I am wanting an alternative to soap for handwashing in our school garden. Currently we station a bucket, scrubbing brush, and bar of soap near a tap. Students need to wash their hands before heading back to class. This uses a fair amount of water (kids aren't very efficient, and sometimes 5-10 kids might need to wash hands). I would love to use this water for something, or alternatively find some suggestions for garden friendly detergents (they need something as the scrubbing brush does not get their hands clean enough easily enough).
We use a goat milk soap and the water is fine for the gardens. Unless the soaps have phosphates and other gunky ingredients they will work just fine for direct watering of gardens and trees. Most soaps function as wetting agents, they make water "wetter" by breaking the bonds that create surface tension, so this wetter water actually soaks in faster and is used easier than "pure" water. We also have a gray water filtering set up which is a sand filter followed by a myco enriched greens bed, the water flows through these before going to the gardens.