For fertilizers and organic matter, thorough incorporation is suggested. This can be done with a spading fork for small areas or, more commonly, a rototiller for larger ones. If adding topsoil, add a 2 inch layer of topsoil first and till it in, followed by the rest of the topsoil. Roots do not like to go from one type of soil to another and the above method provides a gradient rather than a sharp change.
Kansas State University