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What a waste! Phosphogypsum enriches the soil.

The huge, unsightly mounds can stretch across hundreds of acres. Some covered, some uncovered--these mountainous stacks can be an eyesore on the flat, sandy Florida skyline. Whitish gray in color, with a crusty surface look like massive heaps of table salt that tower up to 200 feet high.

This is Florida's stockpile of phosphogypsum. More than 600 million tons of it are already on the ground and an additional 30 million tons accumulate yearly.

When mined phosphate rock is treated with sulfuric acid, the result is phosphoric acid. which is used to make fertilizer. Phosphogypsum (PG) is a byproduct of the process.

Phosphogypsum has been used in construction materials and as a base in building roads. Its largest value, however. might be agricultural.

"This material could be used beneficially as a soil amendment." says Stanley Nemec. "We applied it for 3 years, at up to a ton per acre, on three orange groves with good results."

Not only did the PG improve overall tree health, but it also increased the calcium and manganese levels in the fruit juice and calcium levels in leaves and bark. "Citrus is a heavy user of calcium, needing more of it than many other plants," says Nemec.

An ARS plant pathologist with the U.S. Horticultural Research Laboratory in Orlando, Florida, Nemec collaborated with University of Florida scientists Don Myhre and Ron Sonada.

"One finding was surprising: PG can probably reduce foot rot, a major citrus disease in Florida that causes cankers on tree trunks and rots the roots." Nemec says.

The high concentration of calcium in PG apparently strengthens tree cell walls against invasion by this fungus-borne disease, he says.

"This unexploited source of calcium could also help avoid drought stress by facilitating deeper penetration of calcium--and consequently, of roots--into the acid subsoils of the Southeast," says Doral Kemper, ARS national program leader for soil management.

But the use of phosphogypsum has been controversial because it contains a small amount of radium. Therefore, until consumers understand that they are not endangered by crops grown on PG-treated soil. most citrus growers hesitate to use PG.

The U.S. Environmental Protection Agency has also been cautious. Under a section of the Clean Air Act, it banned the use of phosphogypsum in April 1990 but issued a waiver for agricultural use through the rowing season that ended October 1, 1991.

Then EPA issued a final ruling in the June 3, 1992, Federal Register, says Craig Conklin, health physicist with EPA's Office of Radiation Programs.

"The new ruling explicitly permits the controlled use of phosphogypsum in both agriculture and research and development," he says.

According to Conklin, "controlled use" means that when PG is used for agriculture, the average radium-226 level must not exceed 10 pCi/gram. This amount translates to 0.01 parts per billion, or the equivalent of less than one drop of water in two Olympic-sized swimming pools. Anyone who distributes PG must certify its radium-226 level in writing.

"We tested the fruit from our 3-year experiment with soil enriched with phosphogypsum and found no increase in levels of radium 226 from normal levels in fruit grown in untreated plots," Nemec reports.

Since Florida has set up the Institute of Phosphate Research to look for ways to use this abundant waste product, Nemec hopes his collaborative research results will provide new potential PG uses and complement the Institute's goal. Michael Lloyd, research director for the Institute, supports Nemec's studies.

Soil Benefits

"We've been trying to tell people about the benefits of PG for years. Phosphogypsum is a superior amendment for many soil types and is an excellent fertilizer source of sulfur and calcium," Lloyd says. Any soil that tends to surface-harden and resist water, he contends, would benefit from PG. "It creates a more porous texture that soaks up rainfall."

Although other sources of sulfur are available, Lloyd says PG is ideal because it is only slightly soluble in water and therefore longer-lasting in the soil. Since the sulfur in PG is present in the form of sulfate, it can be absorbed directly by the plant; sulfur from most other sources must first be converted to the sulfate form before the plant can use it.

And, being neutral in soil reactions, sulfur from PG doesn't change soil acidity.

"This would be a real boon for citrus growers. They now use limestone, which raises the soil pH and may be a factor causing plant nutrient problems," Nemec says.

Lloyd explains that gypsum has also been used to reduce toxic elements in the soil. When present in heavy concentrations, almost any soil element can be toxic to plants.

Abnormal levels may occur naturally or may be introduced by the application of fertilizers, insecticides, or fungicides. In some cases, PG can inactivate toxic soil elements or, in other cases, mobilize them so they are easily leached from the soil.

Research is under way to determine how different elements will be affected by PG additions.

"Farmers have used phosphogypsum as a soil amendment for years and have come to rely on it," says Ron Phillips of the Fertilizer Institute, Washington, D.C.

Peanut grower Hubert Dollar of Bainbridge, Georgia, is one of those farmers. "I use from 1,000 to 2,000 pounds of phosphogypsum per acre for each peanut crop. To mature properly, peanuts need lots of calcium. PG is the cheapest source," Dollar says.

He's been adding PG to his soil in similar amounts for about 20 years, and his father did the same for many years. Last year, Dollar had tests run on his soil, peanut plants, and peanuts. Result? "No sign of any radium," he says.

Although the growing piles of phosphogypsum may pose an environmental problem where they are stored, use in the agricultural sector could reduce the stockpiles and benefit growers.--By Doris Stanley, ARS.

Stanley Nemec is at the U.S. Horticultural Research Laboratory, USDA-ARS, 2120 Camden Road, Orlando, FL 32803. Phone (407) 897-7300, fax number (407) 897-7309.
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Author:Stanley, Doris
Publication:Agricultural Research
Date:Oct 1, 1992
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