Residue from Bronners and BS
two words: hard water. if you want to use bs then you'll have to mix it up using distilled or purified water. you can still rinse with your regular water. the minerals and such make it impossible for bs to completely desolve. i've even tried boiling it on my stove and it just creates a cloudy water. if i boil bs with purified water it comes out completely clear in seconds.
dr. bronners is a castile soap. castile soaps and hard water are not friends. i have a stain on my drive way to prove it. it will leave you with a terrible residue thats hard to get rid off. definitely try some organic/natural shampoos.