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Why Wetting Agents Are an Asset for Potted Plants

lia marchi werle headshot
Lia Marchi Werle,
Exacto Field Development Manager

Have you ever purchased a potted plant and noticed that after a few rounds of watering it, the water was going straight down to the drainage holes? That’s a classic case of water repellency or hydrophobicity exhibited by the growing media. The good news is that soil surfactants, or wetting agents will solve this problem.

Potting soil is a type of growing medium used in nurseries and horticulture for planting and growing plants. It is a mixture of various organic and inorganic materials such as peat moss, perlite, vermiculite, and compost. Once potting soil is dried out, it can easily become hydrophobic.

In the context of growing media, like potting soil, hydrophobicity can be a problem as it can prevent water from being absorbed by the soil or media, leading to dry spots and uneven watering. It can cause issues in nurseries as it will be more challenging to maintain plant quality, but even more so for consumers where plant maintenance and watering might not always be on top of their minds.

Wetting agents can help to overcome this problem by improving water absorption and distribution in the soil or media.

What are Wetting Agents and What’s Their Role in Nurseries?

While wetting agents were first invented in the 1950’s for turf use, they have been adapted for agriculture and ornamental use to improve water absorption in soil and growing media. Wetting agents are often pre-mixed into the potting media and fertilizers and function by reducing water’s surface tension, enabling greater uniformity in moisture distribution. By doing so, the use of wetting agents results in greater root development, healthier and more uniform plants, and better water drainage. Ultimately, wetting agents optimize irrigation applications, allowing for efficient water use.

Water alone has a high surface tension and beads up on a surface. Adding a surfactant decreases the surface tension, improving the water’s ability to spread and wet a surface.

The value of wetting agents could be compared to the value of a crop insurance policy. Ensuring that an extensive number of potted plants in a nursery facility receive adequate and uniform moisture is key for reducing the loss of seedlings in early stages of development. Because all commonly used potting media (e.g., coir fiber, pine bark, peat moss and compost) exbibit some degree of hydrophobicity, irrigation events will become less efficient over time and fail to spread thoroughly unless soil surfactants are used. The next challenge is during transport and commercialization. Once these plants leave the growing facilities, it’s crucial that moisture is well kept until the next round of irrigation, which will typically occur at local distributors or big box stores. Therefore, nursery companies rely on wetting agents added to the potting mix to assist with the most important step of the production process, the sale of their goods.

In terms of chemical classification, surfactants have different structures and therefore different modes of action based on how they interact with soil (Zontek and Kostika, 2012). In a broader view, soil surfactants can be divided into three categories: anionic and blends with anionic, nonionic, and cationic surfactants. The nonionic chemistries based on polyoxyethylene (POE) came first, but lost popularity due to the phytotoxicity on fine turf in some situations. Block co-polymers, another subcategory of nonionic surfactants, encompass the most-used soil surfactants in today’s market and are well known for their effectiveness at reducing soil water hydrophobicity and improving volumetric water content.

At Exacto, we routinely conduct a variety of studies that begin in the lab by performing formulation stress tests. Once good candidates for surfactant formulations are identified, they are then tested on experimental turf fields and ornamental plants growing in nurseries. Here are a few highlights of a study conducted by the University of Florida with our premier block co-polymer wetting agent, Lateral.

Philodendron plants treated with Exacto wetting agent

Figure 1: Philodendron plants growing peat media treated with Exacto’s wetting agent. Yellowing is due to the high intensity grow lights. No phytotoxicity observed with the wetting agents.

dry down curve of peat media over a 7 day-period

Figure 2: Dry down curve of peat media over a 7 day-period. Plants were saturated to capacity (day 0) and allowed to drain. Daily weight was recorded to calculate water loss. University of Florida (2018) using potted philodendron as a model.

The wetting agent was pre-mixed with peat-based potting media, used to grow philodendron (Figure 1). Pots were saturated to capacity and daily weight was recorded to determine loss of water (Figure 2). The difference in pot weight is remarkable at day 1, where Lateral-treated pots were nearly 50% heavier than the untreated, despite receiving the same amount of water. This indicates that water was likely leaching out of the untreated pots and not wetting the potting mix.

Figure 3: Daily volumetric water content (VMC %) measured after a re-watering event (day 1) philodendron potted in peat media. Containers were allowed to drain and VWC was measured during the dry down period.

While untreated media maintained the same low water amount for the first four days, treated media weight remained elevated, an indicator that philodendron plants had greater water availability with Lateral. By taking measurements with a moisture probe after re-wetting peat, we can check the volume of water ratio based on volume of dry peat, the volumetric water content (Figure 3).

Figure 4: Change in volumetric water content (%) of peat media measured weekly. Plants were watered to capacity and monitored for 8 weeks. Study initiated on week 1, peat was treated with Lateral whereas untreated pots received only water. Plants received 100 ml of water 3 times a week.

Lateral-treated pots had greater VWC than untreated after re-watering to saturation, and trend continued as dry down progressed over time. By repeating this test over a longer time period, the difference in VWC for peat treated with Lateral against untreated peat was as high as 20% (Figure 4). At the end, these differences mean greater uniformity in plant watering, more efficient irrigation cycles and ultimately healthier roots that will result in healthier plants.

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