The most common water softeners used in homemade laundry soap borax and washing soda, but they’re marketed as laundry BOOSTERS because they are meant to aid a detergent not replace one.
If you have really hard water, like I do in California, using a homemade laundry soap is even worse because the high levels of calcium and magnesium make it almost impossible for the soap to wash free from the textiles.http://www.boatdetergent.com/soap/laundry-soap/