Getting to Know Compatibility Agents

Written by Donald Lester – November 2012

When Donald Lester mentions compatibility agents, his family immediately thinks of unscrupulous dating services. So, he decided to write this article in order to clear up the misconception—not only for his relatives but for anyone else who might be confused.

In a previous article, I discussed the importance of avoiding chemical incompatibilities, especially when mixing pesticides, fertilizers and other chemicals in your nutrient solution tanks. All sorts of undesirable things can happen—everything from common and costly precipitation (settling out of products due to a chemical reaction) to rare explosions and even fires. Even if there are no outward signs of serious problems, an incompatibility can weaken the mixture and greatly reduce the desired effect.

Typically, growers are urged to take small amounts of the products they want to combine, mix them in a small glass jar and look for the tell-tale signs of incompatibility: precipitation, cloudiness in the solution or heating up of the mixture. (It is important to use a glass jar because metal containers can react with many kinds of chemicals.) Some people avoid this simple test and “cookbook” their mixtures at full scale, but this is the kind of scenario that leads to the most serious and most expensive incompatibility problems.

Often, the term compatibility agent is used to refer to an additive that can facilitate a successful mixture. In this article, I will discuss some of the more common compatibility agents and how they can help you to avoid the problems of chemical incompatibility and the subsequent undesired results. Keep in mind, however, compatibility agents do have a legitimate place in greenhouse and hydroponics tank mixes, but they are not a fix for bad mixing decisions. They are also not a magic additive to make impossible mixtures work. For the most part, the old saying “you cannot un-fry an egg” best describes what you can do once a chemical incompatibility happens; it’s not pretty.

In regulatory terms, compatibility agents are referred to as adjuvants. Adjuvants are defined in the horticultural world as any material added to a pesticide—including insecticides, fungicides, herbicides, and even some fertilizers—to enhance its effects. Some pesticide manufacturers add adjuvants to their formulas during the manufacturing process. These adjuvants are referred to as “formulation adjuvants,” whereas adjuvants added by the applicator are called “spray adjuvants.” The United States Environmental Protection Agency (EPA) does not regulate adjuvants. Likewise, most states do not regulate them either, although adjuvants are regulated as pesticides in California.

There are several types of adjuvants: spreaders (wetting agents), stickers (deposition agents), buffering and conditioning agents (pH reducers, water conditioners), drift control agents (foaming agents), defoaming agents, thickening agents and UV absorbents. It is important to note that not all of these additives can be combined into one tank mix and no one product can perform the function of all adjuvants. It is easy to get lost in the terms, so I will discuss the similarities and differences between the different kinds of adjuvants below (however, since foaming and defoaming agents are not often used indoors, I will not cover these additives).

Spreaders (wetting agents) and stickers (deposition agents)

In chemistry, a wetting agent is any substance added to a liquid to lower its surface tension— the force that allows insects to stand on the water—and thus increase its ability to spread across or penetrate into a solid. In other words, spreaders allow a water droplet to spread into a film over a surface. As a college student, I witnessed a demonstration of this phenomenon: the addition of a wetting agent to a pond broke the water’s surface tension, and mosquitoes literally sunk and drowned right before your eyes. Typically, spreaders are specialized detergents or soaps. As such, never add a household detergent or soap to a spray mixture; always go by the pesticide label recommendations. There are conventional and organic versions of wetting agents. The most popular organic wetting agents are yucca-based products, which are natural, biodegradable and leave no toxic residues.

Stickers, as the name implies, make a spray solution stick to the leaf surface in order to reduce the amount that washes off during irrigation or rain.

Foliar-applied nutrients and pesticides need to stick to a plant’s surface and enter the plant to do their work. But some plants, like grasses, have a small surface area, so droplets of spray have a hard time sticking to the leaves. Also, the size of the droplet produced by the sprayer might be too large. Smaller droplets adhere better to leaf surfaces and improve coverage, but there is more chance for spray drift with smaller droplet sizes. Compounding the problem is the fact that some plants have an extra waxy layer or excessive hairiness on the leaves that repel water-based sprays. This is where spreaders and stickers, which are often used together in tank mixes, come in. Be aware that applying too much spreader can cause the drops to roll off the leaves (the opposite of what you want), so always follow the pesticide label and use recommended dosages. Also, just as spreaders and stickers increase the absorption of materials into plants, spreaders and stickers can also increase the absorption of pesticides into the skin! So, when using a spreader and/or sticker and pesticide together, be extra careful to cover up and protect your skin to minimize exposure.

Buffering and conditioning agents (pH reducers, water conditioners)

When mixing chemicals with water, it is always important to know the pH and hardness of the water before adding anything to the tank. Most pesticides are acidic in nature (pH 5.5 to 7.0). Pesticides exposed to high alkaline water (above pH 7.0) degrade quickly and lose effectiveness or even break down completely. Often in hydroponics systems, the pH of the tank solution tends to climb into the basic (alkaline) range over time, so growers add a pH reducer-type material to offset that reaction before the addition of a pesticide. This will adjust the pH to the desired range, but it might not hold it there. A pH buffer will hold the pH in the desired range, and some pesticide manufacturers add buffers to help stabilize their products in a narrow range.

Also, hard water usually has high levels of calcium and magnesium that react with pesticides. Always check the pesticide label for recommendations for hard water conditions.


A plant will absorb pesticides and nutrients through the leaves as long as the solution stays wet. If it dries, the spray will remain on the leaves until rain, dew, or irrigation wets it again. This stop-and-start absorption action can lead to the spray being absorbed too slowly to be effective. Thickeners increase the viscosity (or thickness) of the spray solution to slow evaporation (volatilization) and keep the solution on plant surfaces for a longer period of time in order to increase absorption.

UV absorbents

Ultraviolet light is damaging to many pesticides. Just as we use sunscreen to protect our skin from the damaging rays of the sun, pesticide manufacturers add special absorbents to protect the integrity of their product.

Now that you know the difference between a sticker and a water conditioner, you can be more confident in selecting the right spray compatibility agents for use in your indoor tank mixes. Plus, you can increase the effectiveness of your spray applications and save yourself an afternoon of cleaning out unwanted precipitation sediment from the bottom of your tank.

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