Salt and soap don't mix well. Best to use either one alone. For cleansing, a mild soap solution is best. To rinse any soap solution residue, a followup salt water enema is helpful. Plain water enemas can upset the electrolyte balance in your body and should be avoided.
Also, for a soap solution enema, you should only use bar soap (Ivory is best), or liquid castile soap. Never use regular liquid soap - it is just too difficult to get the proper concentration, and if it is too concentrated can cause damage to the delicate lining of the colon.