Polyacrylamide (PAM) is a synthetic, high molecular weight organic polymer.  Depending on the difference in molecular structure, PAM can be classified as linear or cross-linked polymers. Linear PAM dissolves in water and is used to control soil erosion; cross-linked PAM is a granular crystal that absorbs hundreds of times its weight in water.  Since 1995, linear PAM polymer granules have been used commercially as effective erosion-fighting weapons. It can enhance infiltration in many soils. Linear PAM works best on silt to clay-textured soils. Nationwide, about 400,000 hectares were treated with linear PAM in 1999. In the following discussion, PAM refers to linear PAM, unless otherwise indicated.

PAM can be manufactured as a neutral, cationic, anionic, or amphoteric polymer of varying chemical and physical properties, molecular weights and lengths. There are actually hundreds of specific PAM formulations, depending on the polymer's chain length and the kinds of functional groups substituted along the chain. Shown in Fig.1 is one of the repeating monomer subunits in a PAM polymer. When a percentage of the amide (-CONH₂) groups are substituted with other functional groups, the  polymers will vary in ionicity and chemical reaction in the soil due to the substition. Anionic PAMs, some having more than 100,000 functional subunits, are used extensively nationwide for potable water treatment, dewatering sewage sludge, washing and lye-peeling fruits and vegetables, clarifying sugar juice and liquor, thickening and suspending agents in animal feeds, manufacturing paper, mining and drilling applications, and other industrial applications. PAM is synthesized from natural gas and was originally used about a half century ago for soil conditioning.

Reducing Irrigation-Induced Erosion with PAM

Negatively-charged (anionic) PAM granules are effective for fighting erosion on furrow-irrigated farmlands in the western United States, according to USDA Agricultural Research Service (ARS) scientists at the Northwest Irrigation and Soils Research Laboratory in Kimberly, Idaho. ARS soil scientist R. D. Lentz says growers have told the ARS that "PAM-treated water leaving their furrows is often cleaner than when it came in." The ARS scientist says small doses of PAM can boost water infiltration by as much as 60 percent. The water retained can become available for plant growth.

The form of PAM the ARS scientists use for erosion control is a large (12-15 megagrams/mole) water-soluble (non-crosslinked) anionic molecule that contains less than 0.05 % of the acrylamide monomer (AMD). In their studies on furrow-irrigated soils over a three-year period, application rates of 1 kg PAM/hectare/irrigation eliminated 94% of sediment loss in field runoff, on average, and, in soils susceptible to furrow erosion, infiltration increased 15-50% compared to untreated controls (Sojka et al., in press). Their studies followed the USDA Natural Resources Conservation Service (NRCS) application standard for PAM, which calls for dissolving 10 g/m³ (10ppm) anionic PAM in furrow inflow water as it first crosses a field (water advance), which typically is the first 4-6 hr of a 24 hr irrigation in a production-sized field. When runoff begins, PAM dosing is halted.

Seasonal application rates varied from 3 to 7 kg PAM/ha in the ARS studies, depending on the crop and field conditions, which both influenced the number of cultivations and irrigations. Based on their results, the ARS scientists say high efficacy can be achieved by applying 1-2 kg PAM/ha/irrigation in furrow-irrigated soils. Different PAM forms did not have a significant effect on erosion control: Their results showed that it did not matter whether dry PAM granules were dissolved in irrigation water or emulsified PAM liquid formulations were used.

The cost of PAM in the ARS studies was $7 to $13/kg. A thorough cost-benefit analysis for PAM was not done, but yield increases may result from increased water infiltration, and the needs for furrow reshaping and return-flow ditch cleaning are reduced with PAM.

PAM Patch Application Method

The "patch" application method has become popular with farmers. In this method, dry PAM granules are spread on an area-equivalent rate basis (based on furrow spacing and length) in the first meter of furrow below the inflow point. The "patch" of dry granules turns into a thin, "gel-like mat" when water flows over it and slowly dissolves during the irrigation. Positive benefits on erosion control and infiltration enhancement are comparable for the patch method and the NRCS standard approach of PAM-dosing at a set concentration in the advance water stream. Residual erosion control during subsequent non-treated irrigations is generally better with the patch method than with the NRCS standard advance flow dosing because small areas of the PAM patch can still be intact at the end of a treated irrigation and can provide small amounts of PAM in subsequent non-treated irrigations.

For effective seasonal erosion control, the ARS scientists recommend PAM treatment whenever soil is disturbed, whenever it is loose and highly erodible, such as a pre-planting or pre-plowing irrigation. After an initial PAM-treated irrigation using the NRCS standard method of dosing the advance flow at a rate of 10 g anionic PAM/m³, erosion control on the subsequent, non-treated irrigation will be about half if the soil is undisturbed. Thus, farmers and the NRCS have reported that about 80% seasonal erosion control is common on farm fields in the Pacific Northwest when irrigation of disturbed soil is PAM-treated at 10 g/m³, but remaining irrigations of undisturbed soil are treated at lower rates or left untreated, such that the seasonal application rate is about 3 to 5 kg PAM/hectare. ARS soil scientists have shown that on 1-2% slopes, a constant, low application rate of 0.25 g PAM/m³ throughout the season was about a third less effective at controlling erosion than (a) the higher, initial dosing rate of 10 g PAM /m³ in the advance flow followed by a rate reduction of 50% reapplied every few hours or (b) a continuous rate of 1 to 2 g PAM/m³.

