Greywater and soaps

Q. I would love to hear more about your discoveries as you use your greywater system. What soap/shampoo/laundry soap do you use to avoid salt buildup issues in your garden soils? If you use homemade, would you share the greywater soap recipes? I have seen very little information about what soaps *are* compatible with water re-use (brands, ingredients, etc), have only seen warnings about how detergents are not compatible. Even those of us who do not yet have greywater systems could help, by supporting with our purchases the companies who make the greywater-compatible brands.

A. Thank you for your question, Joanne. First, we haven’t yet installed the Australian made Greywater Diverta. However, for years, we have been re-using our laundry water. With the Diverta will also be able to re-use the bathroom sink and tub water. As for the kitchen, we won’t be re-using the kitchen sink water because of the food particles and other possible containments like cooking oils/fats.

Types of soaps that we do use that would be compatible for water re-use:

In the Bath

Homemade olive oil bar soap (or any natural based soaps)

Castile soap (e.g. Dr. Bronner’s)

Natural, no sodium lauryl sulfate shampoo (e.g. Aubrey Organics)

In the Kitchen

Biodegradable, non synthetic dish soap (eg. Trader Joes “Next to Godliness” Dish Soap)

In the Laundry

Biodegradable, vegetable based laundry soap (e.g. Seventh Generation or Citra-Suds)

(Note you could also use a a soap-less water treatment disk )

Q. What about soaps and detergents? Are they harmful to the soil and plants?
Soaps and detergents are biodegradable, but they can present problems when gray water is used over an extended period. The main problem with most cleaning agents is that they contain sodium salts which, if present in excessive amounts, can damage the soil structure, can create an alkaline condition, and can also damage plants.

Avoid detergents that advertise “softening power,” because they will have a large proportion of sodium-based compounds. The phosphates in detergents can be good for plant growth, but unfortunately, the detergents highest in phosphates usually contain the greatest amount of sodium. If you re-use washing machine water, cut down or eliminate the amount of bleach you use and do not use detergents or additives that contain boron, which is especially toxic to plants.

When doing your household cleaning, use ammonia, or products that contain ammonia, instead of chlorine as the cleaning agent.

What precautions can I take to protect the soil from damage when I use gray water over a long period of time?
As mentioned earlier, a great danger in using gray water is the build-up of sodium in the soil. You can discover if the sodium levels are high by having the pH of your soil tested. A pH of 7.5 or above indicates that your soil has become loaded with sodium. You can correct or avoid this problem by spreading gypsum (calcium sulfate) over the soil at a rate of two pounds per 100 square feet about once a month. Rainfall, or rotating gray water applications with fresh water, will help leach the soil of sodium and excess salts.

{Courtesy of University of Massachusetts: ‘Recycling Gray Water for Home Gardens’}

You can use graywater from your shower, bathtub, bathroom sink and laundry. Only these sources are included in the legal definition of graywater

Don’t use graywater on or near annual vegetables, annual ornamental bedding plants, or for any planting that requires working the soil several times per season. Use graywater around perennials, trees and shrubs only, and wash your hands after working in soil irrigated by graywater.


  1. Alje van Hoorn says:

    I agree completely with your not using fabric softeners or Laundry detergents that contain Phosphors.

    We have been using using grey water to irrigate vegetable gardens for 16 years and we are yet to see the detrimental side effects of using this water.
    What would be you reasons for not using grey water around beddings that require working the soil several times a year?

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  3. Seannon says:

    Not sure the reason that they’ll give you, but when I asked a friend of mine she said that bacteria from human waste that is given two years in the soil is perfectly safe for humans (anything that could make you sick has been out-competed by soil microbes), but if you’re in the dirt more often than every 2 years, you should be a little more cautious. It’s just a matter of giving the soil bacteria enough time to kill off anything that might give you problems.

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