) and compost. Every spring I dig in bags of composted manure around the base of the plants and I mulch twice a year with chopped leaves. Yet every spring the beds seem to have reverted back to their original sandy soil and I feel like I'm starting all over again. My plants never "thrive and flourish" in spite of regular waterings. Many perennials I've tried never make it to their second season, and those that do seem to become smaller and weaker with each passing year before they disappear completely. Am I fighting a useless battle?