Soap Making with Potassium Hydroxide, Part 1, The Process

I prefer to avoid pesticides, at times however I want crops and plants I love are being destroyed. One of the safer pesticides to use is soap.

Additionally I would rather add potassium to my crops, than sodium.    Most soap these days is made from Sodium Hydroxide, it is cheaper and makes a harder soap.  The old stuff was made with potash, Potassium Hydroxide.  It costs a bit more, but for spraying on the tender leaves of my plants, I want a better grade of soap.


Potassium Hydroxide is still a dangerous chemical, and care needs to be taken when using it.


First things first, protective gear.


Vinegar is  an acid that can be used to neutralize the Potassium Hydroxide, or KOH.   It will quickly negate the alkali effects of KOH, but it will also produce heat while doing it.  It is possible to cause burns while trying to prevent burns.  To prevent this,  the vinegar needs to be used wastefully as a wash and not as an ointment.  KOH should be washed off with lots of water and then the residue neutralized with vinegar.  Having a hose turned on and ready is a very good idea.

Safety glasses and gloves are also needed for safety.  The area needs to be well ventilated, children and animals need to be elsewhere when handling dangerous chemicals.   paths need to be clear, actions need to be rehearsed and considered.  Contingencies planned for, I did this outside, in case I needed the hose to clean with.


Note that in this picture, the scale is not protected.  This is probably not ideal.


In this picture, the scale is protected by plastic wrap, while the Potassium Hydroxide is being measured.

Here is the process, on a nice warm day, with a pleasant breeze and with no distractions or issues likely to arise, measure the water into a bucket and then measure and mix the KOH into the water.  The chemical reaction of the KOH melding with the water is exothermic.  Heat will be produced.  Care must be taken, and the KOH must be added slowly.  One of the many things  you want to avoid is melting the plastic bucket while working with caustic chemicals.  In my experience KOH is not as bad as Sodium Hydroxide for generation of heat, but being careful and aware of what is going on, is very important when working with these materials.  The dust from KOH being poured can be quite bad for you.  Bad for your eyes, bad for your skin and lungs.  The vapors created when you initially mix KOH and water are also best avoided.  So attention to detail and careful material handling are big issues.


I am using a cement mixer on a power drill to mix.  When I actually pour the KOH, it is much closer to the bucket.  I mix as I pour, so that the heat of reaction is not concentrated at the bottom of the bucket and instead is diffused through the rest of the water.


After the water and KOH have been mixed, you now have lye water and you want to start adding oil.  The reason that I have specified a warm day is two fold.  Oils pour better when warm, and need to be liquid to mix in any case.  If the lye water temperature is lower than the temperature that the oils you are using stay liquid, you will have problems.  If the mix falls below body temperature, it will probably not saponify.  Saponification is the process of lye water and oil mixing and producing soap.  This reaction may or may not instantly happen, and may cause you considerable stress getting it to happen.  Agitation and warmth are key to making it start.   Sometimes it will start and stop.  In some cases it will take a long time.  In any case, soap does not cure as well or at all when cold.


Here is the mix I made, Water, KOH, Soybean Oil, Palm Oil all carefully measured and added.  Now I see a problem.  The bucket is too full to mix well.   I made too much.  So I carefully mixed it up and then removed a bit, to try and keep the mix even.  If I just removed the lightest materials on top, I am sure to mess up the recipe.


Here I am mixing the stuff.  Note that a bit has spilled, and that the small bucket beside it has separated out.   A sure sign that it has not reacted and made soap.


Finally you can see that the goop has started to gel.  This is the sign that it is beginning to be soap.  Notice that the shadows are getting longer.  Sadly a cold snap was coming in, and part way through, the gelling stopped.  I put the raw partially reacted soap in my green house and the next day when the mix was nice and warm, it quickly mixed up and made soap.  Fortunately my poor timing did not result in a bad batch.

Usually you wait for two weeks or a month before using the raw soap.  You also make sure there is not lye left over that has not reacted.  To avoid this your recipe is usually five to ten percent super fatted.  That means that more oil was used than was needed to prevent making skin damaging soap.  Since I was not worried about making a hard bar of soap, I used extra water.  This helps to prevent all sorts of possible issues.  It also speeds the reactions.

