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What Are Wetting Agents in Potting Soil?

What are wetting agents listed in many of the major branded potting mixes?

Wetting agents combine surfactants, humectants, and emulsifiers to help the potting soil absorb and then hold onto the water so it doesn’t dry out as quickly. I will discuss each of these generic chemical compounds in more detail below.

do all potting mixes have wetting agents?

Do All Potting Mixes or Potting Soils Have Wetting Agents?

No, not all of them do. I will be compiling a list of potting soils or potting mixes that are used for indoor gardening and will be listing the ingredients shown on the product labels.

When the article is published I will link it here.

What are Surfactants in Potting Mix?

Surfactants are molecules with both a hydrophilic (water-loving) and a hydrophobic (water-repelling) end. Basically they are polar molecules as is water.  This is why wetting agents are used in potting mixes that contain an organic ingredient that repels water when dry, such as peat moss.

As polar molecules they are able to attach themselves to the surface of water molecules. This reduces the surface tension of the water, making it easier for the water to penetrate and saturate the soil.

Once attached to water molecule, the polar molecule property now makes it more difficult for the water molecule to be affected by gravity and drain out of the soil, or be drawn upwards by dryer air and evaporate. It’s like a two-for-one deal. I gotta-cha and am not going to let go of you.

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Surfactants are added to potting mixes to help the water spread more evenly throughout the soil. This makes it easier for the plants to absorb the water and helps to prevent the soil from becoming waterlogged.

Many different surfactants can be found in potting mixes. Some of the most common include the following.

If you aren’t sure what a potting mix is, here is an article explaining why it is called potting mix and not potting soil.

Polyacrylamides

These are synthetic surfactants that are very effective at reducing the surface tension of water. They are also relatively inexpensive, which makes them a popular choice for commercial potting mixes.

The most common polyacrylamide in potting mixes is specifically anionic polyacrylamide (APAM). APAM is generally considered to be safe for humans and animals when used in the recommended concentrations. 

surfactant in potting soil

Yucca Extract

This is a natural surfactant that is derived from the yucca plant. It is less effective than polyacrylamides at reducing the surface tension of water, but it is also less likely to harm plants.

The compound in yucca used as a surfactant is saponin. Saponins are generally considered to be safe for humans. Safe enough to be used in anti-cancer treatment. However, if you think about it, treating cancer means saponins play a part in killing cells. 

yucca root produces saponins

Sodium lauryl sulfate (SLS)

This synthetic surfactant is often used in personal care products, such as shampoos and soaps. It is also effective at reducing the surface tension of water and is sometimes used in potting mixes.

SLS is generally considered to be safe for humans when used in small amounts. Some people are concerned about the potential long-term health effects of SLS.

There is some evidence that SLS can damage the skin barrier, which can make the skin more susceptible to infection and linked to cancer. However, this paper suggests that SLS is safe for cleaning or non-consumable products.

surfactant in potting mix

Triton X-100

This synthetic surfactant is often used in laboratory research. It is also effective at reducing the surface tension of water and is sometimes used in potting mixes, but rarely because of its cost.

Interestingly, to me, anyway, Triton X-100 will dissolve in both water and oil. Triton X-100 is generally considered to be safe for humans when used in laboratory settings. However, it can be harmful if it is ingested or inhaled in large amounts.

Polysorbate 80 (Tween 80)

This is another synthetic surfactant that is often used in food and pharmaceutical products. It is also effective at reducing the surface tension of water and is sometimes used in potting mixes. In fact, Tween80 is often used in the following food products.

  • Ice cream: Tween 80 is used in ice cream to prevent the formation of ice crystals. It does this by binding to the water molecules in the ice cream and preventing them from forming large crystals.
  • Chocolate: Tween 80 is used in chocolate to make it smooth and creamy. It does this by preventing the cocoa butter from separating from the chocolate.
  • Salad dressing: Tween 80 is used in salad dressing to emulsify the oil and vinegar. It does this by forming micelles, which are tiny bubbles that trap the oil and vinegar.
surfactant used in potting mixes

What Are Humectants in Potting Mix?

Humectants are substances that draw and hold water molecules to themselves. This helps to keep the soil moist for longer periods of time. 

Potting soil humectants can be derived from both natural and synthetic sources. Some common natural humectants used in potting soil include:

Glycerin

This is a clear, colorless liquid that is derived from vegetable oils or animal fats. It is a very effective humectant and helps to keep the soil moist.

glycerin is a humectant used in potting mixes

Honey

This is a sweet, viscous liquid that is produced by honeybees. It is a natural humectant and helps to keep the soil moist. Of course it is processed and used in very small quantities.

honey can be used as a humectant in potting mixes

Aloe vera

This is a succulent plant that is native to Africa. The gel from the aloe vera leaf is a natural humectant and helps to keep the soil moist.

aloe vera can be used as a humectant in potting mixes

Hyaluronic acid (HA)

This is a naturally occurring substance that is found in the skin, joints, and eyes. It is a very effective humectant and helps to keep the soil moist.

