This is the recipe I've been using for nearly two years now. One batch makes a 5-gallon bucket, which in my small household lasts for 6-months and costs just a penny per load.
I need to note that the Fels Naptha soap is, in my opinion, a rather questionable ingredient. The makers of it (the Dial Corporation) seem to go out of their way on the label not to tell you what's really in it. I'm assuming there are some fairly nasty chemicals.
Here's the ingredients list: "Cleaners, soil & stain removers, chelating agents, colorants, perfume. CONTAINS NO NAPHTHALENE."
That being said, I've used it for all of the batches I've made, except for the very first one. For the first one I had purchased the Fels Naptha soap but was so put-off by the strong chemical scent that I couldn't bring myself to use it. So for that first batch I used a bar of Ivory soap. The problem was that by the end of the first bucket our clothes seemed noticeably dingy and--I wasn't sure--but I thought maybe I detected a very faint sort of funk developing, something oh-so slightly malodorous. In other words, the Ivory soap just wasn't cutting it.
Now I definitely strive to make the most environmentally-sound choices, but in this case I caved and have used the Fels Naptha soap for all the subsequent batches. It does work very well. It gets our clothes clean, and until I can come up with a better substitute, I will continue to use it. I suspect that it contains a fairly high concentration of lye because it is quite harsh on the skin (if you get the detergent on your hands--not from merely wearing clothes washed in it). This probably isn't a good recipe for people with sensitive skin though.
Here's the recipe:
1 cup washing soda
1/2 cup borax
1 bar Fels Naptha laundry soap
few drops essential oil of your choice (optional)
an old cooking pot
an old cheese grater
an old wooden spoon
**Do not use any of the above items for food anymore.**
a 5-gallon bucket
stirring stick of some sort (I use a yardstick)
an empty detergent bottle
Grate the Fels Naptha soap (or other soap of your choice) into your cooking pot.
Cover with water and cook over low heat, stirring to dissolve, until completely melted and blended.
Pour the melted soap mixture into your five gallon bucket, add the washing soda and borax, and fill the bucket with warm water. Add essential oil if you're using it. Stir thoroughly and allow to cool. (Notice how the detergent has washed the numbers off of my yardstick.)
Fill your empty detergent bottle with detergent (rubber gloves recommended if you're dunking the bottle into the bucket like I do). Use 1/2 cup of detergent per load (the cap on the bottle I use holds a half-cup, but I'm not sure if that's how all detergent bottles are sized).
Once the detergent cools it will become a thick gel. You will have to stir it up very well each time you need to fill your bottle, and you'll need to shake up the bottle before each use.
If you have a lot of whites to wash you probably want to buy some liquid bluing (like Mrs. Stewarts) since homemade detergent doesn't contain any optical whiteners.
**If anyone out there has an effective detergent recipe which they know to be environmentally friendly, please post it in the comment section below. Also, if anyone knows the real ingredients in Fels Naptha soap, please share that with us as well.**