ARS scientists have compared different PAM formulations for their effectiveness in controlling erosion in freshly formed furrows. Lentz et al. (2000) found that the order of effectiveness was anionic > neutral > cationic and efficacy increased with increasing charge density and molecular weight. Infiltration was favored by lower molecular weights and medium-to-high charge density. Neutral and anionic PAMs favored infiltration over cationic PAMs.

Additional information about the ARS studies and sample calculations for determining PAM application rates for liquid and dry formulations are available in several publications on the ARS websites:

http://kimberly.ars.usda.gov/pamPage.shtml

and http://kimberly.ars.usda.gov/lentz/tips/Pam-rev.html.

PAM and Runoff Water Quality

One of PAM's most promising applications is for protecting the quality of runoff water. According to recent ARS research, PAM can reduce microorganisms, pesticides, and weed seed carried across furrow-irrigated fields by furrow streams, runoff, and return flows (Sojka and Entry, 1999).

The Washington State Department of Transportation (WSDOT) is testing PAM to determine whether it will be efficacious for (1) removing fine suspended sediments from stormwater runoff at highway construction sites and (2) preventing soil erosion at highway construction sites caused by rainfall and concentrated flows over exposed soil surfaces. To evaluate erosion control, WSDOT is testing direct application of dry granular PAM, PAM-dosed aqueous solutions, and PAM/mulch combinations. WSDOT is also evaluating PAM for its ability to flocculate turbid stormwater runoff within wet detention ponds using a passive, non-mechanical dosing system (polymer get blocks). The PAM research project by the WSDOT is supported by the Priority Technology Program of the Federal Highways Administration (FHWA), according to the website of the WSDOT Environmental Affairs Office.

Toxicity and Environmental Fate of PAM

The chemical and physical properties of PAM differ significantly from those of the acrylamide (AMD) monomer, which is a neurotoxin to humans. In contrast, anionic PAM when used at prescribed rates is nontoxic to humans, fish, animals, and plants. In soil, PAM degrades at a rate of at least 10%/yr as a result of physical, chemical, biological, and photochemical processes. According to Kaye-Shoemake et al. (1998a,b), significant negative impacts have not been documented for aquatic macrofauna, edaphic microorganisms, or crop species for PAM applied at recommended concentrations and rates. Although the anionic PAMs used at recommended concentrations are nontoxic, overexposure can lead to skin irritation and inflammation of mucous membranes. Users should read and follow label precautions, avoid exposure to eyes and other mucous membranes, and should be careful not to breathe PAM dust. PAM spills become very slippery when wetted, so they should be cleaned up with a dry absorbent first, before any attempt to wash down the surface with water.

The effects of PAM on biota are buffered because of the adsorption and deactivation associated with suspended impurities. ARS researchers have determined that only 3-5% of applied PAM leaves fields in runoff because of its affinity for suspended sediments and soil. Lost PAM did not travel very far (100-500 m) before being completely adsorbed on sediments in tile flow or onto ditch surfaces (Lentz and Sojka,1996).

PAM's interactions with Soil Texture and Structure

The potential benefits from using polymers are influenced by the soil texture, structure, and salinity. PAM works by stabilizing the soil surface structure and improving pore continuity. PAM's floccule-forming and erosion-preventing effectiveness in irrigation water is due to its surface attraction for soil particles by coulombic and Van der Waals forces. The surface attractions enhance particle cohesion, which helps to stabilize soil structure against shear-inducing detachment, thereby preventing transport in runoff. PAM applied in irrigation water is irreversibly adsorbed onto the first few mm of soil encountered during infiltration. Thus, to be effective, only a thin veneer of soil  needs to be stabilized. PAM delivery via furrow irrigation is very efficient because only about 25% of the field's surface area to a few mm of depth is treated to prevent erosion. PAM can prevent erosion of furrow bottoms and scaling of the wetted perimeter. In silt loam soils, lateral water movement in PAM-treated furrows has been shown to increase by about 25% compared to non-treated furrows. Results with coarser textured soils have differed. PAM-treated coarser textured soils with fine pores have shown slight infiltration decreases to no infiltration effect, especially at higher application rates (Sojka et al.,1998).

PAM and Sprinkler Irrigation

The effects of PAM under sprinkler irrigation have been less predictable than the beneficial results with furrow-irrigated soils. In sprinkler-irrigated soils, the interest in PAM has focused on its potential to prevent runoff and improve irrigation distribution uniformity rather than its ability to reduce erosion. In bench-top studies using sprinkler irrigation, PAM applied at rates of 2-4 kg/ha reduced runoff by 70% and soil loss by 75% compared to controls, but the effectiveness of sprinkler-applied PAM is more variable than for furrow-applied PAM because sprinkler systems affect water drop energy, the rate of water and PAM delivery.

