When making soap you use fat and lye (NaOH or KOH, either will do, the result is slightly different, but still soap). The trick is to get the proportions right; too much lye and you get a harsh soap that will turn your skin red, too little and you will have too much fat left to get clean. If you google for "lye (calculator OR index) AND soap" you will find a lot of sites with lists of how much lye to use for any given fat. And for real life soaps you will aim for something like 5-8% excess fat (better for your skin).
Any real soap is totally biodegradable, unless someone has decided to add some strange stuff (preservatives, bacteriocides, whatever).
For those of you who may recall any of your school chemistry
The fats are triglycerides, and when you add the lye you will get glycerine and a sodium (or potassium) salt of the fatty acid. The lye index is simply a convenient way to expess the molar ratio; you need three moles of OH^- ions for each mole of triglyceride.
I have set of lab instructions that I use with my chemistry students, I could translate them into English if anyone is really interested. Perfectly doable in the kitchen, but while it only takes an hour or two to make the soap (and you can easilly make soap for your familly for a year in one batch) it will take a few weeks before it is ready to use.
and it is completely bio degradable (at least its what they advertise). it is also cheap 
