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How Plant Soap compares
Full guide and scienceDr Forest Plant Soap (potassium soft soap)
- True soap: saponified from British rapeseed, organic castor, organic coconut and insect larvae oils with a potassium alkali
- Rapeseed base, roughly 63% oleic acid, gives a mild, conditioning wash
- No fragrance, colouring, preservative or synthetic surfactant
- Dilution-rated for living foliage, from 6 ml per litre upwards
- Breaks down completely in soil and water; runoff is not a problem
- Doubles as an emulsifier for horticultural oils
- No expiry date
Washing-up liquid or household detergent
- Synthetic surfactants (SLS, SLES) built for kitchenware, not leaf tissue
- Fragrance, dye and preservative that serve no purpose on a plant and can leave a film
- No plant dilution rate on the bottle, so people guess
- Formulations change without notice, so last year's mix may not behave the same
- Strong enough at guessed rates to lift the leaf's own protective wax
Customer highlight
Great product
Great product does as it should.
Editorial highlight from marketplace and store reviews — separate from the full review list below.
Made by Growers · Backed by Science
Plant soap: a British rapeseed potassium soap for hygienic leaf washing
British Rapeseed Oil True Potassium Soap Four-Oil Concentrate Fully Biodegradable No Synthetic Additives Handcrafted in Stockport
Dr Forest Plant Soap is a potassium soap for plants: a soft soap concentrate that dilutes into water to wash dust, grime, honeydew residue and hard-water scale off leaves. It is a true soap, saponified from oils with a potassium alkali, rather than a synthetic detergent, so it lathers softly, rinses clean and breaks down fully in soil and water. The base oil is British cold-pressed rapeseed, blended with organic castor oil, organic coconut oil and insect larvae oil, and made from scratch in small batches in Stockport, Greater Manchester.
Use it as a routine leaf wash, a heavier wash for built-up deposits, an emulsifier for mixing horticultural oils into water, and a biodegradable cleaner for pots, tools and greenhouse glass. It is a hygienic plant wash and leaf cleaner.
4Oils in the blend
~63%Oleic acid (rapeseed)
6–10 ml/LMaintenance wash
100%Biodegradable
What is a plant soap used for in the garden?
- Routine leaf cleaning — lifts dust, atmospheric grime and mineral deposits so foliage can take in light and exchange gases properly
- Sticky residue removal — dissolves honeydew residue, leaving a clean leaf surface
- Pre-feed leaf preparation — wash first, dry, then foliar feed with seaweed powder or fulvic acid; a clean leaf takes up a spray more evenly
- Mixing horticultural oils — the soap surrounds oil droplets and holds them in suspension so the spray stays evenly mixed
- Greenhouse and polytunnel hygiene — washes glass, covers, cold frames, benches, trays and pots at either end of the season
- End-of-season clean-up — removes a summer's worth of deposits from fruit trees, perennials and greenhouse crops
Why a potassium soap rather than washing-up liquid?
Dr Forest Plant Soap (potassium soft soap)
- True soap: saponified from British rapeseed, organic castor, organic coconut and insect larvae oils with a potassium alkali
- Rapeseed base, roughly 63% oleic acid, gives a mild, conditioning wash
- No fragrance, colouring, preservative or synthetic surfactant
- Dilution-rated for living foliage, from 6 ml per litre upwards
- Breaks down completely in soil and water; runoff is not a problem
- Doubles as an emulsifier for horticultural oils
- No expiry date
Washing-up liquid or household detergent
- Synthetic surfactants (SLS, SLES) built for kitchenware, not leaf tissue
- Fragrance, dye and preservative that serve no purpose on a plant and can leave a film
- No plant dilution rate on the bottle, so people guess
- Formulations change without notice, so last year's mix may not behave the same
- Strong enough at guessed rates to lift the leaf's own protective wax
Handcrafted in Stockport
Every batch is saponified by hand at the Dr Forest workshop in Stockport, Greater Manchester, from British rapeseed and organic castor and coconut oils, and bottled in recycled packaging. If you have been using washing-up liquid on your plants, read why washing-up liquid fails as a wetting agent first; the same chemistry applies to leaf washing. Made by Growers. Backed by Science.
