" Blog da Horta Biológica: Potassium Hydroxide Liquid Soap for Garden Pests and Laundry

Potassium Hydroxide Liquid Soap for Garden Pests and Laundry

At the request of our Azorean readers, we're going to teach how to make potassium hydroxide / coconut soap for garden and orchard pests... and more!

First of all, we'd like to thank our readers for following us and trusting in our work, and we'd like to send a big hug to those of you who follow us from the wonderful islands of the Azores!

At the end of this post, you can also watch our educational video where we show you in detail how to make this soap.

For this post, we've formulated a soap that, in addition to having only only a few ingredients, is very effective at fighting pests in the garden and orchard, as well as washing clothes in the home laundry.

Let's start by looking at the ingredients and understanding why each one is used.


  • 141g Cold-pressed extra virgin coconut oil
  • 94g Extra virgin olive oil
  • 60,11g Potassium hydroxide (KOH), 90% pure *
  • 180,32g Distilled water

* Potassium hydroxide is highly hygroscopic, which means it has a strong ability to absorb moisture from the air. For this reason, as soon as it is weighed, it should be immediately sealed in its container, which must be well stored to prevent its deterioration. After sealing the bottle, we place it inside a tightly closed bag and store it in a cabinet to avoid contact with moisture. See the images in the video for a better understanding.

If you don't have a precision scale, round down, never round up.

Important: This soap does not have super fat, so it is not intended for use on the skin!

Why These Ingredients

First of all, it's important to remember that all the ingredients must be of high quality! Sometimes, when the soap is not made for skin use but for pests and laundry, there is a temptation to use low-quality oils. In this case, we strongly advise against it, since the purpose of this soap is also to nourish and care for the plants. 

On the other hand, we also want to stress that this formula has been calculated on the basis of pure oils. Each oil has its own saponification index, which means that if dubious oils are used (i.e. oils that are not truly extra virgin), the result could be a fiasco.

If you've never made soap before and aren't familiar with the process or the technical terms we are using, please read our post first: How To Make Soap.

Let's take a look at each of the ingredients:

  • Cold-pressed extra virgin coconut oil: coconut soap is extremely effective in controlling garden pests (find out more in our post Soap and Vegetable Oil Spray). Additionally, coconut is highly nutritious and is used as an organic fertilizer for the healthy growth and development of plants.
  • Extra virgin olive oil: the oil can be plain or macerated with pest-repellent plants such as peppermint, rosemary, thyme, etc. Find out more about macerations in our post How To Make a Maceration and more about plants and their uses in our book The 5 C's of Aromatic and Medicinal Plants. The use of olive oil in this formula has two functions. One is to add more beneficial properties to the soap, and the other is to achieve a final consistency that is easier to work with, since 100% coconut soap is much thicker and more difficult to break down and dilute.
  • Potassium hydroxide (KOH) 90% pure: potassium is one of the essential nutrients for plants, and potassium soap, in addition to helping control pests, will contribute to plant protection, nutrition and growth. It has to be 90% pure to guarantee effectiveness in the saponification reaction. 

  • Distilled water or rainwater: is used because it is free of impurities, minerals, and contaminants. Distilled water must be used to ensure that the chemical reactions take place as expected and to guarantee the purity, precision and consistency of the final product.

The Materials

To make this soap, you need to gather some materials. The ones we've marked in blue must be used exclusively for making soap because, for food safety reasons, the same materials cannot be used for cooking food and making soap (due to the chemical reactions between fats and potassium hydroxide, also known as caustic potash). 

None of the materials can be made of aluminum because of the reaction with potassium hydroxide!

This list of materials is in the order of use when making soap. 

  • 1 Digital kitchen scale (must be digital for greater accuracy in weighing ingredients)
  • 1 Stainless steel pan
  • 2 bowls for weighing the oils (we recommend weighing them separately in case of mistakes that require adjusting the quantities)
  • 2 tablespoons to help remove the coconut oil from the jar if it is solid
  • 1 Plastic or glass container for the water
  • 1 Plastic or glass container for the potassium hydroxide
  • 1 Pan base
  • 1 Stainless steel spoon to dissolve the hydroxide in the water
  • 1 Kitchen spatula for scraping the bowls with the oils
  • 1 Spatula for stirring the potassium hydroxide solution with the oils
  • 1 Hand blender (yes, it really must be exclusive for soap making!)
  • 1 Digital kitchen thermometer
  • 1 Small spoon (to help with pH tests)
  • 1 Pyrex
  • pH strips
  • 1 Large spoon (to help transfer the soap to a container)
  • 1 Container with lid to store the soap batter at the end
  • 1 Water container
  • 1 Water boiler, which will be very useful for dilution
  • 1 Funnel
  • 1 Container to store the liquid soap in at the end

These are the materials we use. Some of these materials are mandatory, others can be adapted according to what you have at home. Watch our video for a better understanding.

Safety Equipment

As always, safety equipment is more than mandatory, and here are all the necessary elements:

  • Safety goggles
  • Rubber gloves
  • Face mask. Surgical masks (also known as “Covid masks”) are not effective for dealing with chemical reactions, so more closed masks should be used.
  • A long-sleeved lab coat and closed shoes. All skin must be well protected.
See all the safety warnings on soap making in our post How To Make Soap.

How to Make Potassium Hydroxide Liquid Soap

First of all, you should know that the result is never liquid soap, but a thick batter that only becomes gel-like or even liquid once diluted, depending on the dilution ratio.

This soap is made using the Hot Process, meaning the soap will be cooked and will not need to cure once it has been made. Once again, if you don't understand these terms, read our post first: How To Make Soap.

