Waste wallboard shows promise for the farm

One day those unused scraps of clean wallboard from construction sites and remodeling projects may be crushed and spread on agricultural fields.

Construction industry sources estimate that building a 2,000 square-foot house produces about a ton of clean wallboard waste. An estimated 2 million to 3 million tons of gypsum wallboard scraps are put into U.S. landfills each year. Spreading wallboard on land would be yet another way to extend the life of landfills, while recycling the material’s plant nutrients.

Because wallboard is classified as a solid waste, spreading it on agricultural lands is currently regulated by the Wisconsin Department of Natural Resources. However, recent UW–Madison research may answer regulators’ concerns about the practice.

“In a 3-year study, we found that applying crushed wallboard has neither strong positive nor negative agronomic effects on alfalfa production,” says Richard Wolkowski, a soil scientist in the College of Agricultural and Life Sciences.

“Gypsum wallboard is primarily calcium and sulfur with much smaller amounts of other elements such as magnesium,” Wolkowski says. “The paper is easily screened off after grinding. Therefore, crushed wallboard is very similar to commercial gypsum fertilizer, a soil conditioner and fertilizer that supplies calcium and sulfur to soils.

“A few Wisconsin crops such as potato may respond to the calcium in gypsum fertilizer,” Wolkowski says. “But it is the sulfur in gypsum that is more often beneficial to crops and gypsum is one of the more economical sources of sulfur. Where tests show sulfur is needed, we recommend 10 to 25 pounds of sulfur per acre for corn and up to 50 pounds of sulfur per acre for alfalfa.”

Since other scientists have studied the effect of crushed wallboard on corn and soybeans, Wolkowski turned his attention to its effects on alfalfa, a crop that covers more than 3 million acres in Wisconsin. “Historically we’ve seen responses to sulfur on light-textured soils that are low in organic matter,” Wolkowski says. “These soils can be found in central and northwestern Wisconsin in cropping systems where manure is not applied. Semi-solid dairy manure supplies 1 to 2 pounds of sulfur per ton.”

Working at the College’s research stations at Arlington, Lancaster, Spooner, and Ashland, Wolkowski studied how application rates and methods affected alfalfa yield, stand density, soil test, and forage nutrient content. He applied crushed wallboard at rates ranging from 1 ton per acre to 16 tons per acre before planting alfalfa, or spread the material over the crop in spring at rates that ranged from 250 pounds per acre to 1 ton per acre.

“Alfalfa yields tended to increase with increases in wallboard application rates at three of the four sites,” Wolkowski says. “However, those yields were not significantly greater than yields on plots that did not receive wallboard or agronomic rates of commercial gypsum fertilizer. The rates we applied did not affect alfalfa stand density.”

The highest levels of crushed wallboard tested did alter soil chemistry, according to Wolkowski. It increased the calcium and sulfur in soils while decreasing the magnesium and pH. The analysis of plant samples showed similar results. Tissue sulfur usually increased and was significantly higher in treatments at Spooner all three years.

The calcium in the wallboard binds to soil particles, displacing some of the hydrogen and magnesium found there, according to Wolkowski. The movement of hydrogen appears to make the soil more acidic by 0.2 or 0.3 of a pH unit, he says, although this is misleading and does not mean the soils need lime. The displaced magnesium leached out of the upper soil layer, a more serious concern.

“Even though crushed wallboard contains a fair amount of magnesium, the soil at Spooner nearly became deficient in magnesium where we applied the highest rates of crushed wallboard,” Wolkowski says. “At Spooner the plant magnesium levels decreased, sometimes below threshold. If farmers use crushed wallboard, they should check magnesium levels in alfalfa because feeding forages low in magnesium can lead to grass tetany.”

The study concluded that crushed wallboard was as effective as commercial gypsum fertilizer.

“We’d suggest that if farmers apply crushed wallboard to alfalfa fields that they limit rates to 1 ton per acre on sands and loamy sands, and 3 tons per acre on loams and heavier textured soils,” Wolkowski says. “Application sites must be approved by the Wisconsin Department of Natural Resources. One application at those rates, incorporated before seeding or topdressed after cutting, every 3 to 4 years will be adequate to supply forage legumes with sulfur.”