The Science
The science of potassium soap: why a soft soap cleans a leaf without stripping it
What a potassium soap actually is
Soap is what you get when a fat or oil reacts with a strong alkali. Use sodium (NaOH) and the result is a hard bar. Use potassium (KOH) and the result is a soft, water-soluble soap that stays liquid at room temperature and dilutes into a sprayable solution without heating or grating. That is the whole reason horticultural soaps have always been potassium soaps. The reaction also releases glycerine, which stays in the concentrate and softens the lather.
A soap molecule has a fatty acid tail that sits in oil and grease, and a charged head that sits in water. Diluted on a leaf, the tails bury themselves in whatever film is sitting on the surface, the heads face outward into the water, and the deposit lifts away with the rinse. The same behaviour is what lets this soap act as an emulsifier: add oil to a soap solution and the molecules wrap each droplet so it stays dispersed rather than floating back to the top. If you want the broader picture on surfactants, wetting agent, surfactant or adjuvant: the difference covers it.
Four oils, and what each fatty acid does in the lather
| Oil | Main fatty acid | Job in the soap |
|---|---|---|
| British cold-pressed rapeseed (Brassica napus) | Oleic acid, ~63% (with ~17% linoleic, ~7% alpha-linolenic) | Mild, conditioning base; the character of the wash |
| Organic castor (Ricinus communis) | Ricinoleic acid, ~90% | Dense, stable foam that clings to the leaf |
| Organic coconut (Cocos nucifera) | Lauric acid, 45–53% | Highly soluble, fast-cleaning soap molecules |
| Insect larvae oil (Hermetia illucens) | Lauric acid, roughly a third to a half | Reinforces the coconut fraction; a circular-economy fat |
Chain length decides how a soap behaves. Short and medium chains (lauric, C12; myristic, C14) make soaps that dissolve fast and clean hard. Longer, unsaturated chains (oleic, C18:1) make gentler soaps with a softer lather. A coconut-only soap would clean aggressively and could dry the leaf surface; a rapeseed-only soap would be mild but thin. Blending the four gives a soap that spans C12 to C18, cleans properly at 6 to 10 ml per litre and still reads as gentle on foliage. Cold-pressing the rapeseed, rather than refining it, keeps its tocopherols and carotenoids in the oil [1].
A leaf under a film of dust is a leaf working in the shade.
Mechanisms of action
01
Dust on a leaf costs the plant light and heat balance
Hirano, Kiyota and Aiga (1995, Environmental Pollution) dusted cucumber and kidney bean leaves and measured the physical consequences: the particle layer shades the leaf and interferes with the stomata, and photosynthesis drops [2]. Shah and colleagues (2018, Environmental Science and Pollution Research) found roadside dust significantly reduced chlorophyll a, chlorophyll b and carotenoid levels in three ornamental species, with the loss worst at the highest dust dose [3]. Washing the film off is the direct fix.
02
Surfactants can dissolve the leaf's own wax, so the rate matters
Every leaf carries an epicuticular wax layer that slows water loss. Tamura, Knoche and Bukovac (2001, Journal of Agricultural and Food Chemistry) showed that surfactants can solubilise that wax [4]. Any surfactant will do it if you use enough, which is the case against guessing with washing-up liquid. A soap with a stated dilution range, built on a mild oleic base, is the way to clean the surface without taking the wax with it.
03
The rapeseed base: why oleic acid makes a gentle soap
Rapeseed oil runs 46 to 66% oleic acid with 13 to 41% linoleic and roughly 8 to 15% alpha-linolenic, and cold pressing preserves the vitamin E, flavonoids and carotenoids that refining strips out [1]. The British cold-pressed oil used here sits at around 63% oleic. Saponified with KOH, that profile gives a soft, conditioning soap that can be used weekly on foliage.
