Preservatives for Homemade Laundry Detergent?

You can also use GSE (Grape Seed Extract). It's sold in most Health Food Stores. It's a 100% Natural cleaner & antibacterial made from Grapefruit Seeds. Using the name-brand of "Nutribiotic". It cost about $12 or so for a small bottle, but it should last you quire some time cause you only need a little!

Read more about GSE & Nutribiotic here:
www.pureliquidgold.com/
www.pureliquidgold.com/citricidal.htm

* Lots of Soap makers use GSE to preserve their homemade soap.

You'll only need a tiny bit. Like about half a teaspoon per gallon. With GSE you Laundry soap should last a month or so. At least I think so. You can also keep the soap fresh longer by keeping it cold. Do you have an extra fridge in the garage? If so, this may be the way to go or maybe leave it the soap outside in the winter.

Grapefruit seed extract (GSE) is an antioxidant - this is great to use in oils to prevent them from becoming rancid, but it is not a preservative. To date there is not an all natural preservative. A preservative isn't necessary in a dry laundry detergent, but if you are adding water to a bar soap, it may grow bacteria and/or mold - If you make a laundry detergent using a liquid soap paste that was made with oils and potassium hydroxide, your detergent should be safe based upon the ph of the end product.

HTH :)