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Greywater Problem? Plant Food Solution!

Hacking washing powder into plant food

Moving into a new rental house (again), it was time to unroll the hose snaking from the recently arrived washing machine into the freshly mulched veg patch. The new dusty quarter acre, surrounded by beige surburban brick veneers, comes complete with particularly poor sandy soil, and the washing water beads across the surface like water on an incandescent saucepan. This soil was good, about 80 years ago, with fine carbon still clinging tenaciously to baked sand, the remnants of some native coastal forest sitting proudly on a sandstone hill. Now the few patches of grass cling tragically to small patches of garden shade which the sun and glyphosate has missed.

Now, greywater is great stuff for growing veggies, if you’re careful not to wash the nappies straight onto the bok choy or other unfortunate microbiological disasters. Each wash cycle satisfyingly delivers water to make the basil, tomatoes, pumpkin and other garden denizens grow rampant. Modern washing powders, being the devil’s spawn of user convenience, industrial production and reticulated sewerage (to a place where it’s someone else’s problem), tend to increase sodium in soil over time.

Washing powder contains a lot of salts, principally sodium carbonate, which is granny’s washing soda before it was packaged up and sold to her grandchildren for 10 times the price. One of the big problems with clothes washing water is the danger of soil getting salty or sodic. Sodic is pronounced more like soda in soda water, rather than the British swear word (if you have sodic soils, it probably is a swear word though).

In most places, there is enough rain and freshwater flowing through the garden so the sodium salts get leached out. In dry places the salt build up is slow and even after several years of using washing machine water I haven’t seen any effect on plant growth. Eventually a salty white crust formed on my garden bed which made me wonder how sustainable is using washing water in a dry place? Why is there sodium salt in washing powder, and what is washing powder made of, after all? How does it biodegrade, if at all, and how does it clean?

Washing powder usually contains sodium carbonate which is alkaline and dissolves greases and fats. Natural fibres (cotton, wool, hair) unwind and become slightly sticky with alkaline treatment, which is why your hair tangles with shampoo. However, alkaline conditioners dissolve oil and fats, both from hair and clothes. Other ingredients in washing powders are soaps (surfactants) to help dissolve dirts and keep them in the water (rather than in the clothes), and “builders” which help the soaps do their work (especially if you have hard water).

A final ingredient is “bulkers” which make you think you’ve bought lots of washing powder, but actually you’ve bought inert filler, sometimes sand, but mostly more sodium in the form of sodium sulphate. The main function of sand is to act like grinding paste in your washing machine mechanism, and fillers are great for producing CO2 hauling the washing powder around, making you pay extra for the privilege, and salting up your garden.

Soaps are often sodium based too, whether sodium laureth sulphate (which gives me hives just writing it), or good old fashioned bar soap used for clothes washing. Why the sodium? Its cheap, mate. Take an eyeful of all those breakers next time you’re at the beach and you see why sodium salt is used in laundry powder.

There are other salts which can be used for washing, and in fact potassium soaps have a lot of advantages. For washing, potassium soaps are more effective. Also, plants love potassium, it’s vital for their growth. Potassium is the K in NPK fertiliser. So by replacing sodium with potassium, we get a better wash, amazing plant food and soils that don’t build up sodium.

Regions with high rainfall won’t have a salting problem, and addition of lots of compost will lock up the sodium in less wet regions, but for really dry places, potassium based washing is ideal. And as we’ll now design laundry detergent that is also plant food, you can also use it in wet regions and have verdant plant growth!

For a nasty sodium based detergent, the ingredients would look something like:

Surfactant = sodium laureth sulphate

Builder = sodium carbonate

Bleacher = sodium percabonate

Bulker = sodium sulphate

So the first thing to do is convert to potassium, like this:

Surfactant = potassium soap otherwise known as Castille soap which is made from potassium hydroxide and vegetable oil

Builder = potassium carbonate

Bleacher = potassium percabonate

Bulker = none

We can also improve the builder by using something banned in commercial clothes washes. We can add dipotassium phosphate. This is fantastically effective at helping the soap do its work. But, you say, phosphates are banned from clothes washing powders! Why? When phosphate washes into river and lakes (it passes through the sewerage farm unscathed), the plant life goes bananas or more accurately goes algal. Giant algal mats appear and smother the fish, native plants etc. Right stuff for plants in the wrong place, natural waterways.

To a permaculturist this reeks of opportunity if directed correctly. Phosphate (P) is another of the components in NPK fertiliser. If we have a good size garden, full of nice spongy organic mulch absorbing washing water, none of the phosphorus has a chance to get to a waterway before it is absorbed and turned into plants. It will be taken up rapidly by plants and your bananas will thank you. Or if you have shrimp aquaculture your algae mats may even thank you. So our final washing powder looks like:

Surfactant = Castille soap made using used vegetable oil (30% by weight)

Builder = potassium carbonate and dipotassium phosphate (30% each)

Bleacher = potassium percabonate which can be made from hydrogen peroxide (6% by weight) and potassium carbonate (4%)

Dash of eucalyptus oil which helps dissolve grease and smells great.

Note, I don’t use the polyphosphates found in 1970’s washing powders. It is more effective for washing but also biodegrades more slowly. The simple phosphate chosen is taken up by plants directly. No need for it to biodegrade. The above weights are my best guess based on replicating commercial laundry powder compositions minus bulking agents and sodium. This is strong stuff, so use half the amount you’d use of a good commercial washing powder.

Have a way to measure a set amount, that will save lots of money. Given the above recipe, buying the materials will cost you less than a quality commercial washing powder per load. If you can buy ingredients in bulk, the price works out thriftier than the cheapest, nastiest washing powders available. Not only that, you’re also buying fabulous plant fertiliser. I am not sure this would get an organic rating as the above are mineral fertilisers, but I’m sure using ordinary washing powders in your greywater is certainly not going to get an organic rating. If you are selling organic produce from your washing greywater I suspect you have bigger problems to think about though.

As a final step in the washing stage, washing water is necessarily alkaline to remove the dirt and greases. Luckily washing machines often have a drawer for fabric softener which is released in the rinse cycle. If we add citric acid to this drawer it makes a great fabric softener and it will neutralise the alkaline greywater close to a neutral pH. Your plants will be watered solely with plant food! So pour some citric acid (most supermarkets have it) in the fabric softener drawer as a final beneficial act for your washing (and your plants).

Now theory is all well and good, and in a future article I will describe using this washing powder, what happens, both to the plants, the washing, and the wearers of the washing.

About the author

Larry Croft attended permaculture courses with Jeff Lawton and Bill Mollison. He’s made balcony scale gardens (around five square metres) in Brisbane, Switzerland, Malaysia and on a sailboat. While growing his four square metre balcony garden in Kuala Lumpur, he worked on plant probiotics (compost teas) delivered at a million hectare scale. His previous work used genomics to bring wild vigour (rewilding) into overbred commercial plant varieties while maintaining valuable traits. He’s currently Associate Professor at Deakin University where much of his work is population genetics of endangered plants and animals.



  1. Such good advice. Another option is the soap nuts – which comes from trees. I only use those as I collect my grey water in a pond before distributing, so my ducks tend to drink from it.

  2. Thanks for this wonderful resource! I will have the chance to direct greywater from a washing machine into raised beds for the first time soon and will use your advice! Would it be possible to offer a recipe for your formulation? Also, I am already using soapberry nuts and wonder if there’s a way to combine them with your ingredients usefully. Thanks!

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