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Benefits and Costs of Applying Polyacrylamide (PAM) in Irrigated Furrows

Andrew Nishihara and Clint Shock
Malheur Experiment Station, Oregon State University
Ontario, Oregon, 2001
  

What is polyacrylamide (PAM)?

    Polyacrylamide is a synthetic water-soluble polymer made from monomers of acrylamide.  PAM binds soil particles together.  Once soil particles suspended in water are bound together by PAM, they settle out, so water has a harder time washing them out of the field.  Water-soluble polymers like PAM have been known to benefit soil properties for a long time.  Recently these polymers have gained renewed attention for their use in reducing irrigation-induced erosion, now that the cost of applying PAM has become economically feasible.  Other uses of polymers like PAM include treatment of municipal water supplies, food packaging, adhesives, a boiler water additive, film former in the imprinting of soft-shell gelatin capsules, adjuvants in the manufacturing of paper and paperboard, and the list goes on and on.

    After Soil Science (Bear 1952) published a set of papers that introduced water-soluble polymers as soil conditioners, the Monsanto Chemical Company spent about 10 million dollars producing and marketing the water-soluble polymer Krilium during the 1950's.  Krilium was not adopted by commercial agriculture.  Although Krilium was able to reduce soil erosion and other problems associated with furrow irrigation run-off, it was too expensive to justify applying it on fields and the recommended application rates were just too high to be economically practical.  Since then, more extensive research has been done, identifying water-soluble polymers for agricultural use and effective application rates.
 
Why would you want to use PAM?

  PAM is highly effective in reducing soil erosion off of fields and can increase water infiltration into irrigated furrows (1, 2, 3, 4, 5, 6, 7, 8).  PAM has been shown to significantly reduce soil erosion by 90-95 percent when applied to irrigation water.  Increases in water infiltration rates vary from 20-60 percent from trials.  The increased use and distribution of polyacrylamide products in the past few years has brought down product prices, making PAM a more economical BMP option. PAM's many forms and application techniques make integration into the farmer's irrigation routine smooth and relatively easy once the initial set-up is complete.  Relatively low cost, high reduction of irrigation-induced erosion and soil loss, ease of use and integration, make Polyacrylamide a best management practice worth looking into by any agricultural operation.

How is PAM applied and what forms does it come in for application?

   PAM's three most common forms are dry granules, solid blocks (cubes), and emulsified liquids.

   The application method of PAM chosen depends on the form of PAM selected.  The use of dry granular PAM  into irrigation water is facilitated by the use of an augured metering system and excellent mixing and thorough dissolving before the PAM reaches the irrigated furrows.  PAM blocks (or cubes) are usually placed in wire baskets that need to be secured to the edge of the ditch to avoid washing of the blocks down the ditch.  Liquid PAM can be metered directly from the container into the irrigation ditch, directly into the furrow, or through a pipe line or injector pump.

    Dry granules of PAM can be applied either by dissolving directly in the irrigation ditch before it hits the furrow, or applied directly in the furrow using what is known as the "patch method".  In order for the PAM to dissolve properly in the irrigation ditch it must have proper agitation.  Unlike sugar or salt which dissolve fairly quickly in water, granular PAM needs to be agitated thoroughly in order for it to dissolve.  If not agitated, PAM globules form, and in time the globules can float down the furrow with little effect on the furrow erosion.  A way to make sure the applied PAM is dissolved is to have a drop structure in the irrigation ditch to add turbulence to the water before it hits the furrow.  Another tip to achieve desired dissolving is to place the applicator close to the point where the irrigation water first hits the ditch.  In a concrete ditch, tins or boards will provide sufficient turbulence.  In a earthen ditch a drop dam works nicely.

    The "patch method" involves placing PAM at the point in the furrow where the water first hits; applying it for a length of about 3-5 feet down the furrow to reduce the risk of the PAM becoming buried in the furrow or washing down the furrow with little to no effect.  The patch method creates a sort of gel-slab at the top of the furrow where the water slowly dissolves the PAM and carries it down the furrow.

Table 1.  PAM application by the patch method at the head of the furrow sufficient to provide 1 lb of PAM per acre with different furrow spacings and lengths.  Malheur Experiment Station, OSU.
 

    PAM blocks are usually placed in wire baskets in flowing ditches at turbulent points.  The blocks slowly dissolve, releasing small amounts of PAM into the water.  Of the three forms PAM blocks may not perform as well as well as liquid or granular PAM in furrow irrigation.  PAM blocks, however, have been useful for treating settling ponds to accelerate water clarification and promote flocculation.  They can also be used to dose concentrated runoff areas on fields that otherwise cause uncontrolled erosion.

    Emulsified PAM (special liquid PAM solutions) can be applied like the granular form into irrigation ditches or into furrows using the patch method.  Emulsified PAM doesn't require quite the vigorous mixing as the granular form, but still needs adequate mixing for dissolving.  Emulsified PAM is more voluminous than dry forms, but is easier to dissolve and is the only form of PAM that should be used for sprinkler irrigation systems, due to greatly reduced the risk of clogging the lines.
 
Why would you want to use granular or liquid PAM in your irrigated furrows?

