Gel Holds Water. Can Roots Actually Pull It Back Out?

Ask a supplier how much water their gel holds and you will get a number in the hundreds. Ask how much of that water a root can remove and the answers get vague, because the honest answer is: somewhere between 40 and 70 percent, depending on how tightly the polymer network holds it. The difference between those two figures is the difference between a product that carries a tree through a dry month and one that keeps water in the ground while the tree dies next to it.

This is the least discussed property of water retaining gel and the one that most affects whether it works. Absorption is easy to measure and easy to market. Availability is harder, and it is what the plant experiences.

Water potential, in plain terms

Water moves from wet to dry, or more precisely from high water potential to low. A root hair can pull water out of soil down to about minus 1.5 MPa before it gives up, which is roughly the permanent wilting point of a loam. Polymer holds water at a range of tensions: the water in the loose outer gel is easy to remove, the water bound inside a densely crosslinked network is not. A gel with very high absorption usually achieves it by loosening the network, which means it holds a lot of water at low tension and releases it readily. A tighter network holds less but gives it up over a wider range.

In practice both can work. What fails is the extreme: a very highly swelling product in a small container, where the gel competes with the roots for the same water and holds it at a tension the roots cannot overcome. We have seen a 420 g/g product underperform a 260 g/g product in 3-litre nursery pots, with the roots growing around the gel rather than into it.

The three factors that decide availability

Crosslink density is the first, and you cannot see it on a data sheet. The second is the salt load of the water and the soil: calcium and magnesium crosslink the polymer further and lock water in, which is why absorption measured in tap water is a better predictor than the deionised number. The third is mechanical confinement. Gel swollen inside a compacted clay matrix is compressed by the soil and cannot expand to take up water in the first place.

That third factor explains a result that surprises a lot of growers. In a loose, well-structured mix, gel performance tracks the label. In a dense, compacted mix, the same rate delivers half the benefit and the gel looks shrunken and hard when you dig it up, which gets reported as a bad batch. It is not the batch. It is the pore space.

A pot test that answers the question for your soil

You do not need a pressure plate apparatus to get a usable answer. Take six pots of your own mix, three with gel at the intended rate and three without. Saturate all six, let them drain for 48 hours, then weigh and stop watering. Weigh every second day and record the day each pot reaches the point where a test seedling flags in the evening and recovers by morning, which is the practical wilting point.

The difference in days between the treated and untreated pots is your real benefit. If the treated pots last four days longer, the gel is doing something. If they last one day longer, it is holding water but not releasing much of it, and no increase in rate will fix that, because the problem is the tension, not the volume.

What to specify instead of a big number

Ask for absorption in a defined salt solution, a crosslink or free-swell figure at a stated time, and a particle size range. Then run the pot test. If the supplier cannot tell you the swelling at 2 hours as well as at 24 hours, that is itself information: the 2-hour number is a decent proxy for how the granule behaves when a rain event lasts forty minutes.

A final number worth keeping in mind: in a loam at field capacity, a well-matched gel at 0.2 percent by dry weight typically extends the time to wilting by 30 to 50 percent in a pot trial, and much less in the field, where roots can grow away from the treated zone. Anyone promising a doubling should be asked for the trial data and the soil type.

Run the six-pot test on the next delivery and keep the weights on file. It takes three weeks and it tells you more about whether the product suits your mix than any certificate. The grades and swelling figures for our potassium polyacrylate SAP powders are listed with both the 2-hour and 24-hour values for this reason.