PAM: Just the Facts
If you've had anything to do with irrigated agriculture in the past couple years, you've probably heard about PAM, touted to be the erosion control miracle. The next couple pages highlight some of the important points about this new "miracle product" for erosion control and water management.
What is PAM? Polyacrylamide (PAM ) is a long-chain molecule commonly used to clean waste water. To date, the primary market for this compound has been municipal wastewater treatment facilities. It makes the fine solids in treated water adhere to one another until they become big enough to settle out or be captured by filters to make sewage sludge, and because of this feature, its use in agriculture is growing. PAM seeks out and binds to the broken edges of clay particles, which carry a negative charge. By increasing the cohesiveness of soil particles on the soil surface of a field, PAM makes soil more resistant to the highly erosive shear forces exerted by water flowing over it.
What does PAM do? When used according to the NRCS standard, polyacrylamide (PAM) increases infiltration in addition to nearly eliminating furrow erosion. Increase in infiltration varies with several soil attributes, especially texture. Silt loam soils have shown about a 15% increase in net infiltration and a 25% increase in lateral wetting from shallow furrows between low, flat beds. PAM preserves a more pervious pore structure during the formation of surface seals during irrigation, thus allowing increased infiltration. The greater infiltration associated with PAM-treated furrows can boost crop yields in sloping areas such that it's almost like giving the farmer the added yield equivalent of another irrigation during the growing season. Studies have shown that because PAM holds the top soil in place, it also keeps phosphorus, nitrogen, pesticides, weed seeds, and microorganism out of waste water.
How much PAM does it take to get good results? It takes very little PAM to dramatically cut erosion and increase infiltration. Just 10 parts per million (ppm) added to the advancing stream can reduce erosion by 70-99%. After PAM treated water has reached the end of the field, the irrigation can continue for another 12-24 hours without further treatment.
When should PAM be applied? As a minimum, PAM should be used on the first irrigation and on subsequent irrigations if the soil has been disturbed by traffic or cultivation. As long as the soil has not been disturbed by foot traffic or cultivation, the next irrigation can be delivered PAM-free and still hold erosion to about half that of recently plowed fields. For full protection that second time around, farmers can "refresh" their undisturbed furrows by beginning the next watering cycle with a mere 1 ppm PAM treatment. For crops in which erosion naturally subsides during mid season, such as potatoes when the vines elongate, PAM need not be applied after the natural erosion control properties take over.
What forms of PAM are available? PAM comes in granular, liquid oil emulsion, tablet, and block forms. Dry PAM forms are easy to store, transport, and meter into head ditches and have a longer shelf life than liquid forms. However, liquid forms require less agitation to dissolve. All the forms of PAM have a relatively low toxicity, but one should still be careful not to ingest it or have prolonged skin contact. Also, PAM spills of any kind are extremely slippery.
How should PAM be applied? Regardless of the form, PAM needs TURBULENCE to dissolve in the water. Even though dry forms require more vigorous mixing than liquid forms, all PAM needs to be added to the incoming water at a turbulent point. More concentrated PAM solutions need more aggressive mixing, with dry forms requiring the most turbulence. If using gated pipe, the first length of gated pipe after the point of PAM injection can have one or two baffles to enhance mixing. Metering liquid or granular PAM or suspending tablets or blocks of solid PAM anywhere from 25-300 feet before the first siphon tube, gate, or turnout allows plenty of mixing time.
Granular PAM can be sprinkled in a patch along the first 3-5 feet of furrow, starting right where water from the siphon tube or gate hits the furrow. Depending on the length of run, 1-3 ounces will dose a furrow. Large scale automatic fish feeders are a relatively new innovation in applying granular PAM. The feeders have a wide belt moved by a timer and sprinkle PAM into the water over the course of an irrigation.
Liquid forms of PAM usually come in a container with a built in meter and instructions. Block or tablet forms can be suspended in baskets at a turbulent point in the incoming water, but are used primarily to increase infiltration on land where erosion is not a problem. Consequently, they do not perform as well as other forms for erosion control.
Can PAM go through sprinklers? Studies in Washington and Idaho have shown that injecting PAM directly into sprinkler systems reduces surface ponding and runoff and promotes more even plant growth because of the more even water distribution. In these studies, the cost ranged from $5 to $8 per acre. When considering the potential benefits of PAM, this ability to apply greater amounts of water without runoff or surface puddling and sealing becomes particularly valuable under mid-summer conditions when center pivot irrigators find it difficult to keep up with plant water demands, because of the limitations of sprinkler application rates under center pivots.
Do I need to make any management changes when using PAM? Yes. PAM maintains higher infiltration rates throughout an irrigation than is usual with untreated water, so if irrigation practices are not adjusted, PAM can worsen variability of infiltration from upper to lower field ends. If inflow rates are increased during initial advance and then reduced to the least sustainable flow once runoff begins, erosion is still greatly reduced and advance times shortened, thereby improving infiltration uniformity.
Since PAM use increases net furrow infiltration, water management may need to be adjusted to avoid over watering. In fields with steeply sloping furrows (>2%), infiltration tends to be lower and water normally advances rapidly down the field. Improved infiltration and longer furrow advance times resulting from PAM treatment are not likely to be a problem here. However, on very steep fields, PAM may increase net infiltration enough to warrant reducing irrigation set times, since more water goes into the soil in a shorter time. In fields with gently sloping furrows (0-0.5%) and especially for non-trafficked furrows, infiltration can be relatively high and advance times excessively long with PAM use, leading to nonuniform, down-furrow water application. This is particularly a problem when the inflow stream is not increased. PAM allows irrigators to increase inflows without increasing furrow erosion losses. Enlarging initial stream size greatly reduces advance time and equalizes infiltration opportunity times for the top and bottom of the field.
What if the irrigation water has silt in it? If there is a significant amount of sediment suspended in the water supply, PAM will cause it to flocculate and settle out almost immediately. A small settling pond at the top of the field may be necessary to prevent sediment from filling head ditches. PAM can cause silt to accumulate in gated pipe, particularly if the pipe is on a low grade, has low spots, or if the water is moving slowly. PAM can also cause nozzles in sprinkler systems to plug. Consequently, care must be taken when using PAM in gated pipe or with sprinklers.
How much does PAM cost? Expect to pay $3-$6 per pound of active ingredient. Pay attention to the percent active ingredient because it varies between the different forms and manufacturers. In general, granular form is the cheapest and liquid is the most expensive.