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USE OF CROSS-LINKED POLYACRYLAMIDE IN
FORESTRY
by: Daniel J. Wofford
Abstract
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.
PRESENTATION SUMMARY
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:
South Dakota:
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).
Bareroot Dipping:
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.
Aerial Reseeding:
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.
Animal Repellent:
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.)"
The Future:
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.
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