04
Castor for cling, coconut for cut
Castor oil is about 90% ricinoleic acid, an unusual fatty acid found in almost no other oil, and it is the reason the lather is dense and stays put on a leaf rather than running straight off [5]. Coconut oil is 45 to 53% lauric acid [6], which saponifies into short, highly soluble molecules that lift grease and honeydew quickly. Contact time from the castor, cleaning power from the coconut.
05
Insect larvae oil: lauric acid from a circular source
Black soldier fly larvae are reared on organic waste and their fat is dominated by lauric acid: 28 to 37% in one recent extraction study [7], and from 32% up to 76% across substrates in a wider review, with myristic, palmitic and oleic acids making up most of the rest [8]. It reinforces the coconut fraction and comes from a waste stream rather than a plantation. It does mean this soap is not vegan; that is stated plainly rather than hidden.
06
Hard water turns soap into scum
Calcium and magnesium ions in hard tap water swap places with the potassium on the soap molecule and form insoluble calcium and magnesium fatty-acid salts, the same stuff that rings a bath. The solution goes cloudy, the lather collapses and a white film can be left on the leaf. Rainwater or filtered water avoids the problem entirely; with hard tap water, use the top of the dilution range and agitate well.
Scientific references
- Shen, J., Liu, Y., Wang, X., Bai, J., Lin, L., Luo, F. & Zhong, H. (2023). A comprehensive review of health-benefiting components in rapeseed oil. Nutrients, 15(4), 999.
- Hirano, T., Kiyota, M. & Aiga, I. (1995). Physical effects of dust on leaf physiology of cucumber and kidney bean plants. Environmental Pollution, 89(3), 255–261.
- Shah, K., Amin, N. u., Ahmad, I. & Ara, G. (2018). Impact assessment of leaf pigments in selected landscape plants exposed to roadside dust. Environmental Science and Pollution Research, 25(23), 23055–23073.
- Tamura, H., Knoche, M. & Bukovac, M. J. (2001). Evidence for surfactant solubilization of plant epicuticular wax. Journal of Agricultural and Food Chemistry, 49(4), 1809–1816.
- Mubofu, E. B. (2016). Castor oil as a potential renewable resource for the production of functional materials. Sustainable Chemical Processes, 4, 11.
- Dayrit, F. M. (2015). The properties of lauric acid and their significance in coconut oil. Journal of the American Oil Chemists' Society, 92(1), 1–15.
- Srisuksai, K. et al. (2024). Physicochemical properties and fatty acid profile of oil extracted from black soldier fly larvae (Hermetia illucens). Veterinary World, 17(3), 518–526.
- Suryati, T., Julaeha, E., Farabi, K., Ambarsari, H. & Hidayat, A. T. (2023). Lauric acid from the black soldier fly (Hermetia illucens) and its potential applications. Sustainability, 15(13), 10383.
Fatty acid figures describe the published composition of each oil class; the blend proportions in this product are not published.
How to Use
How to use potassium soap for plants: dilution rates and method
Shake well before every use
This is a natural soap concentrate and some separation between uses is normal, not a defect. Shake the bottle vigorously before measuring so every dose is the same strength.
Soft water, and not in full sun
Mix with rainwater, filtered or dechlorinated water. Hard water reacts with soap, goes cloudy and loses its lather; if hard tap water is all you have, use the higher end of the dilution range and agitate thoroughly. Spray in early morning or evening: cooler air and higher humidity extend contact time, and wet foliage in direct sun can scorch temporarily.
Application rates
Maintenance leaf wash: routine cleaning
The standard dilution for regular leaf cleaning on all plants. Gives a light, even lather that lifts dust, grime and surface deposits. Spray over the whole plant, leaf undersides included. The rapeseed base is mild enough for weekly use on all foliage types.
