Washing-up liquid as a wetting agent: why it fails

Wetting agents · part four

By Joe, Founder of Dr Forest · August 2026 · 14-minute read

It half-works. That is the awkward part, and it is why the tip refuses to die.

A few drops of washing-up liquid in a watering can really does lower the surface tension of water, and the spreading effect is real and measurable. At 0.1%, surfactant took the water contact angle on a waxy evergreen leaf from 77° down to between 10° and 20° [8]. So if you have tried washing up liquid on plants as a homemade wetting agent and thought it did something, you were right. The case against it is what else arrives with the surfactant, and dose.

The guide to what a soil wetting agent is covers the mechanism. What follows is the case against the kitchen bottle, ordered by strength of evidence, including the arguments against it that do not stand up.

What is actually in the bottle

The safety data sheet for Fairy Professional Original, revised 8 August 2025, declares sodium laureth sulfate at 10 to 20%, lauramine oxide at 5 to 10%, ethanol at 1 to 5%, and two isothiazolinone preservatives at 0 to 1% each [1]. Stated pH is 8.4 to 9.4. That is a manufacturer safety data sheet, not a peer-reviewed measurement, and it declares only hazard-relevant substances, so no honest salt figure can be pulled from it. A full Detergents Regulation declaration fills the gap: smol lists five sodium salts in one bottle, laureth sulfate, formate, benzoate, chloride and xylenesulfonate [2].

Soap and detergent are different things

This is the point everything else hangs off. A true soap is a potassium salt of a fatty acid, made by saponification with potassium hydroxide. Borden and Dale at the University of Florida describe most dish soap products as detergents rather than true soaps, and state that the sodium in modern soaps is toxic to plants [3]. Bonnie Dwyer, for the UC Master Gardener programme, is blunter: many products labelled dish soap or laundry soap contain little or no actual soap [5]. Colorado State University Extension warns that dish soaps can injure plants, damage soil and contaminate waterways [4]. Same job, different chemistry, and the counter-ion is sodium.

The phytotoxicity numbers, and the ones that cut the other way

Baales and colleagues sprayed eight surfactants at 0.1% onto cherry laurel leaves [8]. The untreated contact angle was 77°, give or take 8°. After alcohol ethoxylate treatment it fell to 10 to 20°, recovered to 70 to 80° within 24 hours, and rinsing restored it completely, with no irreversible cuticle damage detected. So washing-up liquid does not permanently strip the waxy cuticle, at least at that dose on that species. What did persist was residue: the anionic, cationic and polar surfactants plateaued at 42 to 54°, leaving the leaf abnormally wettable. Washing-up liquid is dominated by anionic surfactant.

Dose is where the case firms up. Bubenheim and colleagues grew lettuce in nutrient solution with an anionic surfactant and saw root browning within hours at 250 mg per litre and above, then clear recovery within three days, which they put down to microbial activity [6]. Report both halves or you have not reported the paper. D'Incau and colleagues found SDS inhibited ryegrass germination only at ten times the critical micelle concentration [7]. Curkovic's review puts injury risk above 1 to 2%, with olive and grape damaged at 0.5 to 1% whilst apple and citrus shrugged off the same [9]. Spreading at 0.1%, injury in a sensitive species from 0.5%. A squirt in a watering can is not a dose.

Sodium, and what it does to soil structure

Separate what is measured from what is inferred. Measured: the main surfactant is itself a sodium salt at 10 to 20%, sodium chloride is declared in at least one UK product, and the liquid is alkaline. Not measured: nobody has published total sodium for a UK washing-up liquid, so you will not get milligrams per litre from me. The nearest proxy is domestic greywater, a compilation of other authors' work putting sodium at 87 to 248 mg per litre [10]. Alkalinity in the bottle is a separate question from the pH of your soil, which moves for its own reasons and is dealt with in how to increase or decrease your soil pH.

The soil consequence is inferred from irrigation science. USDA Agriculture Handbook 60 calls a soil sodic above an exchangeable sodium percentage of 15, a boundary it describes as somewhat arbitrary and tentative [11]. FAO's guidance is more use in a garden: infiltration problems arise from sodium and salinity jointly, and at a sodium adsorption ratio of 0 to 3 severe restriction appears once conductivity drops below 0.2 dS/m [12]. Sodium in low-salinity water is the trap, and low-salinity water is what comes out of a rain butt.

Kitchen water carries its own problem. Travis, Weisbrod and Gross measured oil and grease at 200 mg per litre in untreated kitchen greywater against 7.2 mg per litre in bath water, and found kitchen-irrigated soils at up to 200 mg per kg in the top 20 cm after more than seven years, against under 50 mg per kg under fresh water. Capillary rise in sand fell by roughly 60% at 250 mg per kg [13]. Kitchen water makes soil water-repellent over years, the opposite of the intended effect. This one is a conference paper.