HA is a glycosaminoglycan, which is a type of sugar molecule. It comprises repeating units of sugars called glucosamine and glucuronic acid. HA is a very large molecule that can hold a lot of water.

HA is derived from bacteria or animal cells grown in a culture medium. The culture medium is then extracted, and the HA is purified. It can also be produced synthetically by chemically combining glucosamine and glucuronic acid.

a natural humectant used in potting mixes

Below are some of the common synthetic humectants used in potting mixes.

Propylene glycol (PG)

This is a clear, colorless liquid that is derived from petroleum. It is a very effective humectant and helps to keep the soil moist.

PG is generally considered to be safe for humans when used in food and cosmetics

propylene glycol is used in wetting agents

PEG-40

This is a clear, colorless liquid that is derived from castor oil. It is a very effective humectant and helps to keep the soil moist.

PEG-40 is generally considered to be safe for humans when used in small amounts.

peg-40 is used in potting soils

Butylene glycol (BG): This is a clear, colorless liquid derived from petroleum in a multi-step ethylene oligomerization process. It is a very effective humectant and helps to keep the soil moist.

BG is generally considered to be safe for humans when used in small amounts.

In addition to these natural and synthetic humectants, some potting soils also contain clay. Clay is a natural substance that helps to retain water in the soil.

What are Emulsifiers in Potting Mix?

Emulsifiers are substances that help to keep two immiscible liquids (such as water and oil) from separating. This is important for wetting agents because it allows them to be mixed with water and evenly distributed throughout the potting mix.

The common emulsifiers found in potting mixes include the following and have been discussed previously in this article and serve multiple purposes in potting soil wetting agents.

  • Polyacrylamides
  • Yucca extract
  • Sodium lauryl sulfate (SLS)
  • Tween 80

The other emulsifier added to the potting mix is naturally derived Humic acid.

This is a natural humectant that is found in soil. It helps to keep the soil moist by attracting and holding water molecules.

Humic acids are a complex mixture of many different acids containing carboxyl and phenolate groups so that the mixture behaves functionally as a dibasic acid, tribasic acid, or fulvic acid.

Humic acids are important to plants because they form chelate complexes and help regulate the bioavailability of metal ions.

Are Wetting Agents Harmful? Do Vegetables Uptake the Chemicals?

I will take closer look at that in this article.

Wouldn’t It Be Nice for Better Labels?

Most manufacturers or brands do not list the exact compounds found in the wetting agents they add to their potting mixes. 

I expect the governing agencies and governments are not requiring manufacturers to list the compounds in wetting agents.

And let’s face it, wetting agent sounds much better than sodium lauryl sulfate on the label, even if SLS is perfectly safe.

Think of polysorbate 80 or Tween 80 the next time you eat ice cream, or add a bought salad dressing to your next salad. 

Knowing the nitty-gritty details doesn’t make some things better.

Protective Measures When Using Potting Mixes with Wetting Agents

Here are the PPE and other protective measures to follow when using any potting mix, regardless if the mix has wetting agents in it or not.  Especially if the potting soil does have wetting agents.

  • Wear gloves and a mask: Potting mix can contain harmful bacteria, fungi, or chemicals, so it is important to wear gloves and a mask when handling it. This will help to protect your skin and lungs from exposure to these harmful substances.
  • Open the bag of potting mix in a well-ventilated area: Potting mix can release dust particles into the air, irritating your eyes, nose, and throat. It is important to open the bag of potting mix in a well-ventilated area to avoid breathing in these dust particles.
  • Keep the dust down. Slow, steady movements will stir up fewer dust particles than quickly throwing soil into the trays.
  • Wash your hands thoroughly after handling the potting mix: Even if you are wearing gloves, it is important to wash your hands thoroughly after handling potting mix. 
  • Keep potting mix out of reach of children and pets: Kids and pets can think potting mix is good fun to play in, and they don’t have the common sense yet not to throw it around or even eat it. 
  • Avoid getting potting mix in your eyes: If you do get it in your eyes, flush them immediately with water for at least 15 minutes. If you experience any pain or irritation, seek medical attention immediately.

By taking these protective measures, you can help avoid the risks associated with potting mix.

Stock images by Depositphotos

Author

  • Todd

    Todd is the founder of Home Microgreens & the Home Microgreens store. He also writes for several other websites, including MyViewFromTheWoods.com. Todd worked at a large farm market, garden & nursery center for 20 years. Somehow he snuck off to become a geologist and professor before coming back to his senses to write & lecture about microgreens and gardening. When not at the computer, he can be found in the garden, trout stream, or mountain trail with his new Springer Spaniel Caden.

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