

Hydrogels: Are They Safe? - January 14, 2009

Jeff Schalau, Associate Agent, Agriculture & Natural Resources

University of Arizona Cooperative Extension, Yavapai County



Some gardeners use products called “hydrogels” to increase soil water holding capacity and reduce irrigation frequency. Hydrogels are sometimes referred to as “root watering crystals” or “water retention granules” because they swell like sponges to several times their original size when they come into contact with freely available water. These products have been used successfully by the landscape industry to reduce transplant shock and increase containerized plant growth. However, some people are concerned about the safety of hydrogels, so I thought I’d present a little of the scientific information about their efficacy and safety.



There are two broad classes of polyacrylamide (PAM) hydrogels: soluble (linear) and insoluble (cross-linked). Linear PAM dissolves in water and has been successfully used in reducing irrigation-induced erosion in agricultural fields. Cross-linked PAM does not dissolve, but forms a gel when water is added and is often used in garden, landscape, and nursery situations as a way of retaining moisture. Insoluble PAM products are marketed as “superabsorbent gels” or “hydrating crystals.” Instead of dissolving, these gels absorb water, swelling to many times their original size. As they dry, water is slowly released to the soil. Cross-linked PAM products are the focus of this column.



How PAM gels will act in any given situation can be hard to predict, as the chemical interactions between the gels, soil components, and dissolved substances are complex and occur simultaneously. Without getting too scientific, specific soils have unique combinations of physical and chemical properties. Some soil factors include electrical charges, water holding capacity, and forces that cause individual molecules to attract or repel each other (called van der Waals forces). These characteristics modify the affinity of the gel for other compounds. PAM gels also contain a complex array of positively charged, negatively charged, and neutral chain segments, all with varying affinities for other molecules.



The stronger the attraction between the gel and surrounding soil minerals, nutrients and/or salts, the greater the ability of the gel to absorb water, create aggregates, and stabilize soil structure. Unfortunately, on-the-ground conditions can prevent PAM hydrogels from functioning optimally. Fertilizers and other dissolved substances can interfere with hydrogel water-holding capacity. Hot, dry weather conditions can lead to increased degradation and decreased effectiveness of PAM hydrogels. And for every success story, researchers have found situations where hydrogels have failed to function.



The documented impacts of cross-linked PAM hydrogels on plant survival and establishment are variable. Some researchers report enhanced growth of crop and tree species. According to other researchers, however, PAM hydrogels did not improve plant survival compared to control or other treatments, especially if performance was evaluated over time. In several cases, PAM-treated plants performed worse than the untreated controls and exhibited measureable nutrient deficiencies. PAM gels will also break down over time because of naturally occurring soil microbial activity.



Aside from these variable results, some most gardeners are concerned with the safety of PAM hydrogels. As the name suggests, polyacrylamides consist of many linked acrylamide subunits. Acrylamide is a known neurotoxin in humans and is suspected to be carcinogenic as well. During the manufacture of PAM gels, residual acrylamide is present as a contaminant and strictly regulated in the United States to levels no more than 0.05% or 500 ppm for agricultural use. However, an international study recommended that polyacrylamide gels used in cosmetics contain a residual monomer level of only 0.1 to 0.5 ppm. Therefore, the PAM hydrogels manufactured for agricultural and garden use can contain much greater concentrations of (1,000 to 5,000 times) toxic acrylamide than that found in personal products causing concern among some users.



Additional health issues can be presented by exposure to the more or less intact polyacrylamide gel where toxic effects have documented. Health risks associated with the breakdown products of PAM hydrogels are entirely unknown, but exposure risk could be great to gardeners and green industry workers that are exposed over time.



Given the findings presented above, I cannot recommend the use PAM hydrogels for use in home gardens. There are several alternative cultural practices that are safer and equally effective. These include mulching, periodic addition of compost, wind protection, etc. For those wanting more scientific information about risks associated with PAM hydrogels, I have included several links below.



The University of Arizona Cooperative Extension has publications and information on gardening and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at cottonwoodmg@yahoo.com and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://cals.arizona.edu/yavapai/anr/hort/byg/.



Super-absorbent water crystals – are they really so “super?” by Linda Chalker-Scott, Ph.D., Extension Horticulturist and Associate Professor



The Myth of Polyacrylamide Hydrogels: "Polyacrylamide hydrogels are environmentally safe substances that reduce irrigation needs" by Linda Chalker-Scott, Ph.D., Extension Horticulturist and Associate Professor



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