Stopping Erosion with Gypsum and PAM

In the late 1700s, Benjamin Franklin, whose interest in scientific experiments is now legend, demonstrated the value of a natural geological substance called gypsum, used in making plaster, as a soil amendment.

On a prominent hillside, Franklin applied gypsum in a word pattern that read, "This land has been plastered." The increased grass growth in the area on which the gypsum had been applied served as an effective demonstration of its value as a fertilizer.

Two hundred years later, gypsum is again being studied—this time as a way of controlling erosion by increasing water infiltration.

"New technologies to improve air quality have produced more and more gypsum byproducts with potential for beneficial use in agriculture," says ARS soil scientist L. Darrell Norton. He is at the National Soil Erosion Research Laboratory (NSERL) in West Lafayette, Indiana. "The removal of sulfur from flue gases in coal-fired power plants has resulted in immense stockpiles of these byproducts that can supply sulfur to crops and may also serve as a liming agent."

"Each year, power plants produce about 100 million tons of gypsiferous material, high in calcium and sulfur, as a byproduct of capturing sulfur dioxide emissions," says W. Doral Kemper, who formerly led the ARS national program in soil management research. "That's enough to apply a ton per acre to a quarter of U.S. farmland."

Norton and Kemper believe that recycling these low-cost byproducts of industry to control erosion and increase yield is a win-win situation. They say gypsum can be used on many soils nationwide to improve water infiltration and help plant growth.

The two believe the increased yields are explained by studies conducted by ARS soil scientists K. Dale Ritchey at Beaver, West Virginia, and Ronald F. Korcak at Beltsville, Maryland, and their coworkers. These studies have shown that calcium from gypsum applied to acidic soils gets down into the subsoil where it is needed, so crop roots can grow deeper and access more water.

Norton says, "Using gypsiferous byproducts would give farmers a low-cost remedy for acid, sodic, and erosion-prone soils."

Several years ago, ARS scientists at the NSERL worked with world-renowned soil scientist Isaac Shainberg, who is now director of the Volcani Institute in Israel. They wanted to determine how electrolytes, which are natural electrical conductors in rainfall and runoff water, could affect estimates of soil erodibility. During Shainberg's visit, the idea of using gypsum and other soil amendments to control soil erosion by water from agricultural fields was also discussed.

"We suspected that gypsum could reduce surface sealing and improve water entry and reduce erosion," says Norton. "In lab studies, we found the powdered waste product releases electrolytes that keep clay particles clumped together, reducing crusting."

About the same time, Shainberg became interested in another white powder—PAM, short for polyacrylamide—a material used in water treatment plants as a flocculent to clean up the water by precipitating small particles.

"According to the literature, you could stabilize a soil with PAM, but it was very expensive for practical use when mixed in the entire plowed layer," says Norton. A series of lab studies conducted by Shainberg and ARS scientists on some small flumes confirmed that as little as 5 to 10 parts per million of PAM mixed with water almost eliminated rill erosion—the tiny gullies caused by water moving over the soil.

At the NSERL, Norton and Shainberg, working with three ARS colleagues—soil microbiologist Diane E. Stott, agricultural engineer John M. Laflen, and soil scientist Joe M. Bradford—studied how adding PAM both to simulated rainfall water and to the soil surface affected erosion. They soon came to recognize that if the soil surface could be stabilized down to just a very small depth, erosion might be greatly reduced.

"Most important," says Norton, "was the finding that PAM didn't have to be mixed into the soil. Only the surface layer—less than the top one-sixteenth inch or less of soil—has to be treated, to let water into the soil." NSERL agricultural engineer Dennis C. Flanagan, working with Norton and Shainberg, conducted field tests to examine how effective both gypsum and PAM could be at controlling soil loss on a steep and erodible silt loam soil. They tested surface applications of 2.2 tons per acre of gypsiferous byproduct from the Purdue University power plant in West Lafayette, Indiana, as well as a liquid solution of 18 pounds per acre of PAM sprayed on the soil and allowed to dry.

They found that the byproduct improved infiltration and could potentially reduce runoff and erosion problems on similar U.S. soils. The PAM surface treatment was very effective at controlling rill erosion, even for water inflows up to 16 gallons per minute per rill.