A Brief Overview
During the summer of 2000, irrigators in the Milk River area and Buffalo Rapids Irrigation District, which covers the area of the Yellowstone roughly between Miles City and Glendive, MT, got some first hand experience with PAM, the long, chain-like synthetic polymer which has been touted as the best thing to come along since sliced bread. PAM, the acronym for ‘polyacrylimide,' has become widely accepted and used for erosion control on irrigated soils, particularly silt loam soils like those common throughout the irrigation districts in Montana. PAM works by essentially casting a fine, webbed net around tiny soil particles and binding them together into larger aggregates. The net result is a more stable soil surface that can handle greater flow volumes with less erosion, while maintaining relatively high infiltration. So, in theory, an irrigator should be able to do one of the following:
  • Irrigate as usual and measure much less erosion or
  • Increase the stream inflow volume at the upper end of the field without excessive erosion, thereby improving the uniformity of water application across the field through better infiltration.

This sounds great, and, in fact, lab and field studies have shown it can be done. But, as you'll see, it doesn't always work this way. In a nutshell, PAM works wonderfully under the right conditions, and makes very little difference in other cases, as you'll see in the next couple pages. These trials employed 85% AI (active ingredient) flowable powder granules; 40% AI tablet, and 35% AI oil emulsion. The most suitable situation to use granular PAM is in row crops with the granules banded at the head of the row in a 3-8' long band, just below where the water enters the furrow from either gated pipe or siphon tubes. The tablets are about 1.5" in diameter by 5/8" thick. These were placed in the row at the head of the field, similar to the dry PAM. On one site, liquid emulsion was dribbled directly in the row, but that was not overly effective. Dripping the emulsion directly into the irrigation supply stream above the turnout to the field seems to be the most effective option.

A Quick Run Down of What We Learned
  • PAM doesn't make much difference on a field that has already been irrigated, packed by wheels, or sealed by more than ½" of rain. A rough or tilled soil surface is best. If a row has been packed by wheel tracks, cut back on the irrigation stream if PAM is applied, or avoid banding PAM in trafficked rows.
  • PAM increases infiltration rate, stabilizes (not seals) the soil surface, and reduces erosion under the right conditions, so you've got to make a few changes when you apply PAM. First, increase the inflow volume so the water can reach the end of the field. Second, reduce the length of time of the irrigation set.
  • It seems that the longest furrow length that can be effectively treated with a single dose of PAM banded at the head of the field is between 600' and 900'. If the runs are longer than that, the PAM rate must be increased to prohibitively high levels or split applied.
  • You can easily lose the effect of the PAM by burying or coating it with sediment, making it necessary to have good dispersion and mixing before the PAM enters the irrigated row.
Individual Results
Dena Hoff banded 85% AI (active ingredient) granular PAM at the rate of 1 pound/acre on 600' rows of corn, 1500' bean rows, and just below the siphon tubes on alfalfa in border dikes. Here's what she had to say: "On 1500-foot bean runs there was quite a bit less silting and washing where the PAM was applied. On 600-foot rows of corn on sandy soils with significant side slope, the PAM rows watered perfectly, with quite clear water throughout the entire irrigation. There was no silting up and little erosion. Water penetration in the middle and bottom third was pretty uniform, though the very top and bottom were different. The alfalfa was hard to draw any conclusions about visually."

Jim & Sherry Finneman used 85% AI granular PAM at 1 pound/acre on 500-800' rows of pinto beans on heavy, steep ground with side hill rows that typically erode quite badly. "We were totally amazed at the difference in the soil washed away. We didn't have the means to test for infiltration, but just the fact that the soil didn't wash and the water soaked to the top of the row sold us on this product. Hopefully, next year we'll be able to use it more."

Arnie Jerke tried 40% AI PAM tablets placed in a wire cage below a drop inlet to an earthen ditch, approximately 50 feet up-gradient from the first siphon tube. The goal was to introduce and mix the PAM in the incoming water before it reached the siphon tubes. This rate equated to about 2 pounds of PAM per acre on the 1450' beet rows. This trial was done on a field that had received about 3" of rain since the last field operation. Therefore, the entire field had a thick, stable crust, which protected the surface from erosion. Because of the stable crust caused by the rains, there was no recognizable benefit from the PAM application.

Jerry Buxbaum's 1700' corn rows got a dose of 4 oz./ row (or 1 lb/ac) of 50% AI "Retain" oil emulsion PAM dribbled in the furrows over the first 6-8'. Adjacent rows were banded with 85% AI granular PAM at the rate of 1 lb/ac. Gated pipe was used on a 12-hour set. The field had a hard ½ inch rain 7 days prior to the PAM demonstration. Also, in this demonstration, comparison was made between PAM treatment of furrows with wheel tracks versus those without wheel tracks.

Basically, this trial showed that PAM doesn't do much good where the furrows have been packed by wheel traffic. However, in rows which were still rough with alot of loose soil material, the PAM oil emulsion and the PAM granular had significant effects. The rate of advance of the irrigation water down the furrow slowed down, and infiltration increased. The average infiltration of the control rows was 22.1%, while that of the oil emulsion and granular rows was 50% and 53%, respectively. Despite the increased infiltration, soil loss was basically unaffected. The PAM treated rows ran quite clear for a short time at the beginning of the set, but then became loaded with sediment.

