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Erosion takes a powder.

An inexpensive white powder can help save valuable topsoil and stem water pollution. Agricultural Research Service scientists say the powder--called polyacrylamide (PAM)--prevents irrigation water from carrying away soil particles as the water flows down a furrow. That could help halt the loss of more than 2 billion tons of soil that wash off the world's 600 million acres of irrigated croplands each year.

"Sediment loss has been reduced by up to 97 percent and water infiltration increased by about 25 percent during our tests, which started in 1991," says Robert E. Sojka of ARS' Soil and Water Management Unit in Kimberly, Idaho. Soil that erodes from fields costs farmers in lost productivity and it damages downstream environments.

"The PAM treatment also keeps fertilizers and other chemicals--as well as the soil--on fields where they belong," he adds. "That means the chemicals can't run off and harm fish and other aquatic life."

Sojka and fellow soil scientist Rick D. Lentz have found that as much powdered PAM as there is in a shake of salt from a salt shaker can treat a gallon of water. That's about 1 pound of powder per acre-inch of water. At about $1.25 per pound, an average irrigation costs $2 to $3 per acre.

They have run tests on corn and edible dry bean fields to learn the best time to apply minimum amount of PAM needed to stop the most soil loss.

The treatment has the potential in the United States to save soil on millions of furrow-irrigated acres of crops. With this system of irrigation, shallow trenches between crop rows transport water across fields.

PAM is a polymer--a chemical made up of molecules strung together in a chain--like polyethylene. Using polymers in irrigation water for erosion control was originally proposed by a visiting Israeli scientist, Isaac Shainberg.

PAM is nontoxic and nonirritating, according to scientists at Cytec, a division of American Cyanamid Corporation in Stamford, Connecticut, which makes the powder. It breaks down into water, carbon dioxide, and ammonia. A host of environmental factors control the breakdown rate, including the types and amounts of various microorganisms that degrade organic molecules and the availability of water and oxygen.

Water treatment facilities around the country already use polyacrylamide to remove suspended sediment as part of the purification process, explains Lentz. It works by making suspended particles stick together and fall to the bottom of the body of water, so they can be easily removed.

"We need to learn more about how PAM protects soil in irrigated furrows. The substance doesn't change how the water looks or feels, but it may alter the water's physical properties in subtle ways," says Sojka.

The scientists know that PAM-treated irrigation water greatly increases the bonding among soil particles in the wetted furrow. This strong bond prevents soil loss as water flows along the furrow.

The PAM treatment will be targeted to applications that will prevent the most erosion. These would be on fields with steeper slopes or for crops like edible dry beans and cotton that produce little soil-holding residue.

The researchers believe that at least 2 to 3 years of additional studies by soil scientists, chemists, and microbiologists are needed to test the long-term effects of the new treatment on soil and the fate of any treated water that may, run off fields.

They are currently studying whether the treatment can benefit furrow-irrigated Russet Burbank potatoes. Some growers are switching to sprinkler irrigation because high-quality potatoes result only if they receive timely and even water delivery. Furrow-delivered water infiltrates soil less evenly, sometimes failing to sate thirsty tubers.

PAM-treated water from furrows might get to potato roots more evenly and produce high-quality potatoes comparable to sprinkler-irrigated ones. This could cut production costs, because furrow irrigation usually requires a lower capital investment and uses less electrical energy or fossil fuel.

Researchers are cooperating with Sherm Brewster, a local Idaho entrepreneur, to develop inexpensive equipment for adding PAM to water. One promising machine is powered by a water wheel that is rotated by waterflow in irrigation canals. They will also explore solar, battery, and direct-wired applicators.

The scientists believe PAM application is technologically easy enough to help developing countries that lack money and expertise needed for more complex anti-erosion measures such as land grading or changing to other crops.--By Dennis Senft, ARS.

Robert E. Sojka and Rick D. Lentz are in the USDA-ARS Soil and Water Management Research Unit, 3793 N, 3600 E, Kimberly, ID 83341; phone (208) 423-6562, fax (208) 423-6555.
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Title Annotation:polyacrylamide powder prevents loss of soil particles during irrigation
Author:Senft, Dennis
Publication:Agricultural Research
Date:Sep 1, 1993
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