USE OF CROSS-LINKED POLYACRYLAMIDE IN FORESTRY
by: Daniel J. Wofford
Gel-forming cross-linked polyacrylamide, a synthetic, long-lasting, water-absorbing polymer capable of absorbing up to 400 times its weight in (deionized) water, is rapidly developing a significant role in survival tree plantings in the United States. Many millions of seedlings are being planted annually with the polymer, to improve survival and enhance early growth.
The basic planting technique is to dip the roots of all bareroot stock in a thick slurry solution of a powdery grind of the polymer and mix 1-2 cups of pre-hydrated coarse polymer with the backfill of the planting hole. The barefoot dip prevents drying out of seedlings during planting and the gel particles give the seedling a ready water supply to tap into and draw on during its crucial establishment period. This simple technique, costing less than five cents (U.S.) per seedling, is spreading rapidly across the U.S., and we predict its use will spread quickly abroad to countries with survival problems, as this experience is publicized.
Polymer-injection equipment for trees, systemic (bio-degradable) game repellents loaded on polymer, seedlings grown in a polymer/soil mix, nursery bareroot-dipping for transport, fertilizer/micronutrient-loaded polymers and pelletized seeds (made possible by the polymer) for large-scale, aerial reseeding are now a reality in many locations.
At least three million seedlings were planted in 1990 with cross-linked polyacrylamide (c-l poly.) in the Western Great Plains and Rocky Mountains, and the total is growing rapidly each year. This simple, inexpensive, easily applied technique (“Survival Technique”) involves bareroot dipping (cost ½ ¢ ) with a slurry, and placement of a cup to a pint of hydrated polymer crystals (cost 3¢ ) into the back fill of the planting hole of both bareroot and container stock at time of transplanting.
Despite widespread use of this survival technique, few well-documented tests with proper replications and controls have been conducted. However, virtually every one of which we are aware clearly demonstrates superior survival with the use of c-l poly. The following two tests are typical examples:
A 1990 test conducted by USDA/SCS’ Thomas Hurford near Cottonwood, South Dakota, showed 26% mortality for 180 control seedlings (10 different species) as opposed to only 13% mortality for the 180 seedlings planted with a cup to a pint of hydrated polymer in the backfill at planting. Cost was only 3¢ (U.S.) per seedling or $5.40 extra for the 180 treated seedlings. (Bareroot dip was unavailable at planting time.) Some species did better than others, especially the most difficult-to-establish species (honeylocust, hackberry and Arnold Hawthorn, for instance). The site received about 75% of its normal 12″ of rainfall for the five-month period following the 1 April 1990 planting.
Central Arizona (canal) Project:
Fall 1988 tree-shrub tests by Dr. Stuart Bengson (ASARCO, Inc. for the U.S. Bureau of Reclamation) at a site along the 600-mile Central Arizona Project (CAP) showed that 4 ozs. of (dry) crystals increased survival by 50% over the control, 8 ozs. increased survival by 40%, and 1 oz. increased survival by 33%. Planting of the one-gallon containerized plants was done in 6″ diameter, 18-24″-deep augured boles, and each hole received 5 gallons of water for the “mudding in” process. Wide variations in survival occurred according to species, with most significant increases appearing in species normally most difficult to establish (i.e., Paloverde, false mesquite, Acacia spp.) Subsequent, more extensive tests with grass plugs at the same site with (dry) rates of 1,2, 4 and 8 oz. (with controls) showed the best survival rates at the 2-oz. rate. Bengson feels that the 2-oz. rate would prove most successful for seedlings as well.
Cost of the 2 ozs. of dry polymer was 40¢ (U.S.), but this was only a small fraction of the cost of a similar planting at the same site with drip irrigation. We feel that this cost can probably be lowered to perhaps 5¢ by growing seedlings in a polymer/soil mix in the nursery prior to transplanting. This test was planted on 1 December 1988 at the onset of what proved to be the driest winter in recorded Arizona history. The site received a light snow around Christmas, but no other moisture until a rain near Easter 1989 (some four months after planting).
Several million bareroot seedlings are bareroot-dipped annually in the Western United States with a c-l poly slurry. The technique has caught on more because of perceived value and very low cost (½ ¢ ) than systematic field and university testing. Some state nurseries (i.e., Texas) routinely bareroot dip at the nursery for transport, and several Whitfield harvesters have been field-modified with 12-volt spray pumps to handle the bareroot-dipping at time of lifting. R.A. Whitfield Manufacturing (Mableton, Ga.) can supply a harvester equipped with the bareroot spray capability, and is contemplating production.
Dr. Alvin Aim (University of Minnesota Dept. of Forestry Resources, Cloquet and St. Paul) has done considerable research with c-l poly barefoot dip. His 1990 tests involved exposing controls and bareroot-dipped red pine, jack pine and white spruce in the sun for 5, 10 and 20 minutes before planting, and resulted in across-the-board survival increases. His work tends to support a theory held by numerous experienced bareroot users that the principal value of bareroot dipping is to reduce seedling loss in transport and storage.
Little is known about the differences between effective and ineffective bareroot (polymer) dips among the dozens on the market. Research has been proposed to study the various polymer root dips with respirometers, electron scanning microscopes and other scientific devices in an effort to identify physical clues by which we can select and validate effective polymer dips from among the many on the market. One theory suggests that gel-forming polymers, even when crushed into a polymer powder for bareroot dip use, may retain their angular shape, thus allowing the roots to “breathe” better. Because of the increasing interest in gel-seeding of native grasses in arid and semi-arid regions, basic research into the various gels is much needed.