Here is the horrible test that I use to see if my soap is going to be harsh.  Before I tell it to you, please remember that I do not advise anyone to be as crazy as I am and use my methods.  A strip of litmus paper is a much better way to go.   I just like to do it the way the old timers did,  I taste my soap.  If it does not burn my tongue, then it is not caustic.  This is not a compliment to the soap really, but this was, hands down, the best soap I have ever tasted.   I still washed my mouth out with water.


23 comments to Soap Making with Potassium Hydroxide, Part 1, The Process

  • Skip J.

    Hello Bob;

    This one is without a doubt your best Gardening article, congrats! Pretty good one for Chemistry and Recipes too, maybe the best….

    Sooo, there’s going to be a Part 2 and a Part 3 and so on??????

    I just couldn’t bring myself to taste the soap…. this is going to have to be another one of those I order from you …..


  • Yes, making and using soap in the garden are too complex an issue for a single post. As far as tasting a soap, I would rather injure myself than cause indiscriminate injury to another.

    I think garden soap will be the first thing I put up for sale on this website. Because of the nature of soap making, I am not sure everyone has the right environment to make it themselves. I still want to encourage the practice and spread the knowledge however.


  • Skip J.

    Ahhh yes, well… I’m not going to put in the research effort to figure out the diamonds either; even tho it is something I could do. Any kind of Chemistry is wayyy out of my league. I will experiment with the wax mix as you taught me to.

    However, while most of the plants I raise are tough enough; some are very tender indeed when they are young shoots. So I have a definite need for a delicate but effective solution for the appropriate bugs.

    I do wish you well with your venture!


  • Skip J.

    Well Bob; it has been a month and I have had time to think about your article. Actually, this particular article.

    You have introduced a number of soap-specific terms such as saponify and super-fatted that you have explained well. Still, the likelihood someone will follow through and learn your process – and various forms of the soap recipe – are somewhat slim. It would be interesting to read your future articles on the details though!

    But I am ready to hear a few words on preparing the mix to apply, application methods, and the end-product mix’s horticultural benefits if you can….. My own interest is for low-toxic pest control on bonsai potted trees; but, trees like acid, not alkali….. I am usually applying an acidifier to neutral soils for them.


  • I have however sprinkled crushed lime on plants before and had them benefit from it. So some strong alkali can be OK in the right circumstances. In this case however the soap mix is indeed slightly acidic, so there is no issue there.

    Neat thing soap. When I made this soap, I made a lipid out of a strong alkali and an oil. The oil is a fatty acid, so it reacts and combines with the alkali. The result is a fairly neutral molecule that will bond on one side with water and the other side with oil. this molecule is a lipid and a salt. When I put it more oil than it needs, I am avoiding having raw lie remaining in the mix. The result is that the mix has a bit of extra oil so it has a slightly acidic PH balance. This makes is much safer.

    The lipids that we call soap will attach to oils and partially to completely encapsulate them. This is why soap cuts grease. The typical microbe has cell walls made of a double layer of lipids. Soap tends to destroy these cell walls, which is why soap is so good at preventing disease. So in two ways soap is a really good cleaner.

    The extra oil in the soap wastes a certain amount of the soaps strength as a cleaner, but it makes it safer and more pleasant for us to deal with. As a pesticide, the soap and oil are a good combination and will clog breathing tubes and damage cell walls.
    The soap lipid in this case being a salt, will typically draw moisture from the critter and then the air will take it from the soap. This can dehydrate and further damage the bug.

    Bugs have a much higher surface area to internal mass than we do. The cell damage caused by having soap left on us might cause a rash. Soap left on a bug is much harsher.

    It is always wise to test a plant to be sure it is not sensitive, but a lot of plants will not only do fine with a spraying, but will actually take nutrients from it.

    The real downside possible for this and any other pesticide, is that it could also damage a predatory insect. Lady bugs and praying mantis for example live on other bugs, so they are really nice to keep safe and free of poisons. A safe soap has the advantage of being easily removed, and target applied and does not pass toxins up the food chain. Often people add extra toxins to their soap for a bit more kick, but I don’t want to hurt the bees and the lady bugs.


  • Skip J.

    Thanks! I needed that!

