Potassium soap: an effective alternative against pests

The world of phytosanitary products for pest and disease control is increasingly limited. The control of the limits of residues and the active ingredients makes today the classic and effective solutions against various pests shine.

This is the case of potassium soap , an active substance based on the use of soap that we all know to clean the leaves of residues and directly affect the activity of numerous soft-bodied insects.

In this guide we want to praise the use that we can give to potassium soap and its variations for the control of whitefly, aphids, mealybugs and a large number of mites. Its application is currently listed as an ecological product, with formulations that have between 40% and 50% concentration in fatty acids.

We will tell you how to use it, what application dose and what is the most effective way to reduce the presence of pests in your crops using this product.


Potassium soap is born from the combination of fats of vegetable origin mixed with an alkali (product with a very high pH) to achieve solubilization. Various commonly used alternatives can be used as vegetable fat, such as olive oil, coconut, soybean, sunflower, corn, palm, canola, etc.

As these types of fats are extremely unctuous and sticky, when mixed with a solubilizing substance, such as potassium hydroxide , a completely liquid final product is obtained and suitable for foliar application. This mixing reaction is known as saponification .

As you already know, any type of soap has the ability to clean any type of grease when it is in the presence of water, due to the peculiar structure of its molecules. These, on the one hand, have a fat-soluble part and on the other a water-soluble part. The fat-soluble paret makes the soap mix with the fat, dissolving it, and the water-soluble component makes the soap dissolve in turn in the water.

Therefore, potassium soap is a very interesting compound for washing stained surfaces, such as the leaves of a plant when there is production of molasses and sugars due to the presence of sucking insects.


If we concentrate potassium soap in large concentrations and with enough foliar broth, it can have a softening effect on the cuticle in soft-bodied insects, also causing suffocation by obstructing the pest’s airways.

This product, applied several times on a crop with an alternation of 3 to 5 days, is able to completely clean pests that are difficult to eliminate such as aphids , thrips, whiteflies , mites (red spider) or mealybugs .

However, the only tool we need is patience, since it is very difficult to achieve successful treatment with a single application. In the intensive horticulture of Almería, very limited by the use of phytosanitary products and with a maximum value of residues, the use of different types of soaps is widely used, in intensive treatments maximum every 5 days to reduce or control the advance of the fly white .

A well-known product is the famous Fairy or dish soap. This detergent is also widely used and is perfectly valid for use in our garden. However, in our opinion it should only be used on ornamental plants, as it may contain quaternary ammonium in sufficient quantities to cause a food alert.

This, however, does not happen with potassium soap, at least if the provenance or manufacturer is professionally guaranteed.

Castalia – Black Soap, 1 Liter

  • Made with natural products
  • Castalia brand product
  • The best product for the care and well-being of your body


One of the most famous uses of potassium soap or phosphoric soap is against all types of aphids. However, there are so many classes and morphology that it doesn’t work equally well for all species. For the green aphid, the application of this product based on fatty acids reduces its mobility and causes death within a few hours, as long as enough soap has been applied.

Therefore, it is important to carry out the foliar treatment with sufficient pressure and aiming directly at the focus of the pest. In addition, repetitions of the treatment must be carried out to guarantee that the number of eggs, nymphal stages and adults that have not been affected in previous applications is reduced.

  • 50% potassium soap: 250-500 ml / hl (every 100 liters of water).
  • 20% potassium soap: 1000-2000 ml / hl (every 100 liters of water).


The biggest drawback the whitefly has is its ability to fly when it detects changes in humidity or movement. For this reason, when the foliar treatment is started, most of the adults fly away and land once the application is finished.

For this reason, it is very important to constantly and programmed fight against this insect, so that treatments with all kinds of soaps must be repeated every 3-5 days. In this way, we slowly reduce the population, cleaning the leaves of molasses produced (which would cause the entry of sooty mold or bold fungus) and reducing the level of egg laying.

  • 50% potassium soap: 250-500 ml / hl (every 100 liters of water).
  • 20% potassium soap: 1000-2000 ml / hl (every 100 liters of water).


The spider mite usually affects plants in conditions of low humidity and high heat, so it is common for some of our plants to have chlorotic spots in the form of points or even cobwebs with the entry of spring and during summer.

One of the biggest enemies of this mite is humidity, so the single foliar application of water on the plant reduces the activity of the pest. The same happens with sulfur , which also irritates the pest and drives it away.

The mobility of the spider is reduced, and both the adults and the eggs are found on the underside of the leaf, that is, the opposite part. Therefore, when we apply potassium soap on the plant, we must treat with enough pressure and mobility to guarantee that the underside of the leaf also gets wet.

The use of sulfur-based products is also effective, so both products can be combined to achieve a synergistic effect.

  • 50% potassium soap: 250-500 ml / hl (every 100 liters of water).
  • 20% potassium soap: 1000-2000 ml / hl (every 100 liters of water).


The mealybug and all the species that make up the coctelids are a real headache for a large number of pests. They are difficult to eliminate since some have a hardened exoskeleton (grooved scale) and others have protective means so that they are not affected so much by the foliar treatment (cottony scale).

Therefore, the application of potassium soap will help us clean the plant of molasses and sugary residues, increasing its capacity to carry out photosynthesis and reducing the progression of the disease.

Against the cochineal, in addition to the use of potassium soap, it is interesting to use a pressure volume high enough to get rid of the pest from the stems, since they are constantly sucking sap and cause the general weakening of the plant.

For this pest, it is interesting to use the active principle azadirachtin, known as neem oil . It is a mixture of various acids and oils such as palmitic, linoleic, stearic, etc.

  • 50% potassium soap: 250-500 ml / hl (every 100 liters of water).
  • 20% potassium soap: 1000-2000 ml / hl (every 100 liters of water).

In front of the mealybug, use a sufficient volume of broth until the leaves drip.


The most famous brand that currently markets potassium soap is Castalia . We can find different formats depending on the size of our garden and the volume of plants. From 250 ml (for about 25 liters of water), 1 L (for about 100 L of water) or 5L (500 liters of water).


Potassium soap and other kinds of soaps have been homemade for many years. The main problem in making potassium base is that it is an industrial product that is not very accessible to everyone, and it is certainly dangerous due to its high pH (14).

In any case, if we have the materials, the preparation of these fatty acids is carried out as follows.

For its preparation, the mixture of alkali (potassium hydroxide or, failing that, sodium hydroxide) and fatty acids (soy, corn, olive, palm, etc.) is required in a 1 to 1 ratio. That is, 1 kg of fatty acids and 1 kg of potassium hydroxide.

By mixing both components, a sticky mass with a very high pH, ​​but somewhat soluble, is achieved. With this mixture we would have a potassium soap at a concentration of 50%. From here, if we dilute it in alcohols or distilled water, we reduce the concentration of the soap but increase its solubility.

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