Otherwise, you can add some weak acid at some point in the washing process. Since you need just a tiny amount, even cheap vinegar will do, without leaving a smell. You could make a soap goo out of soap and water, and add a bit of lemon juice. You could have a jug of water with a teaspoonful of vinegar in it, to rinse your hair with after soaping. Or how about pouring it into a plastic squirty bottle, so you don't spill it.