    Bob said: “The real downside possible for this and any other pesticide, is that it could also damage a predatory insect. Lady bugs and praying mantis for example live on other bugs, so they are really nice to keep safe and free of poisons. A safe soap has the advantage of being easily removed, and target applied and does not pass toxins up the food chain. Often people add extra toxins to their soap for a bit more kick, but I don’t want to hurt the bees and the lady bugs.Bob”

    I encourage lady bugs and mantis in my low-toxic yard, and am happy when I get a hatch of little mantis’es… soooo I could apply the mix on the one tree with bugs, inspecting to make sure no good bugs are already on the job first. Then let’em die and come back and spray the tree with water to wash’em off and it won’t hurt the tree…. great! Sounds like magic to me…..


  • Maureen King

    I am looking for Potassium soap, as I have heard that mixing this soap with birds eye chillies and water I can spray it on my veges to keep away the white moths eating my green veges etc.
    can you help? Maureen

  • Thank you for mentioning Potassium hydroxide can be neutralized with vinegar.
    One of my Energizer batteries leaked – i wanted to clean the ‘stuff’ that leaked out – vinegar did the trick.

  • marian appau

    that is wonderful, can you please give me the quality of water to the potasium salt and the oil part. thanks

  • Bob Strawn

    Here is a good source for saponification information,

    Here is another table with saponification values;

    Since oils are variable and often adulterated with cheaper oils, it is good to do a small test before you go large from a given source. I don’t mind a bit of extra oil for an insect spray, so I go a bit heavy on oil to be sure the KOH is all converted.

    1.4 times as much potassium hydroxide is needed than the more commonly used sodium hydroxide. Typically you use 0.38 times the weight of the oil in water. Less water will prevent total conversion to soap. Since I am making a liquid soap here, I put in more water than needed, just to be sure the process runs smoothly.


  • Hi Bob – Thanks for the tutorial. I’ve been using homemade (wind-powered) bubble machines & soap bubbles to deter mosquitoes for years. They’re generally aimed at certain areas or the perimeter & it’s more for the gardener than for the garden. But when using regular dish soap one has to be careful not to get too much blowing directly onto the garden itself, especially with certain plants.

    I think that this type of ‘insecticidal soap’ using potassium hydroxide would be much better to use since it’s less harmful to plants & can be directly applied to them. Any thoughts on using a slightly higher concentration in order to make bubbles, instead of a more diluted spray? Also have you tried it with any other additives like Diatomaceous Earth (D.E.)?

  • Bob Strawn

    Bubbles for Mosquito repellent! Great idea!

    I use DE quite a bit, but I limit use of it when the bees are going flower to flower. Soap makes DE stick better, but I have no problem with reapplying DE, and would rather have the DE level low when the flowers are blooming. Additionally wasps patrol my greens so I would rather not use DE on them either. Bubbles to deter mosquitoes sound like fun, I will have to research that!

    The formula I have approximated for optimum bubbles is 19 parts sodium oleate to 750 parts of water . Glycerol is added to make the entire mix 1000 parts. I say approximated as I have used less scientifically refined ingredients, Dawn or Joy with distilled water and glycerine. I was never a fan of corn syrup, in my environment it never seemed to help.

    I have no data on how well potassium soap does for bubbles, but I definitely plan to try it! I will have to order one of your bubble machines so I can review it! I suppose I should try and make one out of cedar. That would be pretty as well as functional.


  • James

    Bob, thank you very much for writing such an informative article.

    I just have a question.

    In your article you state, “I put the raw partially reacted soap in my green house and the next day when the mix was nice and warm, it quickly mixed up and made soap”. Does this raw soap mixture stay liquid all during your suggested waiting period of two weeks to a month before using or does it congeal at some point during that time having to add more water in order to dissolve it?

    Thank you.

  • Bob Strawn

    Potassium soap is not as hard as sodium soap. As I recall, I put in a bit more water than needed in the original mix so it became a gel at it’s hardest. The gel is easy to add water to so that it can be a syrup consistency or a bit thinner so that it can be sprayed.


  • James

    Bob, after pouring your lye water into your oils what is the sign or the consistency that lets you know you have soap whereby you stop mixing with your power drill (for example, your power drill cannot further mix the mixture because its too thick), and let your mixture rest for two weeks to a month before adding more water to get the desired consistency of liquid soap?