Heavy-duty wash: stubborn deposits
For plants with heavy build-up: visible honeydew residue, hard-water scale, or thick dust and grime. The denser lather has more cleaning power. Apply, let the foam sit for a few minutes, then rinse or wipe heavily soiled foliage.
Oil emulsifier: mixing horticultural oils
Add the soap to the water first and agitate to a lather. Then add the horticultural oil while stirring continuously. The soap molecules surround and disperse the oil droplets into a stable emulsion that stays evenly mixed while you spray.
Leaf shine: gentle wash for windowsill and conservatory plants
A gentle wash for plants kept under cover, where they pick up household dust, cooking residue and airborne particles. Spray and wipe the leaves gently with a soft cloth. No leaf-shine film, just a properly clean surface reflecting light as it should.
Tool, pot and surface cleaning
Cleans secateurs, propagation trays, seed trays, pots and greenhouse benches. A biodegradable, residue-free alternative to synthetic cleaners, and worth doing at the start and end of the growing season as general horticultural hygiene.
Greenhouse glass and polytunnel cleaning
Washes greenhouse glass, polytunnel covers, cold frames and propagator lids. Removes algae build-up, mineral deposits and atmospheric grime that cut light transmission. Fully biodegradable, so runoff poses no risk to the plants or soil beneath.
Rate table
| Situation | Rate | Frequency |
|---|---|---|
| Maintenance leaf wash | 6–10 ml per litre of soft water | Every 1–2 weeks |
| Heavy-duty wash | 20–30 ml per litre of soft water | As needed |
| Oil emulsifier | 5–10 ml per litre | Each mix |
| Leaf shine (plants under cover) | 6 ml per litre of soft water | Every 2–4 weeks |
| Tools, pots and surfaces | 15–20 ml per litre | As needed |
| Greenhouse glass and polytunnels | 20 ml per litre | Seasonally |
Step-by-step preparation
- Shake the bottle vigorously. Natural soap concentrates can separate between uses. A thorough shake gives even dosing.
- Measure into soft water. Rainwater, filtered or dechlorinated water gives the best results. Add the soap and stir or agitate until frothy. Hard water causes cloudiness and reduces lather.
- Spray or apply to foliage. Use a pump sprayer set to a fine mist. Cover every surface: upper, lower, stems and growing tips. For potted plants kept under cover, spray and wipe with a soft cloth.
- Apply in early morning or evening. Cooler temperatures and higher humidity extend contact time. Avoid spraying in direct sunlight, as wet foliage in full sun can suffer temporary scorch.
Works well combined with…
Clean foliage takes a foliar feed more evenly. Wash, let the plant dry, then follow with Organic Seaweed Extract Powder as a foliar spray or Fulvic Acid Powder as a foliar feed for the best uptake. When you are mixing horticultural oils, this soap does the emulsifying and the Natural Wetting Agent improves coverage and spread across the leaf. The rest of the range is in plant soaps and wetting agents.
Storage
Keep the lid on, in a cool place out of direct sunlight. There is no expiry date: a true soap concentrate is inherently stable. Some separation over time is normal; shake before use. Keep out of reach of children and pets, as with any concentrate.
Questions & Answers
Frequently asked questions about potassium soap for plants
Potassium soap is made by reacting oils with a potassium alkali (KOH) rather than the sodium alkali (NaOH) used for bar soap. The potassium version stays liquid, dissolves readily in water and gives a soft, spreadable lather, which is why it is the traditional base for liquid castile soaps and for plant washes. Dr Forest Plant Soap is a potassium soap made from British rapeseed, organic castor, organic coconut and insect larvae oils.
It cleans leaves. Diluted in water it lifts dust, grime, mineral scale and sticky honeydew residue off the leaf surface so the plant can take in light and exchange gases properly. It also emulsifies horticultural oils into water and cleans pots, tools and greenhouse glass. It is sold as a hygienic plant wash and leaf cleaner, nothing more.