    In an experiment done at the Malheur Experiment Station in 1995, tests on two different application techniques of PAM (liquid and granular) showed both reduced sediment loss and increased water infiltration into the soil (7).  The experiment was designed to determine if granular PAM could be as effective at reducing erosion in furrows when applied starting at the beginning of the head ditch (where it has not yet thoroughly dissolved) as when applied to the furrows further down the head ditch.  Since applying granular PAM tended to be easier for farmers to handle rather than liquid PAM, research needed to be done to determine whether or not there was a significant difference between the two.  The two forms of PAM were supposed to be applied at similar rates, but liquid PAM ended up being applied at a rate of 0.9 lb/acre and the granular PAM at a rate of 1.8 lb/acre.  The difference was caused by the changes in volume of water flowing in the head ditch during the experiment and by other changes in irrigation management on the commercial farm.  For soil erosion the check furrows lost 322 lb/ac of sediment off of the field in the runoff water during a single irrigation. Furrows irrigated with granular PAM lost 7 lb/ac of sediment off of the field, while those irrigated with the liquid solution of PAM lost 104 lb/ac.  Remember though, the granular PAM was applied at a rate double the liquid.

    In increasing water infiltration, the check furrows lost 37.5 percent of the water as runoff and 62.5 percent was infiltrated. Out of the total water applied treated with granular PAM, 26.5 percent was lost as runoff and 73.3 percent of the water infiltrated into the soil. Out of the total water treated with liquid PAM, 29.1 percent was lost as runoff and 70.8 percent of the water infiltrated.  Granular PAM used as a "patch" was effective to control the loss of sediment and increase water infiltration.
 
How much does this practice cost?

    The cost estimates from Simplot Soilbuilders* in Ontario, Oregon, for PAM are listed in the table below (Cost estimates as of October 1, 2002):
 

* The naming of a supplier and products is not to be considered an endorsement or criticism of other similar suppliers and products.

    Since the recommended rate of PAM in water is around 10 ppm to be effective for reduced soil erosion when the water is first advancing through the field, trial and error for each field is necessary with different irrigation rates and applicators.  Assuming that PAM is applied at a rate of one pound an acre for each irrigation, and an added labor or purchased applicator cost of $1.00 an acre for PAM application, the cost of applying PAM is relatively low.  A per acre estimate for one irrigation using granular PAM is $2.86.  PAM, however, needs repeated applications, especially following cultivations, which increases the total cost of erosion control during the season.  Assuming the the initial irrigation and three irrigations following cultivations receive the full pound rate and 10 other irrigations receive a half pound rate, the total cost for the season would be 9 lb/ac of PAM at $1.86/lb and 14 applications at $1/ac, for a total of $30.74/acre for a year.
 

Project Number TXA&I- 92- 1
September 1, 1992 - August 31, 1994 Grant Number 14- 08- 001- G2048

The research on which this report is based was financed in part by the U.S. Department of the Interior, U.S. Geological Survey, through the Texas Water Resources Institute. Non- Federal matching funds were provided by the Texas A&M University- Kingsville. This report was adapted from a thesis by Bernardino Mendez- Gonzalez submitted in partial fulfillment of the M.S. degree.

Contents of this publication do not necessarily reflect the views and policies of the Department of the Interior, nor does mention of trade names or commercial products constitute their endorsement by the United States Government or the Texas A&M University System.

February 1996

All programs of the Texas Water Resources Institute and the Texas Agricultural Experiment Station are available to everyone regardless of socioeconomic level, race, color, sex, religion, handicap, national origin or age.

Abstract

Using wastewater for irrigation of crops represents an attractive alternative to disposal. Typically, municipal wastewaters are high in sodium, and the resulting high sodium absorption ratio (SAR) alters the soil structure making it more impermeable to air and water. The present study tested the hypothesis that gypsum applied after disking and anionic polyacrylamide (PAM) applied in solution reduce crust formation and improve the infiltration rate of water into soil irrigated with water high in salt and sodium. Two soil amendments were applied to plots furrowirrigated with wastewater. The amendments were gypsum (11 Mg ha^-1), and PAM added to irrigation water at rates of 25 mg L^-1 PAM applications were made during every irrigation and during every second irrigation. In addition, two column experiments were performed to measure hydraulic conductivity. In Experiment 1, PAM was applied to undisturbed soil profiles at rates of 5, 10, and 15 x 10^-6 g PAM cm^-3 of soil. In Experiment 2, two levels of PAM (0 and 25 ml L^-1 ) and three levels of wastewater (20, 40, and 60 mm) were applied to disturbed soil profiles.

Field saturated infiltration (K_fs) rates of PAM- treated plots were approximately double those of the control, and significantly different (P<0.05) from non- irrigated plots. Gypsum was also beneficial but not as effective as PAM. The effectiveness of PAM persisted several weeks after the last PAM application. The results suggest that the deleterious effects of irrigation with this wastewater on soil permeability can be effectively ameliorated using anionic polyacrylamide polymers. The ability of PAM to improve saturated hydraulic conductivity (K_s) in laboratory column studies was variable and not always correlated with levels of polymer added. Also, combinations of PAM and irrigation levels interacted significantly (P<0.05).

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.).

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