Just to interject a couple of things here. A 21' pool with 4' of water in it will weigh in the neighborhood of 90,000 lbs.
Building up the ground with "sandy loam" (which I assume you mean by sandy land) won't work. I'd feel better if it was gravel/clay mix, but not much better. We're all bringing this up because we're concerned for you and your pool and house. In the end it is your descision and you are free to do whatever you want, but my suggestion is to do more research and make it as safe as possible. Use a good foundation and make sure it'll hold the weight without settling and/or shifting.
Building up the ground with "sandy loam" (which I assume you mean by sandy land) won't work. I'd feel better if it was gravel/clay mix, but not much better. We're all bringing this up because we're concerned for you and your pool and house. In the end it is your descision and you are free to do whatever you want, but my suggestion is to do more research and make it as safe as possible. Use a good foundation and make sure it'll hold the weight without settling and/or shifting.