Yes, at the stated dilution rates. It is suitable for vegetables, fruit, herbs, flowers, orchids, succulents, trees and shrubs. It contains only saponified vegetable and insect-derived oils, with no synthetic chemicals, preservatives or additives. Rinse edible crops with clean water after washing if you prefer, though the soap breaks down naturally and leaves no lasting residue.
Once diluted and applied, it is a mild soap that breaks down fully in soil and water, so runoff around beds, ponds and lawns is not a concern. The neat concentrate is still a concentrate: keep the bottle out of reach of children and pets, and spray in the early morning or evening when bees are least active and the foliage dries slowly.
You can, and plenty of people do, but it is a poor substitute. Washing-up liquid is a synthetic detergent with fragrance, dye and preservative, formulated for plates and with no plant dilution rate on the bottle. Used too strong, surfactants can dissolve the leaf's own protective wax. A potassium soap made from mild oils, with a stated rate of 6 to 10 ml per litre, is the version designed for foliage.
Calcium and magnesium ions in hard water react with soap molecules to form insoluble calcium and magnesium fatty-acid salts, the same compounds that make bathroom soap scum. The lather drops and a white film can be left on the leaf. Use rainwater, filtered or dechlorinated water. If hard tap water is all you have, mix at the higher end of the dilution range and agitate well.
Yes, and it is one of its main uses. Oil and water separate within seconds without an emulsifier. Soap molecules surround each oil droplet and keep it suspended. Add 5 to 10 ml of soap per litre of water, agitate, then add the oil while stirring continuously, and the mix stays even while you spray.
It is made with organic ingredients: the castor and coconut oils are organic, and the rapeseed is British cold-pressed. It does not carry organic certification and is not sold as though it does. It is not vegan, because one of the four oils is insect larvae oil from black soldier fly larvae reared on organic waste. If you need a fully plant-derived option, the Natural Wetting Agent is soap nut and aloe vera based.
British rapeseed is non-GM, cold-pressed to keep its natural tocopherols and phytosterols, and around 63% oleic acid, which saponifies into a mild, conditioning soap. It has far lower food miles than imported tropical oils and gives a noticeably gentler wash than a coconut-dominant soap. It is the oil that sets the character of this product.
No expiry date. Saponification turns the oils into soap and glycerine, both of which are shelf-stable indefinitely when the bottle is kept sealed. Natural separation can happen over time; shake well before each use. Store in a cool place away from direct sunlight.
Yes, if it's a plant soap at the right strength. Mix 6–10 ml of this soap per litre of soft water or rainwater and wash or mist the leaves every one to two weeks to lift dust, grime and sticky residue. Spray early in the morning or in the evening, not in strong sun, and test a few leaves first on anything delicate. Washing-up liquid is a poor stand-in: it's a synthetic detergent with fragrance and dye and no plant rate on the bottle. Rinse edible crops before eating.
Here. This potassium soft soap comes in 500 ml, 1 litre and 5 litre bottles and is posted to any UK address. Look for three things whichever soap you buy: it should be a potassium soap (it stays liquid and dissolves easily), it should be made from mild oils, and it should print a plant dilution rate on the label. Ours is made from British cold-pressed rapeseed, castor, coconut and insect larvae oils and is diluted at 6–10 ml per litre of soft water for a routine leaf wash.
A potassium soap made from mild oils, with a plant dilution rate printed on the bottle. Potassium soaps stay liquid and dissolve readily in water, which makes them easy to dose and rinse. Mild oils matter because harsh, short-chain soaps are harder on leaves. This one blends British cold-pressed rapeseed (around 63% oleic acid, which makes a gentle soap) with castor, coconut and insect larvae oils, and it cleans properly at 6–10 ml per litre. Avoid detergents and anything perfumed or coloured.
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