PAM Benefits for Irrigated Cropland
Could “PAM” translate into good news for irrigated sunflower producers? Though the great bulk of U.S. sunflower is grown under dryland conditions, there are thousands of ’flower acres — particularly in the High Plains — under either sprinkler or furrow irrigation. Some of those fields doubtlessly could benefit from a close relationship with PAM.
In this case, however, “PAM” has nothing to do with the female gender. It stands for polyacrylamide — polymer chemicals whose use on irrigated U.S. cropland rocketed from around 50,000 acres in 1995 to an estimated 400,000 to 500,000 acres in 1996. Significantly more growth is expected during 1997.
These chemicals, available in either dry granular or liquid emulsion form, have displayed an impressive ability to reduce sediment loss during irrigation while simultaneously preventing soil crusting and increasing the efficiency of water infiltration into the field. Use of PAM is considered an acceptable soil conservation practice by USDA’s Natural Resources Conservation Service (NCRS) in several western states, and it currently has “interim standard” status on a national basis.
While the use of PAM to date appears to have been highest in the Pacific Northwest and along Colorado’s Front Range, agricultural interest in polyacryl-amides is indeed national — one reason being that PAM products work well under a wide range of soil types and application methods. Irrigated production has driven ag usage thus far, but some researchers envision dryland applications as well — one example being soil erosion control and stand establishment benefits for fall-drilled wheat fields.
Robert Sojka is a USDA-ARS soil scientist at Kimberly, Idaho, whose ties to sunflower-related research stretch back to his days at North Dakota State University. Sojka has investigated PAM agricultural applications since 1991. While initially skeptical about some of the claims made on its behalf, he now says his own research, coupled with that of other scientists — and the experiences of farmers with whom he’s visited — has made him a believer. Here are a few of the key findings:
• USDA trials under furrow irrigation resulted in reductions in sediment loss of as high as 99 percent when a field was treated with PAM. (In the first three years of tests at Kimberly, field sediment loss was slashed by an average of 94 percent.)
• Net infiltration of irrigation water averaged 15 percent more with PAM (compared to untreated furrows). That was paralleled by an approximate 25-percent increase in lateral wetting of furrows, due to minimized “down-cutting” at furrow bottoms.
• Other scientists have recorded substantial reductions in pesticides, nutrient concentrations and oxygen demand in return water flows — a very positive environmental benefit, particularly as nonpoint pollution source regulations become more strict.
• These types of benefits have been achieved with a very modest amount of product: the equivalent of one pound of granular form per acre per treatment, i.e., a desired concentration level of 10 ppm in the advancing irrigation stream (only).
Up to this point, the most common method of applying polyacrylamides in furrow-irrigated agriculture has been placing the dry formulations “in the head ditch through a metered applicator,” Sojka explains, “and then, depending on mixing in the ditch, to predissolve the material prior to entry into the furrow.”
Another avenue has been the “powder patch” method. “That’s a direct application of a patch at the head of the furrow in an approximate one-foot circle right under a spigot,” Sojka notes. “Or, sometimes growers will sprinkle it down the furrow for several feet.”
While the majority of applied PAM has been in the form of dry granules, liquid formulations are becoming increasingly popular. These inverse emulsions use a petroleum distillate as a carrier for the PAM, thereby allowing the attainment of either a 30- or 50-percent active ingredient in the liquid form. “They also have a small amount of surfactant in it, so when the reverse emulsion is in contact with water, it releases the PAM solution,” Sojka notes. Though the liquid products do cost more than dry materials, their ease of use and thorough dissolution in water make them attractive to many producers.
Overhead irrigation is a potentially huge market for the liquid PAM formulations. “Right now, the big interest is in center pivots and linear move systems because they’re the most amenable to injection with these kinds of materials,” according to Sojka. During visits last summer with farmers in Washington’s Columbia Basin, he found them typically putting on the liquid at the equivalent of two to four pounds per acre in one or two initial irrigations.
One of PAM’s most important attri-butes for furrow irrigators — the ability to prevent “sealing” of the soil surface, thus allowing increased water infiltration — also was apparent under the Washington overhead irrigation systems. “At several sites, because a seal was forming on the first one or two irrigations, [growers previously] could never get water to go in on some of the steeper slopes,” Sojka explains. “Or, on the far end, where they were putting on large amounts of water, it would just run down the row, resulting in a very uneven distribution of water.” Where PAM was used, however, growers saw no running water; plus, because of the improved infiltration, they were able to reduce overall irrigation amounts.
Sojka and colleagues Kris Aase and Dave Bjorneberg have initiated laboratory studies to document some of the observations they’ve made in commercial fields. They’re using an irrigation simulator to compare various application strategies, soil types, slopes and other parameters to come up with refined data for sprinkler application.
Preliminary discoveries have been encouraging. “We’re getting good results at between 0.75 pound and 1.5 pounds of polyacrylamide per acre,” Sojka reports. The USDA researchers also have documented a 50-percent reduction in sediment in the runoff and a considerable increase in tension infiltration values through the soil surface — plus, a large reduction in overall water runoff.
Sojka says the use of polyacrylamides is very consistent with the accelerating trend toward precision farming — and actually takes the site-specific concept one step further. Precise placement of inputs (e.g., water, fertilizer) under sprinkler irrigation systems is only part of the answer, he illustrates.
“You can spend lots of time and money implementing precision placement,” the USDA soil scientist explains. “But if the water is running ‘willy-nilly’ up and down your field and infiltrating in oddball patterns, all your efforts to diagnose your field and precision-apply materials can be completely upset — thereby failing to meet the potential of that technology.” — Don Lilleboe
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