Before we start the step-by-step instructions, we strongly recommend that you have a log book every time you make your own soap, so that you can write down (and later remember) all the important details of each production batch. Here is the notebook we use and recommend to everyone who makes soap at home, whether for personal use, to give away or to sell:

Log book that we use and recommend

So now we're going to share with you how we make this soap at home. Here are all the details:

  1. Start by weighing the empty stainless steel pan (ours weighs 374g). Knowing the weight of the pan will be crucial at the end of the whole process.
  2. Weigh the oils separately, each in its own bowl.
  3. Weigh the water.
  4. Put on all the safety equipment before starting to handle the potassium hydroxide.
  5. Weigh the potassium hydroxide.
  6. Add the oils to the pan (use the kitchen spatula to remove all the oil from the bowls, ensuring there is no waste or significant differences in the amount of oil).
  7. Turn the stove to low heat. (There's no need to heat it up too much, just until the coconut oil melts completely).
  8. While the fats are heating, pour the hydroxide into the water (never the other way around!) and stir with a stainless steel spoon.
  9. Once dissolved, add the water solution to the fats in the pan;

From here on, it's crucial to constantly monitor the temperature while stirring the solution!

    10. Measure the temperature and stir the solution with the spatula that is exclusively for soap making. The temperature of the solution should not exceed 70ºC. For those who have a Crockpot (a pan that maintains the temperature), this won't be a problem, but for those who don't, as in our case, it's necessary to keep putting the pan on and off the heat to make sure it doesn't go over 70ºC.
     11. After stirring the solution a little with the spatula, use the hand blender. Blend the solution for a few minutes, always monitoring the temperature, in a combination of stirring and a little heat from the stove.
     12. When the batter is too thick for the hand blender, use the spatula again. For us, the batter took 8 minutes to reach this point.
       13. When the batter has thickened and is beginning to steam a little, turn off the stove and continue cooking the batter using only the residual heat from the pan.
       14. Once the batter has reached the consistency of a paste where it resembles mashed potatoes, but much thicker, it is probably ready. For this we do the pH test.
      15. To do the pH test, put water in a Pyrex, add just a small amount of the soap paste (for a more reliable result, take it from the inside of the paste and not the outside) and stir well. Then, add a little boiling water as this helps to dissolve the soap paste.
        16. Place a pH strip in the water and check the result. The ideal pH for soap is between 8 and 10. Our paste has a pH of 8, which means that the paste is cooked and the soap is ready.
         17. Allow to cool completely before moving on to the next stage.

Storage and Dilution

To store and dilute, it requires a bit of mathematics, but nothing too complicated.

We got a total weight of paste of 429g. How do we know this? We know because we weighed the tare of the pot at the beginning (374g) and weighed the pot again after the soap paste had cooled (803g).  803-374 = 429g of paste.

For this post and video, we decided to divide the paste in half so that we could demonstrate how to store it and how to dilute it, both for use against garden pests and for home laundry. Let's break it down:

  1. Once the paste has cooled down, we removed about half of it from the pot using a spoon and a spatula.
  2. After this removal, the pan weighed 632g. So, 632 - 374 = 258g. This was the amount that remained in the pan to be diluted.
  3. 429 - 258 = 171g. This was the amount we stored for later dilution.


The 171g for further dilution has been stored in a plastic container with a lid. It can be stored in a cool place at room temperature.

Laundry Dilution

The 258g that remained in the pot were diluted to make laundry detergent. 

Now, how to dilute it?

The ideal ratio for this is 1:1.5

So 1 part soap to 1.5 parts water (258g x 1.5 = 387g water).

  1. Weigh out 387g of water.
  2. Use a kettle to quickly bring the water to a boil. The hot water will help break down and dissolve the thick paste. Some water is always lost to evaporation when boiling, but for this purpose, it doesn't make much difference.
  3. Add the boiling water to the pan, use the spatula to break up the paste a little, distributing it better throughout the water. Let it sit for a few minutes.
  4. Use the hand blender until it becomes a homogeneous liquid.
  5. Essential oils can be added at this stage. We prefer to use neutral detergent.
  6. Leave it to cool completely.
  7. After it has cooled, use a funnel to pour it into a container suitable for use in the laundry.
The use of this detergent is the same as for conventional detergents, meaning the recommended doses should be used according to the hardness of the water.

Dilution For Garden Pests

The 171g we saved for later were diluted to spray the pests in the vegetable garden and orchard. 

The dilution ratio for spraying varies depending on the type of soap, the pests to be combated and how heavily the plants are infested. In order to obtain a mild spraying liquid, for lightly infested plants, we will use a maximum of 10L of water for dilution. The 10L makes this easier as we have a 10L pot or two 5L water bottles can be used as an alternative. 

Many sprayers for home gardens are also 5L, so we advise you to make the dilutions according to these proportions, only adjusting the amount of soap according to the severity of the pest attack in the garden.

As this soap is difficult to break down, we recommend starting the dilution in the same way as we did for the laundry detergent, i.e. pour boiling water into the pan in a ratio of 1:1.5, break down and dissolve the soap, and then pour the solution into the pan or divide it between two bottles, filling them to the top with water. That's how you get a soapy spray!

How to Apply in the Garden and Orchard

Transfer the liquid to a sprayer.

The spray should only be used if pests are observed. It should not be used merely as a preventative method.

It should be applied directly to soft-bodied insects such as aphids, lice, mealybugs, caterpillars, whiteflies and moths. Remember not to spray directly on beneficial insects such as ladybugs and bees.

It can be applied once or twice a week, early in the morning or late in the day. Apply to the stems and leaves, not forgetting the back of the leaves where there is a higher concentration of pests.

Avoid applying on windy days and during the hottest hours of the day.

This post is already lengthy, and we hope it has been helpful to our readers! For a better understanding of this entire process, watch our video now, where we show each step in detail.

A big hug to our readers who follow us in the Azores and around the world!

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