Against that, the strongest counter-evidence I hold. Misra, Patel and Baxi irrigated tomatoes sixteen times over nine weeks with laundry greywater and with surfactant at 15 and 150 mg per litre, and found little or no significant effect on growth, though greywater plants carried 79% more sodium [14]. I could verify the authors and not the journal or year, and it still counts.

What happens when it goes down the drain

Linear alkylbenzene sulfonate, the other common household anionic surfactant, is acutely toxic to aquatic life at single-figure milligrams per litre. HERA puts the freshwater no-effect concentration at 0.27 mg per litre, with acute values of 4.1 mg per litre for Daphnia magna and 9.1 for algae [15]. Acute fish LC50 is 3.2 mg per litre there and 1.67 in the Australian regulator's evaluation [16]. Alcohol ethoxylates carry a Canadian guideline of 70 µg per litre [17].

Now the fair correction. Detergents biodegrade. LAS shows primary biodegradation above 99% and a half-life of roughly 3 hours in river water, and no-effect concentrations for terrestrial plants run from 68 to 200 mg per kg of soil, far above any garden loading [15]. The persistent fraction is the preservative: sodium laureth sulfate shows 66% ready biodegradation on the Fairy sheet, benzisothiazolinone 0% [1]. One gap, stated openly: no environmental assessment for the SLES family was retrievable, so no no-effect concentration for it appears here.

What the RHS actually says

It is not advisable to use household products, such as bleach, vinegar or washing-up liquid, as pesticides.

That is the Royal Horticultural Society, on avoiding chemical use [18]. The same page adds that gardeners mixing their own solutions from household products often damage plants, soil and soil organisms, and may harm the wider environment.

RHS grey water guidance is more permissive [19]. Soil and compost filter out soap and detergent residues, which the RHS says can sometimes act as a mild fertiliser. Store it no longer than 24 hours, apply by watering can, keep it off edible crops, and note that the salts in dishwasher water and softened tap water can damage soil structure, particularly in clay. The RHS trial on border perennials and sub-shrubs found no significant effect on plant function over several weeks, although after six weeks Stachys byzantina and others benefited from a rinse through with tap water as they began to show salt stress.

You will also read that doing this is illegal in the UK. HSE does not say that [20]. Its position is that intended use determines regulatory status, that home-made remedies are not necessarily without risk or always legal, and that supplying one to another user may be illegal without authorisation.

Baby shampoo

Baby shampoo turns up constantly in this cluster of questions, usually as a lawn treatment at some half-remembered dilution. I went looking and found nothing. No published trial in my sources measures baby shampoo on turf, on soil or on any plant, at any dose, with any outcome. It is a formulated detergent, and being mild on human eyes is a different endpoint from sodium in soil. Anyone quoting you a rate for it made the rate up. The fuller treatment, including what the UK formula actually contains and where its pH sits, is in homemade wetting agents.

If you are going to do it anyway

Plenty of people will, so here is the least-bad version the evidence supports. Use a real soap rather than a detergent and keep it at or below 2%, which Dwyer gives as four teaspoons to a quart [5]. Keep it off plants already stressed by drought or humidity, and off everything above 32 °C; Borden and Dale name fuchsia, bleeding heart, lilies and sweet peas as sensitive, along with delicate ferns, waxy succulents and hairy-leaved plants [3]. Rinse the foliage afterwards, since rinsing restored contact angles completely in the cherry laurel work [8]. Alternate with rainwater so salts do not build up. And know your concentration: if you cannot say what percentage went on, you cannot repeat it.

A purpose-made product gives you a known surfactant at a known rate, without the sodium salts, the alkaline pH, the fragrance and the preservative that never breaks down. Our Natural Wetting Agent is made with organic Indian soap nuts and aloe vera grown in-house in Stockport. The lawn rate is 1 teaspoon per litre at 1 litre per square metre, which is 0.5% by volume, printed on the bottle rather than guessed at the sink.

I will be straight about our own chemistry too. Soap nut saponins do not beat good synthetics on surface tension: the best measurements put the minimum at about 52 mN/m for Sapindus mukorossi and 47 mN/m for Sapindus trifoliatus [21], against the 32 to 37 mN/m Rai and colleagues give as the mark of an effective surfactant [22]. Saponins win on how little you need, 0.24 to 0.32 g per litre in deionised water, though tap water pushed that to 2.4 g per litre. Our concentrate's saponin content is not published, so apply at label rate. The chemistry is in the piece on soap nut saponins; the application guide has rates by job, and short answers to the questions that come up most often sit in the wetting agent FAQ.

Final word

The trick half-works, which is why it survives. Dry lawn, bottle under the sink, and the water spreads better tonight. You are also putting five sodium salts, an alkaline pH and a preservative that does not biodegrade onto ground you mean to garden for years. Once in a drought, nothing dreadful happens. Every fortnight from April to September and you are running an experiment nobody has published.

Frequently asked questions