    About how long do you have to mix with your power drill from finish to end before letting it rest for two weeks to a month?

    Thank you very much.

  • James

    Sorry, I meant to write about how long do you have to mix with your power drill from beginning to end before letting it rest for two weeks to a month?

  • Bob Strawn

    When the mix starts out it is quite liquid. When it starts to hold shape for a moment it is beginning to make soap. The term used for this is tracing. I have never had to agitate for more than 30 minutes. When making small test batches in a blender at high speeds, it often takes less than a minute.


  • PeterBayard

    I really enjoyed reading your article and all comments.
    I have a suggestion for the bubbles: Try using glycerin as an additive to create large bubbles.. I’m not a chemist but that was the additive we used to make big bubbles from ordinary soap.


  • Erica Nader

    Bob, What Quantities liquid and weights dry, did you use for each, Water, KOH, Soybean Oil, and Palm Oil? Thank You

  • Bob Strawn

    While I did record the amounts I used, following my recipe without knowing the reason for it is probably not the best way to make your soap. Oils are not precise in behavior and often they get substituted. Also the goal of your soap can change the formula a lot. I used soy because it is cheap and it tends to leave a residual film. Good for killing bugs, and above average for washing hands. I used palm oil because I wanted it to suds in cold water. If you don’t care about suds, you might not care about the palm oil. Typically I go with 25% by weight palm oil. So the basic recipe would be 75% soy and 25% palm. If I were doing 3 pounds of soy oil, I would probably do 1 pound of palm oil. A pound of palm oil, if you can trust it and it matches the profile, can be fully saponified with 0.1914 lbs of potassium hydroxide. 4 lbs of soybean oil, if you can trust it and it matches the profile can be fully saponified with 0.8 lbs of potassium hydroxide. Most recipes will add 5% more oil or 5% less lye to try and be safe. Typically you multiply the weight of the lye by 3.333333 to figure out how much water you need. I was not wanting to make a dry bar of soap so 5 times the water was just fine with me. Another advantage to a liquid or gel soap is that it is much easier to adjust later if you have too much lye in it.

    Even with that margin of safety you may find yourself needing to go through the total pain of melting your soap and adding more oil in an attempt to save a batch. You are better off mixing a small single oil batch in a blender first in order to find out if the saponification values are correct for the oil you have.

    The problem is that you cannot trust that the oil is what it says it is. Chemistry does not lie, businessmen are another story. If you don’t test your oils, you will not be able to make consistent soap or know that you are being deceived. If you do test your soap you will discover the few brands that have managed to honestly control their product, suppliers and management policies.

    A lot of the data used by soap makers is based on varieties that are no longer common. Another problem is that early accidents in publishing and testing have been repeated over and over again. Many years ago I tested a wide range of oils by making small blender batches of pure oil soaps. Everyone and I mean everyone says that castor oil makes a soft soap.

    Every brand I could obtain at the time I did my testing made a rock hard soap.

    Don’t get me started on olive oil. So many fancy brands, all of them go rancid if left open for 3 months. Some of the most expensive fancy brands have a ‘characteristic’ mildly rancid taste. Guess what? Real olive oil does not go rancid or in any way taste rancid after being left open for years.


  • Gahigi

    I know this is an old post so I can only hope I’ll get an answer. I’m not trying to make soap but I had some KOH that was in the same plastic container I bought it in. I bought it about 7 years ago and never did anything with it but this morning it blew up, got on my carpet, and put a dent in my dresser like a huge bruise that cracked. I tried to clean it up with water and a towel being careful not to burn myself but I didn’t get it all up. It won’t stick to a wet towel very well so I thought maybe some vinegar would help since I believe KOH is negative and vinegar is positive but I don’t know how it would react with it. I was fortunate that most of it remained in the container but there’s still a little on my carpet and some in my dresser. I don’t even know what to do about the clothes. I just need some ideas on how to clean it up. Thank for any help

  • Bob Strawn

    I use vinegar. I would be careful about heat and fumes when you use it to negate KOH. I try to do all my work with Lye outdoors and store it outdoors.


  • Zweli Joseph Nkosi

    Which colorants can l use to make a green washing liquid, and what can l use for perfume.And thank u so much for the great tutorial.

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