You can go ahead and use homemade lye soap for your laundry. Usually double the amount of washing soda and borax is used in hard water. If you melt the soap and mix in the washing soda and borax to pre-make liquid laundry detergent (soap really), it would be runny rather than the semi-solid mass you'd get with the Fels Naptha.
Traditionally, laundry soap is just soap with borax added. You can add borax (and washing soda too if you prefer) to the recipe when you make the homemade lye soap initially, and save yourself some time and trouble. That way, you could just use the bar to rub on stains and stuff.
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