Garden Edition: June 2002
Water-saving products for the garden, Part II: Superabsorbers
by Linda Coyner
They go by various names‹ superabsorber, hydrogel, water-absorbing crystal. Technically, they're synthetic, cross-linked polymers that are widely used now forestry, gardening, and landscaping as a means of conserving water. When dry, they resemble coarse salt crystals, but they will absorb many times their weight in water, swelling into gelatinous globs that gradually release their moisture as the soil around them dries. Besides conserving water and a gardener's time, they reduce leaching of fertilizers and nutrients into ground water, a major benefit to the environment.
In the old days before superabsorbers, peat moss was the soil additive of choice to hold water, to the point where the demand was destroying the world's peat bogs. What gardener could have imagined that someday we'd have a product that could store water in the soil and then release it as the plant's roots need it?
There are many brandnames: Broadleaf P4, Soil Moist, Moisture Mizer, TerraSorb, Hydrosource, Watersave, Water Crystals, etc. Most recently there are potting mixes with moisture-holding polymer already blended into the mix such as Smart Soil and Moist Soil. (But save your money by buying regular potting soil and adding polymer.) These magic crystals are also used hats, bandannas, and headbands to absorb perspiration.
Uses for superabsorbers are many: as a soil amendment for container planters; in seed germination; as a dip for bareroot plants; as a transplant aid for turf, bedding plants and woody ornamentals; and as an in-soil water storage medium for turf. Horticulturists often recommend adding hydrogels to container plants, especially hanging baskets and window boxes. And testimonials from gardeners on the Internet abound.
Apparently there are two types of absorbent polymers in the trade. Both are synthetic polymers, either polyacrylamides or polyacrylates.
Polyacrylate is the active material in baby diapers This superabsorber is capable of absorbing 400-800 times amounts its mass, but rapidly breaks down. Polyacrylamide is the gardener's friend. It was developed in the 60's to grow plants in arid environments. Since then, it has been refined to last longer and absorb water at higher rates. It's estimated that one pound of crystals will absorb and hold from 15 to 40 gallons of water in the soil.
Polyacrylamide works in the soil for several years and then breaks down to carbon dioxide, water, nitrate and lactic acid, according to the Hydrosource Web site (http://www.conserving-water.com/docs/hydrosource.html). Another report says the crystals break down into water, carbon dioxide, and ammonia.
Long-term safety is an important question, but little has surfaced during the 30-plus years since the original development of the polymer that would give any special cause for environmental concern. Now that it's in widespread use, you can bet they're re-verifying its safety. One report indicated that superabsorbers are used in organic farming. In any case, care should be used not to inhale airborne particles of the powdered form. Use good sanitation practices (wash hands after use) and store away from children, food, and animals.
The lifespan of the product varies depending on which manufacturer's Web site you're reading. Sometimes the claims are vague, saying the product "will last for years." Water Crystals promises 12 or more years; Hydrosource, 12 to 15 years. My guess is that it's highly variable, depending on soil, hydration cycles, climate, etc.
Superabsorbers are unscathed by normal pH's encountered in soils, composts, and irrigation water. At pH levels of less the 5, however, the water absorbency is reduced. Also, the crystals in some cases may increase the effectiveness of the herbicide and fertilizers. So go easy on both. Salt content of soil and water represents one of the few potential threats to the crystals' longevity and may limit its usefulness near the coast. UV light will degrade hydrated crystals sitting on the surface of the ground, so keep the product buried in soil or mulch.
These products are manufactured as powder, dry granules of different sizes, or hydrated as a slurry. You can use the dry crystals right out of the package or hydrate them overnight. Even distribution of crystals in the soil is important as you don't want to create wet pockets. If you use the product dry, apply extra water for a few days until it's certain the product is hydrated. Dry crystals can actually take moisture from the plant until they are hydrated.
Lowes and Home Depot carry superabsorbers, and it's easy to buy them online. Fortunately the price is coming down. One pound of polymer crystals costs less than $20. Watersorb.com charges $15 plus shipping for a two-pound package, the smallest size it offers on its Web site.
While none of these products is cheap, note that it only takes a tiny bit to do the job. For instance, Watersorb recommends that you need 3 teaspoons for a 10-inch pot. Using more isn't better, and can turn plants into soup very quickly. Also, constantly wet areas may invite algae. In climates like Southwest Florida where I live, crystals are a godsend in the dry season but, once the rains arrive, could drown plants by holding in the moisture. High humidity is likely to be a factor as well.
For existing plants, you can make holes in the soil with a pencil and pour prepared product in them. On in-ground plants, spread a thin layer of the hydrated product under the mulch to avoid UV exposure.
An in-depth discussion of how to use superabsorbers can be found at the FS Stores in Ontario Web site*. The site covers how to use superabsorbers with bare-root plantings, new sod or seed turf, houseplants, repotted plants, tree and shrubs, existing plants, flowering plants, and vegetables. Expect to experiment as the needs of soils and plants will vary. Whatever you do, read the label and follow the manufacturer's directions.
*(http://www.fsstores.com/growers/techniques/crystals.shtm)
