Pick up some PAM to preserve the soil surface

A recent edition of Agricultural Research highlighted efforts by scientists from the US Department of Agriculture to establish just how effective PAM is in preserving soil. “PAM has a long history of use,” USDA researcher Rick Lentz told edie. “A different version of the polymer was used way back in the fifties to help stabilise the soil, but it was expensive and not cost-effective. We’ve developed an application method that requires only a kilo of PAM to treat a hectare of land. More and more farmers are now mixing it into the water they use to irrigate crops.”

PAM is large linear molecule that binds the soil surface by attracting soil particles to the negative charges that occur along its length. It is usually sold as a dry granule which is mixed into irrigation water. When untreated water is released into furrows to irrigate crops, the flow of water typically carries the soil surface away with it, leading to erosion and nutrient run-off into nearby rivers. PAM holds the soil in place and acts as flocculating agent for stray particles floating in the water.

“Soils having low aggregate stability are easily eroded, leading to phosphorus and agrochemical pollution of nearby streams,” says Dr Lentz. “Agricultural land is also prone to soil erosion at the start of the irrigation season, when furrows are newly formed and the top soil is loose. We advise applying PAM every time furrows are formed, usually once or twice a year, depending on the crop.”

But with more than a million acres of US land being treated with PAM every year, is there a danger of the chemical building-up in the soil? “Research indicates that PAM degrades by about 10% a year, and eventually breaks down into harmless CO2,” says Lentz. So no fear of the polyacrylamide reverting to its starting component – acrylamide, the cause of much concern following the Swedish study of acrylamide levels in roasted foods (see Food Standards Agency).

While single acrylamide is carcinogenic, the polymer is non-toxic, and there has been little evidence to suggest that the polymer is likely to breakdown into its carcinogenic cousin. “We did a study monitoring the percolation of nutrients, pesticides and acrylamide through the soil following PAM application. There was no difference in the amount of percolating soil water or chemicals between treated and untreated land. In other words, PAM did not encourage the leaching of phosphorus, nitrates or pesticides, nor was their any evidence that acrylamide had regenerated from PAM and moved downward in drainage water.”

Nevertheless, Dr Lentz advises using food grade PAM on crop land. “We have safety limits established from water treatment practices, where food grade polyacrylamide is used, which contains less than 0.05% acrylamide. The same type of PAM should be used in agriculture, and its instructions for use carefully followed.”

As well as drastically reducing soil erosion, PAM has had powerful effects on microbes commonly leached from farm feeding and animal waste sites, such as E coli and fecal streptococci. Microbes floating in irrigation water will be settled out by PAM, leading to cleaner streams and rivers around grazing areas.

But in terms of chemical pollution, PAM checks pollutants only insofar as it prevents soil particles from being swept into the waters. The molecule has little effect on soluble nitrates – in the long run a desirable property since it is also less likely to interfere with essential nutrients in the soil. Researchers have recently been experimenting with combinations of PAM and aluminium sulfate or calcium oxide to further contain phosphorus in the soil. These combinations have also reduced microbial populations by up to 90%, meaning few pathogens in the run-off.

Polyacrylamides are best known for their flocculating abilities in waste water treatment. The EU uses 50,000 tonnes a year to treat municipal drinking and waste water. PAM can also be found in soaps and cosmetics, where they act as thickeners. Produced from natural gas, PAM is frequently used as an additive in papermaking, grouting, gel setting and plastics. The EU has set stringent limits for the amount of acrylamide allowed to migrate from plastic packaging into food, currently at no more than 0.01 milligrams per kilogram of food.