Any soil with a clay content higher than 30 percent will be unsuitable for roses unless its structure is modified. Take out the topsoil down to a depth of 40cm and put to one side, and the subsoil down another 30-40cm to the other side. To this add 3kg of gypsum per cubic metre. Return the subsoil. Sandy soil is comprised of 3-10 percent clay and 85 percent or more sand.
Sandy soil dries out more quickly than clay and nutrients leach out faster.
To help with sandy soil dig deeply to a depth of 80cm and add lots of organic material such as compost, finely milled bark and well matured manure. This will not only enrich the soil but assist moisture retention and enable roots to spread. The addition of wood shavings or clinker ash will help make the soil more friable and less likely to become compacted. It is important to test the pH level of your soil. Roses prefer a fairly acidic soil with a pH between 5 and 6.5, but they can also grow in soil with a pH up to 7.5.