seriously though, don't use soap. Just get a nylon scrubber or stiff bristled nylon(or whatever) brush and use salt. Soap is going to remove the seasoning and probably leave a soapy flavor behind
Every cleaning solution that doesn't include a surfactant is worse. It's slower, more labor, intensive, more water-intensive, and it leaves baked-on crud behind. The latter two are both much worse for cast iron than soap.