Growing Nursery Seedlings in a Polymer/Soil Mix:
Bedding plants grown in a polymer crystal mix grow faster, healthier, and larger, and are much less susceptible to transplant shock. Based on some informal tests of ponderosa pine seedlings in Colorado, it appears that most of the same benefits may also be true of tree and shrub seedlings. Agriculture Canada’s PFRA Shelterbelt Centre has begun testing the technique with Siberian larch and buffaloberry — both chosen because of transplant difficulties.
With 50-200 hydrated crystals already attached to the root system at transplant time, it seems logical that transplant shock would be significantly reduced, and that normal growth would commence much sooner after transplant. Cost of this basic technique (including transplant with the “survival technique”) would be about 5¢ , but research will have to determine the number of crystals which would have to be attached to the root system to insure survival under various rainfall conditions. Each pound of standard cross-linked polyacrylamide crystals contains approximately 67,000-70,000 individual granules, thus it would appear technically possible to keep the transplant cost below 5¢ per seedling.
The percentage of hydrated polymer crystals to soil mix will be crucial, as indicated by a European test on chrysanthemums using hydrated polymer rates of 10%, 20%, 30%, 40% and 50%. Maximum bloom occurred with the 10% hydrated polymer / 90% soil mix rate, and maximum growth resulted from a 20% hydrated polymer / 80% soil mix. Examination of hundreds of sites and conversations with experienced polymer users appear to verify these findings. Above 20%, both blooms and growth drop off significantly, and overdosing with polymer is by far the most common mistake seen for both trees and flowers.
Water Catchment Systems:
The polymer offers a unique water storage mechanism, and more and more users are devising small, very simple water catchment systems to enhance survival and promote early growth. One lb. of c-l poly normally absorbs and holds 48 gallons of rainwater, and 20-35 gallons in most soils depending on salt content.
Compressed-Air and Mudpump Polymer-Injection Guns:
The growing popularity of c-l poly has spurred development of several injection guns. A typical compressed-air gun will fracture 3′ deep and 5-15′ in diameter, but requires a large compressor. Coast redwoods (40′-60′) injected with 4 lbs. of polymer had an average of 20.5″ of new growth per year, those treated with air only 17.1″ and the controls 16.0″ at a Fresno, Calif., city park monitored by UC/Davis researcher Pam Elam. A mature 20-acre, sandy grape vineyard in the Fresno area, injected with 6 ozs. per vine, resulted in more than a 50% increase in growth after one growing season.
O1athe Manufacturing, Inc. (Olathe, Kansas) has a 110-gallon trailer-mounted mudpump, 5 hp (gasoline) unit which pumps 9 gallons of hydrated polymer per minute at 500 psi. Due to the mobility of this unit, it shows considerable potential for tree/shrub injection work, including tree planting.
Other small, compressed-air guns are being developed for tree planting or injecting dry crystals into established trees, but the smaller units cannot operate at a high speed due to the time the 8-12 cfm compressors take to recycle. These normally fracture 1′ deep and 3′ in diameter.
A Surrey, B.C. company (Ani-Pel Silvaculture, Ltd.) is experimenting with a pelletized seed containing polymer for aerial reseeding. Some experiments have shown as high as 70% survival under optimum conditions in this high rainfall province. We are also looking at ways to duplicate nature by using polymer/soil mix for direct seeding by hand.
Ani-Pel also handles a line of animal repellents using denatonium benzoate (Bitrex), one of the ten most bitter substances in the world, and the only one which is water-soluble. Several delivery methods are possible with this systemic repellent, including loading on polymer. No one has yet completed a test with the Ani-Pel-loaded polymer, but we should be able to get above- and below-ground protection for perhaps one year (??).
Proper Identification of Polymers in Scientific Papers:
Seldom are polymers properly identified as to type, rate per volume or screen size (a very important factor). This makes literature review a confusing, difficult issue. To remedy this serious problem, we are working with a university researcher to produce simple, standardized identification codes for all polymers. For example, the code for a particular c-l poly might read: “Gel-forming, cross-linked polyacrylamide. Absorbs 410 X its weight in deionized water in a 4-hour soak at 25 degrees C, and X times in a standard “XYZ” saline water solution of 2000 ppm. Granule size: 500-1500 microns (or appropriate screen sizes.)”
In this author’s opinion, both bareroot and container stock of the future will be nursery grown in a polymer/soil mix to ensure that 100-200 (?) hydrated crystals are attached to the root system of each seedling prior to transplant. These crystals will be pre-loaded with a fertilizer/nutrient package, a systemic game repellent, and other appropriate materials (i.e., fungicides). Bareroot stock will be spray-dipped at time of lifting to further ensure against transport loss. An additional cup to pint or more of hydrated crystals (250-500?) would be mixed into the backfill around the root system, and the planting watered once. The polymer cost of this type planting can be kept under 5¢ (U.S.).
© June 1991 Daniel J. Wofford, Jr. and Connie Lockhart Ellefson, P.E.
Daniel J. Wofford, Jr., 54, is a retired Foreign Service officer with a farm background and a strong interest in research relating to problems of agriculture and water/soil conservation. He holds a B.A. in Asian studies from the University of Oklahoma, and completed four years of graduate work in Far Eastern Studies and Geography at the University of Washington. On retiring from government service three years ago, he started Western Polyacrylamide, Inc. (Hydrosource), and has devoted his efforts to researching and developing this promising water-conserving product, cross-linked polyacrylamide, and keeping the price of it affordable for a wide variety of applications.
CONTACT: Daniel J. Wofford, Jr. – Western Polyacrylamide, Inc. – Click Here For Current Contact Information
Copyright 1991 by Daniel J. Wofford, Jr, and Dale Greenwood.