Claims about Cross-linked PAM (CL-PAM) and Water Conservation

Cross-linked, water insoluble PAM (CL-PAM) is a long-lasting, water-absorbing soil amendment used in gardens, houseplants, and landscaping to increase the soil water-holding capacity and decrease the watering frequency.  Chemically, it is a copolymer of acrylamide and acrylate (anionic, usually potassium salts) with a spatial structure like a net, in which the monomers are cross-linked so as to minimize the water-soluble fraction without compromising on water absorption and release capacities of the co-polymer product. Usually, one pound of this kind of PAM can absorb up to 50 gallons of rainwater or snowmelt, and 15-35 gallons of salt-containing water.

Despite its great capacity in absorbing water, the claim that cross-linked PAM  conserves water is not justified, according to research by University of California, Riverside (UCR) soil scientists reported in the May-June 1992 issue of California Agriculture. The UCR scientists were responding to bold claims by one of the California Water Districts, which had distributed packets of CL-PAM to their customers and stated that CL-PAM would enable their clientele to care for their potted plants more easily, save water (up to 60% or more), and increase the time between irrigations up to 2-3 weeks for a 6-inch potted plant. The UCR scientists decided to test the validity of the claims.

 Greenhouse experiments with container-grown marigolds at UCR showed that adding CL-PAM to the soil mix did not promote water conservation because the polymer treatment did not affect evapotranspiration. The primary utility of the water-sorbing polymer was that it allowed for extending the time between irrigations and increased the soil's water holding capacity in some of the treatments.

 "Use of polymers does not conserve water. Water loss  through evapotranspiration was the same for all treatments. Extending the time between irrigations does not conserve water because more water has to be applied at the time of irrigation to recharge the container to full water-holding capacity. In this experiment, the polymer treatments had no effect on plant growth," wrote John Letey, soil physicist in the Department of Environmental Sciences at UCR. The use of polymers should be coordinated with irrigation scheduling but is not a water conservation practice, Letey concluded. In a properly managed irrigation scheme, the total amount of water applied annually will be about the same with and without polymers. Only timing and amount are altered. Because CL-PAM promotes increased water retention in coarse-textured soils, irrigations can be applied less frequently but in greater quantity when applied.

 The potting mix in the UCR study was 50% plaster sand, 25% bark shavings, and 25% peat moss.  It was amended with 0, 1, 2, and 4 lb CL-PAM/yd³ and fertilizer nutrients (N, P, K and micronutrients) were also added. Pete Clark, staff research associate, and Christopher Amrhein, soil scientist, cooperated in the UCR study.

Literature Cited

Kaye-Shoemake, J.L., W. E. Watwood, R.D. Lentz, and R. E. Sojka.  1998a.  Polyacrylamide as an organic nitrogen source for soil microorganisms with potential impact on inorganic soil nitrogen in agricultural soil.  Soil Bio. and Biochem. 30:1045-1052.

Kaye-Shoemake, J. L., W. E. Watwood, R.E. Sojka and R.D. Lentz.  1998b.  Polyacrylamide as a substrate for microbial amidase.  Soil Bio. and Biochem. 30:1647-1654.

Lentz, R.D. and R.E. Sojka.  1996.  Five-year research summary using PAM in furrow irrigation.  P 20-27.  In R.E. Sojka and R.D. Lentz (ed.) Managing Irrigation-induced Erosion and Infiltration with Polyacrylamide. Proc., College of Southern Idaho, Twin Falls, ID, 608 May, 1996. Univ. of Idaho Misc. Publ. 101-96.

Lentz, R.D., R.E. Sojka, and C.W. Ross.  2000.  Polymer charge and molecular weight effects on treated irrigation furrow processes.  Intro. J. Sediment Research (in press).

Letey, J., P.R. Clark, and C. Amrhein. 1992.  Water-sorbing polymers do not conserve water.  Calif. Agriculture. 36(3):9-10.

Sojka, R.E. and J.A. Entry. 1999.  Influence of polyacrylamide application to soil on movement of microorganisms in runoff water.  Environmental Pollution (in press).

Sojka, R.E., R.D. Lentz, I. Shainberg, T. J. Trout, C.W. Ross, C.W. Robbins, J. A. Entry, J. K. Aase, D.L. Bjornesberg, W.J. Orts, D. T. Westermann, D. W. Morishita, M.E. Watwood, T. L. Spofford, and F. W. Barvenik.  Irrigating with polyacrylamide (PAM)-nine years and a million acres of experience.  (in press).

Sojka, R. E., R. D. Lentz, T. J. Trout, C.W. Ross, D. L. Bjorneberg, and J.K. Aase. 1998.  Polyacrylamide effects on infiltration in irrigated agriculture. J. Soil Water Conserv. 53:325-331.