Layering materials

Is there any way to cause materials to layer and blend into each other? Example; say I had a landscape and I wanted to have a section of dirt next to a section of sand, but I didn’t want a seam where the materials meet, rather a transition between the two materials.
Any ideas?

no, that is only possible with textures

so, it can be done if you find a way to create an alpha channel for one area (big greyscale image texture?) and map it as stencil and put it between the two texture layers for the other textures

Thanks for your help.