Date Last Edited: 08/24/2001
Hawaii Cooperative Extension Service
Hydrophilic gels are generically known in the trade as hydrogels, moisture-holding compounds, cross-linked polymers, super-sluppers, and by various trade names. These compounds absorb many times their weight in water (300 to 1,500 depending upon the produ ct) and release it as the environment becomes dry. Interest in these materials has waxed and waned several times since their introduction to the agronomic and horticultural market almost 20 years ago.
There are two types of absorbent polymers in the trade.
Manufacturers suggest that hydrophilic gels can
These properties would be very advantageous in plant production and landscape management. Water-holding amendments could be used to increase water availability to plants, reduce labor, and to conserve water.
These products have found their place in several facets of agriculture:
There are reports in scientific and trade publications of controlled research performed at various universities and experimental fields. This research has also been conflicting and is often controversial. Past studies differ widely in the product(s) teste d, plant species evaluated, evaluation procedures, environmental conditions under which the study was conducted, and quality of research techniques. Just because study was conducted at a college or university does not always assure the quality of the resu lts.
It is up to the manager to digest the available information and make an informed decision. These materials are expensive and present advantages and disadvantages. I am personally hesitant to believe some of the fantastic success stories and testimonials r epresented by promoters.
A few months later, I saw an advertisement from the company with the headline, "PRODUCT X INCREASES PLANTING SURVIVAL by 300%." There was my name and a reference to the study. The utter failure had suddenly become a resounding success through the miracle
of marketing. The advertising was withdrawn at my insistence.
There have been a number of research trials using of hydrogels as an amendment in nursery and greenhouse production and in landscape management. The results have been conflicting, however. Research studies are designed to evaluate particular products unde r specific, controlled conditions. The results should not be universally applied to every plant species, material, or situation. Incorporation of gels at manufacturer's rates does appear to sometimes be advantageous in improving plant water status for som e species under limited moisture conditions.
One of my former graduate students, Dr. Janet Henderson, now at Oklahoma State University, found that mixing a hydrogel into sand provided a brief delay in drought in a model system. This benefit was not evident, however, when the water-holding capacity w as increased by adding a small amount of soil.
There have been other bona fide research projects indicating that the addition of some gel products to container medium reduced water stress and increased time to wilt. Other reports, however, have shown no advantage, and some even indicated damage from u
se of the materials.
Dipping bareroot plants in solutions of crosslinked polymer before planting has not been highly rewarding, but research results vary. There is some evidence that the gels may cause the roots to cling together, actually reducing the potential water-absorbi ng area for a few days. I have found no published research, beyond testimonials, that report any influence on plant survival or growth as a result of sprinkling small quantities of these products in the bottom of planting holes.
There is not sufficient information available to predict which plant species might be aided by use of gels and which will not. Similarly, the effects of various environmental conditions on hydrogels are largely unknown.
There has been a lot of interest among turfgrass and landscape managers in using cross-linked polymers injected or mixed into the soils as a water reservoir to reduce irrigation frequency or amount. Several research projects have been initiated by equipme nt manufacturers and hydrogel producers and marketers. The results of these trials should reach the scientific journals soon. Some of the figures presented in the trade press deserve consideration.
One company's representative suggested that their product could store an inch of water in the turf root zone, releasing 95 percent of it upon plant demand. This, according to the article, represented nearly a 4-day supply of water if the evapotranspiratio n (ET) rate was 0.24 inch per day.
Accomplishing this degree of water storage would require uniformly mixing 30 pounds of polymer per 1,000 square feet (1,300 pounds per acre) into the upper 6 to 8 inches of soil. These materials cost between $5.00 and $7.00 per pound (or more). The abilit y to reduce irrigation frequency a few times over the season for 2 to 4 years may be a bit pricey.
An interesting study evaluated the amount of water actually absorbed by one product. The manufacturer indicated that the product could absorb 500 times its weight in water. The maximum amount of distilled water absorbed after 6 hours was slightly over 350 times of its weight. This was less than promoted, but it is still a significant amount of water. The researcher then soaked the product in 0.01 molar CaCI2 solution. This is considered to be similar in salt content to an average soil solution. This reduc ed the amount of water absorbed by 90 percent. Apparently, divalent ions in the soil salt solution lock the two parts of the polymer together preventing water absorption.
An agronomist has presented calculations representing the contribution of a hydrophilic water-holding compound on the soil water budget. If the dry material was applied to soil in seed or root areas at the rate of 10 pounds per acre (apparently the maker'
s recommendation), it would hold 5,000 pounds of water, based on manufacturer's claims. Although this is a great deal of water, an actively growing crop can be expected to use about 40,000 pounds of water per acre per day. The 5,000 pounds of water would
account for only 12.5 percent of the crop's daily water use. This could conceivably be used up in a few hours. An acre of soil (six inches deep) can hold approximately 400,000 pounds of plant available water. The gel's contribution would be less than 1 pe
rcent of the total, at the rate applied.
Fertilizer salts dramatically reduce absorption by polyacrylamide gels, according to some research. Fertilizer ions may restrict expansion and water absorption by "locking" the polymer in place. Gels apparently act like a soil particle in their ability to retain certain chemical compounds.
We found that a gel would bind ammonium (NH4 +), but not nitrate (N03-) ions. Other studies have shown that gels absorb calcium and magnesium. At least some herbicides are also bound by gels.
The quality of irrigation water should be a factor when considering hydrophilic gels. Water containing substantial amounts of minerals or salts decrease water absorption of some gels. Poor quality water negates any possible advantage presented by these ma terials. There are, according to manufacturers, differences between products in the effect of salts on absorbency.
Using hydrophilic gels may have some advantages where water is limiting. They are not, however, a panacea. While gels may increase the water status of some plants under low moisture conditions, they will never substitute for good management.
Each manager must determine if these effects are real or truly economical in their situation. Most of the literature, especially from promoters, consists of primarily testimonials, rather than hard scientific facts. These, in some instances, should not be taken as gospel.
There remain significant differences among products and serious questions about water quality, fertilizer, and pesticide effects on gels and vice versa.
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