SCIENCE TO THE GROWER: Hydrogels do not decrease water use in container plant production

by Richard Evans

When drought strikes California, ornamental crop producers are urged to incorporate water-absorbing polymers (hydrogels) into container media. It’s as sure to happen as the appearance of yard cleanup service ads on my porch whenever I neglect my garden. I should use those ads to mulch my flower beds 4 inches deep and suppress the weeds, but I’m too lazy. But enough about me. Let’s talk about hydrogels.

The typical claim is that these polymers, which can hold several hundred times their weight in water, increase the water-holding capacity of container media and decrease crop water requirements. These claims are backed by some reports indicating that hydrogel addition increases time to wilt, reduces required irrigation frequency and decreases plant water use. However, there are many reports that hydrogel incorporation has little or no effect. In this article I will review what we know about how hydrogels behave in container media.

The attraction of hydrogels comes from their ability to hold astounding amounts of water, and the idea that adding them to a potting mix would increase water retention. The usual way of increasing water retention is to use a finer mix that has smaller pores, because water retention in pots is inversely proportional to the average pore size of the medium: small pores tend to hold water, and large pores don’t. Most commercial container media have pore sizes that enable them to hold a lot of water, typically between 50% to 80% by volume. Only media with very coarse pores might be expected to benefit from hydrogel addition.

Unfortunately, the ability of hydrogels to hold water isn’t as impressive after incorporation into a typical potting mix. Research in my lab (Evans and others 1989) and at UC Riverside (Letey and others 1992) showed that addition of a polyacrylamide gel to a potting mix at the recommended rate had no effect on water retention. Both studies reported that doubling the rate of addition increased water retention slightly, but, since plant water use was unchanged, water was not conserved.

Why don’t the hydrogels retain water? We found that polyacrylamide gels exposed to typical concentrations of fertilizer solutions hold as little as 10% of their maximum water-holding capacity (Bowman and others 1990). We think that the positively charged ions in the fertilizer solution—especially multivalent ions of calcium, magnesium and iron—interfere with the ability of hydrogels to attract water molecules, and constrain hydrogel expansion. We also showed that the inhibition of gel hydration by fertilizers can be partially reversed by applying a solution of a potassium salt, followed by repeated leaching with water (Bowman and Evans 1991). As you can imagine, that exercise would not be practical in commercial horticulture.

The picture for hydrogels hasn’t changed much since then. Fonteno and Bilderback (1993) reported that hydrogel addition to an unfertilized, coarse mix of pine bark and sand increased water holding capacity slightly, but at the expense of lower air-filled porosity. They noted, however, that the small increase in water retention may not benefit plants because of an “oasis effect,” whereby the water held by gel particles may not be hydraulically connected to roots and therefore not available to the plant. More recently, Green and others (2004) confirmed the earlier findings and reported that the response to fertilizer salts limits the usefulness of hydrogels in field soils, too.

The most important point regarding hydrogel use during droughts is that they do not decrease the water requirement of crops. If irrigation is managed properly, the total amount of water applied will be the same in hydrogel-treated soil as in untreated soil.

Richard Evans is UC Cooperative Extension Environmental Horticulturist, Department of Plant Sciences, UC Davis.

References

Bowman DC, Evans RY. 1991. Calcium inhibition of polyacrylamide gel hydration is partially reversible by potassium. HortScience 26: 1063-1065.

Bowman DC, Evans RY, Paul JL. 1990. Fertilizer salts reduce hydration of polyacrylamide gels and affect physical properties of gel-amended container media. Journal of the American Society for Horticultural Science 115: 382-386.

Evans RY, Sisto I, Bowman DC. 1989. The effectiveness of hydrogels in container plant production is reduced by fertilizer salts. Flower & Nursery Report, Summer 1989. pp. 5-7.

Fonteno WC, Bilderback TE. 1993. Impact of hydrogel on physical properties of coarse-structured horticultural substrates. Journal of the American Society for Horticultural Science 118: 217-222.

Green CH, Foster C, Cardon GE, Butters GL, Brick M, Ogg B. 2004. Water release from cross-linked polyacrylamide. In: Proceedings of the Hydrology Days Conference, March 10–12, 2004, Colorado State University, Ft. Collins, CO. pp. 252–260.

Letey J, Clark PR, Amrhein C. 1992. Water-sorbing polymers do not conserve water. California Agriculture 46 (3): 9-10.