Adding sand alone to a clay soil is not a good idea. Clay is made up of fine particles, which is why it doesn't drain well. (Water drains through the spaces between the particles; the smaller the spaces, the slower it drains.) Sand, on the other hand, drains well because it is made up of larger particles. When sand and clay are mixed, the smaller clay particles stick to the larger sand particles, eliminating the spaces and creating a dense layer of soil that's something like concrete. If you get the right combination of sand and clay, you'll end up with adobe bricks. It's a much better idea to add organic matter-such as peat moss or compost-to a clay soil. As organic matter breaks down, it helps create larger spaces between the clay particles and enriches the soil.