It sounds kinda like minerals from the soil, but you usually only get that kind of build up in potted plants. But you might want to have it tested. The only way I can thik of that you are getting that kind of deposit on top of the soil would be if you are flooding your rose garden and the water is standing on top. If you scratch the surface of the soil is it crusty, hard and dry? If so, your soil might benefit from the addition of some compost and a mulch layer on top. Cheryl