Decyl Glucoside does not soften water. Decyl Glucoside can also be used in hard water.
I am experimenting with applying Decyl Glucoside to about 60 crops this year.
Right now, the recommended usage figure is 500L (132 gallons) 3-10L.
If you convert this to 20 gallons, it seems to be 0.12 to 0.4 gallons.
