In a large pan or stock pot, heat 7 cups of water on medium-high heat until just starting to boil, then turn down to simmer.
Add the washing soda and borax to the pot of simmering water, stirring until the powders are dissolved. Add the liquid castile soap and stir again. Turn off heat.
Add 7 cups of room temperature water to the pot and stir. Allow mixture to cool for about 10 minutes. Then add essential oils, if using, and stir again. You can also mix with an immersion blender.
Once the soap is cool enough, transfer to a gallon jar or two half-gallon jars.
Add a label to the jars, and allow to cool for 24 hours before using. After 24 hours, the laundry detergent should have thickened and become more of a gel consistency (it will also appear whiter). If the gel is more watery, or if the mixture separates, shake the container (or stir) before using.
Notes
To Use:
Shake or stir before each use.
Use 1/4 cup per regular load and 1/8 cup for smaller loads.
For HE washers, use 2 tablespoons.
This soap will last for 6-12 months.
Tips:This recipe makes a gallon of homemade laundry soap. If you'd like to make less, it's easy to halve the recipe and make a smaller batch. You can also make a large batch in a 5-gallon bucket if you want to stock up. It is normal for this detergent to separate or become a bit clumpy, which is why I shake the jar or bottle before each use.I keep a 1/4 cup-size measuring cup next to the jar of detergent.*We have not had any issues using this homemade laundry detergent in our washing machine, but you should always check the manual for your specific washing machine to make sure it is safe for your particular machine.