An adjuvant is an agent that alters the effect of other agents. Surfactants are the adjuvants most used in the home garden.

Water is an essential element in gardening, but water often doesn’t stick or penetrate as well as needed. To get water, or anything mixed with water, to stick or penetrate more effectively, and save money on garden chemicals, adjuvants called surfactants must be added.

surfactants change the surface tension of water

What Kind of Adjuvants are Oil and Soap?

There are several different types of agricultural adjuvants. Oil and soap act as surfactants. Surfactant is a blend of the words Surface Active Agent, and was coined by Antara products in 1950.

How Do Surfactants Work?

Surfactants change the surface tension of water, making it “wetter” or “stickier”. Surfactants influence the wetting and spreading properties of liquids, and change the precipitation, suspension, and dispersion of another substance in the water.

In layman’s terms, surfactants make whatever you mix with water disperse into the water and break down into smaller particles, so that it spreads better on the surface. When the liquid is sprayed onto a surface, the surfactant makes it “stick” to the surface.

Type of Surfactants

The three types of surfactants are nonionic (no electrical charge), anionic (negative charge) and cationic (positive charge). Soaps and oils are anionic surfactants.

How Oil and Soap are Used as Surfactants

There are two uses of surfactants in the home garden:

  1. To allow agents to stick to the surfaces of plants
  2. To make water stick to the soil.
  • If pesticides do not stick to the plant surface for a long enough period of time, the insects will not be affected. In this circumstance, oil and soap act as extenders, which are surfactants which increase the period of time a substance remains on a surface.
  • Soap breaks down the waxy coating on the bodies of some insects, allowing the insecticide to penetrate their hard outer shells. Oil seals the pores on the bodies of insects, causing suffocation.
  • Adding soap to liquid fertilizer when using foliar feed prevents the fertilizer from running off of the plant, and also allows what does run off to stick to the soil around and under the plant, increasing the availability of the fertilizer.
  • In soils that are very porous or sandy, where it is difficult to get the soil to hold water, soap added as a surfactant causes the water molecules to “stick” to the soil particles, thereby increasing the amount of water absorbed by the plant roots.
  • In soils with a high surface tension, where water stands on top and doesn’t readily absorb into the soil, soap breaks the surface tension, allowing the water to be absorbed.

water optimally absorbed by plants due to surfactants

Surfactant Usage Rates

When using commercial products, always follow label instructions to the letter. When using household products to make homemade pesticides, many recipes can be found online.

One good recipe is 1/3 cup Oil Soap (like Murphy’s), 1 tablespoon baking soda, and 1 gallon of water. The baking soda acts as a fungicide as well as changing the pH to a more alkaline mixture.

Cautions When Using Surfactants

While soap and oil sound relatively safe, there are some cautions you should use:

  • Never spray any mixture containing oil during the hot part of the day, as it could burn your plants. Always spray in the early morning or early evening.
  • Do not use laundry detergents, or grease cutting dishwashing detergents, because they contain additives that counteract the surfactant properties of the soap, or that may be harmful to plant life.

Using surfactants is green and thrifty. It is a simple way to make your gardening products more effective, while also saving money.