Dendritic salt (Buy Direct) is a very fine grain salt which has been crystallized to provide greater surface area. The increased surface area helps to retain fragrance, requires less pigments and reduces clumping. By adding approximately 5% dendritic salt to your bath salts, your bath salts will retain their fragrance longer than without the inclusion of dendritic salt.
Natrasorb Bath (Buy Direct) is an oil absorbing, hydrophilic tapioca starch which has a unique function in bath powders, salts, bath bombs and other products. It allows you to load your product with fragrance, essential and/or nutritive oils without making the product greasy or "wet". The starch absorbs the oils into pockets and releases them into the bath water. Natrasorb Bath dissolves completely in water, giving it a silky feel and leaving no film or residue. This product is useful as a fragrance fixative in bath salts and powders as it absorbs a high load of fragrance oil and also keeps bath powders/salts free flowing. Note with the Natrasorb - a little (in weight) goes a LONG way. 1 cup of the stuff weighs just a couple tenths of an ounce.
The trick here is to add the fragrance directly to the dendritic or natrasorb, and then add that mixture to the rest of the salt. For this recipe I would use about 1/2 cup of natrasorb, or 1 cup of dendritic. Slowly add the fragrance to it and stir well.
Note: If you don't have dendritic salt or natrasorb, that's o.k. - your salt will still be great. The additives just allow it to hold the fragrance better/longer.