Surfactant and Gel in the Same Pore: What Each One Takes

A grower with 40,000 two-litre pots added a soil wetting agent to his peat and bark mix in March, on the advice of a substrate supplier, and kept his hydrogel rate unchanged. By June the pots were drying out faster than the previous season, and the gel, when he dug into a pot, was sitting in a shrunken lump at the base of the root ball. Two products, each doing what it says on the label, working against each other in the same 2 litres of media.

Surfactants and polymers are both sold as answers to the same problem, which is water that will not go where it is needed. They solve it by opposite mechanisms, and the interaction is not well documented on either label.

What a surfactant does to a gel granule

A wetting agent lowers surface tension so water enters dry, hydrophobic media instead of beading on the surface. A hydrogel granule, once dry, has exactly the same problem: its surface can become hydrophobic after repeated drying, and water runs off it. A surfactant helps re-wetting, which is why some growers add one specifically to revive dried gel.

But the same surfactant reduces the cohesion of the swollen gel. The polymer network is held together by physical entanglement and ionic crosslinks, and a surfactant that changes the interfacial energy at the water-polymer boundary makes the gel softer, more prone to syneresis, and more likely to slump under its own weight. In a pot, a slumped gel releases water into the media, where it drains. In a coarse bark mix, that water is gone within a day.

At label rates the effect is modest. At double rate, which is what happens when a grower adds a wetting agent to a mix that already contains one, it is not modest at all.

The case where the wetting agent was the whole fix

The 40,000-pot grower had two problems and treated them as one. His media was hydrophobic after three weeks in a heated greenhouse, so irrigation water channelled down the sides of the pot and left the centre dry. The gel was not failing; it was never wetted in the first place. A wetting agent fixed the channelling, and the correct response was to keep the gel rate and fix the irrigation practice, or to drop the gel by 20 percent and accept that re-wetting was now handled.

What he actually did was add the surfactant and keep everything else, which produced a media that wet easily, drained fast, and held less. The pots looked drier because they were drier in the root zone, and the gel looked useless because it had been pushed to the bottom by the extra water moving through.

Three combinations that work

The general rule: decide which problem you have before buying either product. If water runs off the surface or down the pot wall, you have a wetting problem and the fix is a surfactant. If water drains through and the profile is dry two days later, you have a retention problem and the fix is polymer. Buying both at full rate is what happens when nobody diagnoses which one it is.

Mixing order, if you use both

Never pre-mix surfactant into a gel stock solution. The concentrate will foam, the gel will not disperse evenly, and the mixture has a short shelf life. Add the surfactant to the irrigation water or the media, and add the gel dry into the mix as you would normally. Keep at least one irrigation cycle between the two if you are applying through a drip line, and expect the gel to need a longer wetting-in period, because the surfactant will have changed the infiltration rate.

If pots still dry unevenly after both are in, weigh ten pots at saturation and again at 48 hours. A spread of more than 12 percent between pots means the mixing is uneven, not that the chemistry is wrong. The pot trial method and the recommended rates for our potassium polyacrylate gel assume no surfactant in the mix, so adjust before you scale up.