Research and on-farm experience with Agricultural PAM (polyacrylamide) has shown that when used correctly in surface irrigated fields, it will help preserve topsoil, improve infiltration and lateral wetting, avoid sediment-related management problems and improve water quality in return flows and downstream rivers. Grower experience has shown a number of practical benefits other than water quality improvement for this product. Conversations with growers indicate that these economic and irrigation management benefits outweigh the product cost (usually less than $20/acre) in the minds of many growers who have used the product.

A wide range of PAM products have been used for a variety of purposes, including food processing, municipal drinking water treatment and soil conditioning for a number of years. In this case, the molecular weight and negative (anionic) charge have been designed for use as an erosion control treatment. PAM can be used in granular, solid block or liquid oil-emulsion forms.

How PAM Works: Polymers, or long-chain repeating pattern molecules can occur naturally or be manufactured. Starch and cellulose are two examples of naturally occurring polymers. Formulations of PAM used in agriculture are commonly produced from natural gas and have over 100,000 repeating units per molecule. This negatively-charged polymer works by binding adjacent soil particles together and stabilizing surface soil structure. This stabilization means that only a small fraction of soil particles typically eroded by flowing water are now able to be dislodged and moved in the flowing water. It also means that pores between soil particles are maintained and not filled in by small eroded sediment. As a result, soil erosion can be reduced by up to 90%, and movement of water into the soil (infiltration) is increased by an average of 15% by proper PAM usage.

Application Methods: PAM can be applied as an emulsion, or dry powder in the irrigation water supply ditch. It must be metered into the flow upstream of some structure that will produce turbulence, enhancing mixing of the product with the irrigation water. This is particularly important for the granular material. Application in the supply ditch is convenient since only one injection point is required for all sets on the field. However, this is not a good alternative for sediment-laden supply water since the PAM will settle sediment from the flow and quickly fill the supply ditch. Another alternative is to place the correct number of solid blocks of PAM in an area of turbulence in the supply ditch. The rate of PAM metering is less controlled but use is simple and treatment control is maintained.

For sediment-laden supply water, or for an initial low-cost trial, many producers sprinkle a measured amount of PAM into the top 5-10 feet of each irrigation furrow. This method works well in most cases. It is critical, however, to spread the PAM along the bottom of a short length of furrow rather than to apply at just one point. This enhances the rate of PAM dissolving into the irrigation water and improves produce effectiveness. Commercial applicators are available to conveniently meter a specific amount of PAM into the head of each furrow.

Other benefits of PAM: In addition to an average 95% reduction in soil erosion and 15% increase in infiltration, PAM has several other benefits such as improved irrigation uniformity on some fields and reduced need to re-corrugate furrows. On some sloping fields, sediment eroded from the top of the field deposits in the downstream end of furrows. With the furrows filled with sediment, water flows in an uncontrolled fashion across furrows, over-irrigating some areas and under-irrigating others. With PAM however, soil erosion is minimal, furrows do not fill with sediment, and water remains in the correct furrow, improving the uniformity of irrigation. Clean irrigation water flows through crop residue, while sediment-laden water fills it with sediment, forming dams in the furrows.

Increased infiltration can have a significant effect on crop production and profitability. For example, Dennis Pettigrove, who farms near Hansen, Idaho, attributed a 6-sack increase in dry bean production on one field to improved uniformity of irrigation and improved infiltration on the sloping portions of the field. Similar improvement can be seen in sugarbeets. With PAM, the rate of lateral wetting from the furrow to the crop row was faster with the wetting front advancing farther in a given set time. This was particularly important on the sloping areas of the field where the initially broad furrow shape eroded into a narrow channel. As a result, lateral wetting was minimal and crop production was lower than on the uneroded portions of the field.

Possible improved seedling emergence: Under conditions where sugarbeet must be irrigated before germination or emergence, a surface crust can form, reducing seedling emergence and in some cases, forcing replanting. A surface crust can form when water droplets from sprinkler irrigation or intense rainfall break down soil into very small silt-sized particles. Droplet impact causes these small particles to puddle and become compacted. A hard surface crust is then formed as the soil dries. High-silt soils, low organic matter soils, or soils tilled sufficiently to weaken or destroy soil structure are most prone to surface crusting. Because PAM stabilizes particles on the soil surface, it should be helpful in reducing surface crusting in soils. This surface protection is most needed during the 1-3 irrigations prior to seedling emergence.

Possible surface runoff control benefits: Formation of a surface crust reduces infiltration and increases surface runoff problems. This is particularly true on the outer spans of center pivots where water application rates are many times greater then the pre-surface crust infiltration rate. Prevention of surface crust formation by PAM then helps minimize runoff problems. To be an effective runoff control practice, however, more than one PAM application is needed to provide treatment until canopy cover is nearly complete. Cost of multiple applications and the extra effort required for PAM use in sprinklers tend to discourage sprinkler use.

Laboratory and field evaluation of PAM in sprinklers: Drs. David Bjorneberg and Robert Sojka and Kris Aase conducted laboratory tests at the USDA-ARS Laboratory, Kimberly, Idaho. Soils having a variety of soil textures were exposed to simulated irrigation with characteristics like that from typical agriculture sprinklers. PAM applied in sprinkler water a 2-4 lb/acre gave a significant reduction in soil erosion and runoff for that irrigation. Treatment effects were about halved for the next irrigation, and little effect was noted after the second irrigation.

Drs. Bjorneberg, Sojka and Aase also conducted a field evaluation under a linear-move system. Visible differences in the clarity of ponded water occurred during PAM irrigation. However, due to long slope length with uphill and downhill tillage, runoff was severe in spite of any PAM effects. This emphasizes that PAM alone will not stop runoff in many situations, but may be an effective component of a PAM/tillage/crop residue management strategy to reduce runoff. Multiple applications are needed for season-long benefits.

Although no controlled trials have been conducted with sugarbeet emergence, other laboratory and field evaluations of small-seeded crops such as lettuce and celery in California trials indicated that application of PAM in sprinkler water could improve seedling emergence. This is consistent with limited grower reports from Southern Idaho that indicated that PAM was effective in improving seedling emergence.

Product composition and purity vary among suppliers. PAM from reputable agricultural suppliers will have a minimal short chain (Monomer) content. Polymers like PAM have been shown to be safe and food grade polymer is used routinely in water treatment and food processing. However, short chain segments have been shown to have health risks and are not effective in reducing soil erosion. Most PAM used in agriculture is food grade and has very low acrylamide monomer content (less than 0.05%). This product is more expensive than the lower grade polymer with a higher monomer content that is produced during certain stages of polymer manufacture. In the case of PAM, extreme caution should be used before purchasing a "bargain" polymer. It may not work well and it may be unsafe.

For more PAM information, check the Northwest Irrigation and Soils Research Laboratory web page at: http://kimberly.ars.usda.gov and click on "Research Projects" or contact

Dr. David Bjorneberg at (208) 423-6521 email: bdavid@kimberly.ars.pn.usbr.gov

Dr. Robert Sojka at (208) 423-6562 email: sojka@ kimberly.ars.pn.usbr.gov

or contact Dr. Howard Neibling at (208) 736-3631 email: hneiblin@uidaho.edu

Howard Neibling is an Extension Water Management Engineer with the University of Idaho Biological and Agricultural Engineering Department. He works at the Twin Falls Research and Extension Center in Twin Falls, Idaho.