 

 
Average Infiltration
Soil Loss
Oil Emulsion
50%
5.52 tons/acre
Granular
53%
8.6 tons/acre
Control
22%
5.46 tons/acre

 

One can conclude from this that the PAM was either not reaching the end of the field or not in a high enough concentration to be effective. The 1700' run length offers somewhat of an explanation in that we can assume that it was too far the PAM applied in this fashion to be carried.

Trials at Bill Feickert's comparing 40% AI PAM tablets placed at the head of the row and 85% AI granular PAM banded in the row showed a significant difference between the two. Both the granules and the tablets were applied to the 600' rows at 2.8 lbs./acre, nearly three times the recommended rate. The tabs dropped in the furrow slowed erosion for a short time, but never made the water run clear. Within an hour, all of the rows with tabs were cloudy with fine sediment, and the furrow bottoms weren't visible. This suggests that the low AI % and/or the covering over of the tablets by sediment nullified any significant benefits of the PAM.

The granules worked for a longer time. At 45 minutes, 80% of the rows were still running completely clear. At 4 hours, 30% of the rows were completely clear, and by 5 hours, 25% were still running clear. Future trials could incorporate the use of a split application of PAM by placing part of the dosage at the top of the row and part half way down the run. This would give the water a recharge of PAM, hopefully improving the results. The conditions at this site were less than optimal in that the PAM did not appear to mix well with the water, leading to the conclusion that even mixing is necessary for good results.

The trial on Ray Strasheim's employed 35% AI oil emulsion "Pristine" PAM, which was added at the turnout with the calibrated drip mechanism, and a surge valve with the PAM-treated half on a 60% set and the non-PAM part of the field on a 40% set. The Pristine was added at a rate of 5 oz./furrow, equivalent to about 0.65 pound active PAM per acre. Ten of those rows had 1 oz./furrow of 85% AI granular PAM banded in a 2-3' band in addition to the "Pristine," making the total dose about 1 pound/acre of active PAM. The 1800' furrows contained sugar beets.

It took about 6 hours for the irrigation water to advance across 50% of the PAM treated rows and about 4 hours to advance across 50% of the non-treated rows during the initial wetting cycle.

 

 
Time 1st Row Wetted
Time 50% Rows Wetted
Emulsion + Granular
3 hrs. 20 min.
6 hrs.
Emulsion
3 hrs. 35 min.
6 hrs.
Control
1 hr.
4 hrs.

 

The average infiltration (the percentage of the applied water which actually entered the soil) was 72% for the Pristine + Granular, 82% for the Pristine only, and 60% for the non-treated rows. The difference between the Pristine + Granular rows and the Pristine only rows appeared to be due to differences in the slope or soil types between the two locations.

Not only did the Pristine and Pristine + Granular rows have slower advance times and greater infiltration, but they also had significantly less soil loss than the control rows. The PAM rows ran completely clear until 1400-1500 feet, where they started to cloud up. The control rows carried a great deal of sediment from top to bottom. Runoff from a PAM Treated Furrow Runoff from an Untreated Furrow

The following table shows the amount of soil lost from the measured rows in the first 2.5 hours of the irrigation.

 
Soil Lost in First 2.5 Hours of Irrigation
 
Emulsion + Granular
  Emulsion  
     Control     
Pounds of Soil Lost
200
254
3504
(1.8 tons)

 

This means that in the measured rows, erosion was reduced by an average of 94%.

Probing 24 hours after the irritation was complete showed that water had soaked in past 3 feet throughout the PAM treated rows. In the control rows, water had penetrated to 3' until 1400 to1600 feet down the field. From 1600 feet to the end, water had penetrated only 1-2 feet.

John Lacey, a Milk River irrigator, tried the PAM tablets on alfalfa in August, just after his second cutting. He irrigates large panels of alfalfa with graded borders and turns the water in with 5" siphon tubes. John reported that he initially had trouble getting the tablets to dissolve, because he put them in a fine mesh basket in front of the siphon tube outlet. He then constructed a cage made out of refrigerator racks, which worked much better. In his initial attempt, where he didn't get good dissolving action, it took 6 hours to irrigate one panel. By the time he figured out how to make it work, it took 8.5 hours to irrigate a panel. John concluded, "I assumed more water infiltration. Where else could it have gone?"

How About a Recap?
Overall, here's what we learned from the summer 2000 PAM demonstrations.

 

    • PAM doesn't make much difference on a field which has already been irrigated or which has been packed or sealed over by more than ½" of rain and then dried out before the PAM application. You'll get the most miles for your money using PAM with the first irrigation after your last field operation, assuming it doesn't rain first.

 

    • If you use PAM and want to take advantage of all the benefits, increase the volume of water you turn out into each furrow to which you apply PAM . Otherwise, your water probably won't make it to the end of the field. With more water infiltrating the soil, reduce the length of time of the irrigation set to avoid overirrigation.

 

    • It looks like the longest furrow length we can affect with a single treatment of PAM at the head of the field is between 600' and 900'. If the runs are longer than that, either increase the PAM rate or split apply the PAM.

 

    • Make sure you have good dispersion of the PAM and good mixing before the PAM enters the irrigated row. You need to have a method of applying the PAM to insure that it is well-mixed in the water or strategically distributed through the entire length of the irrigated furrow. The best mixing seems to come from use of the emulsion in the headgate or supply canal.

 

  • Don't apply PAM (or waste your money) in furrows which have been packed by wheel traffic. In those rows, cut back on the irrigation stream if PAM is applied in the supply stream.