Nutrient double locks up phosphorus and boosts soil
Gypsum dances to a molasses tempo. As the ultimate slow-release fertilizer, its use in agriculture dates back to colonial times. Beyond recognized soil benefits, gypsum is proving to reduce phosphorus runoff and curb the spread of pathogens such as E. coli.
Phosphorus is under indictment as a significant cause of algal bloom and dead zones in U.S. waterways. With the looming possibility of nutrient runoff mandates, gypsum might offer farmers a practical means to ease regulatory pressures.
Research leader Allen Torbert and his soil science colleagues at the National Soil Dynamics Laboratory in Auburn, Ala., are making major research strides in reducing phosphorus runoff. In essence, they’re using gypsum as a glue to bind phosphorus to soil and reduce runoff by 50% to 60% when combined with manure.
When manure applications place a huge quantity of phosphorus on the soil surface, soluble forms are subject to runoff. However, a gypsum application can hold phosphorus in the soil.
“When gypsum is spread, you apply a soluble form of calcium,” Torbert explains. “The calcium and soluble phosphorus bind into insoluble forms.”
Gypsum needs to go on within three days of litter application, he adds. Efficacy is greatly reduced if it’s applied before litter. He suggests 2 tons per acre. (Torbert’s latest research focuses on poultry litter, but earlier work found consistent results with dairy manure.)
“We used a pure gypsum form, and it spreads similar to lime,” Torbert says. “It bridges more than lime and you need to give the spreader more attention, but application is no problem.”
In addition to providing calcium and locking up phosphorus, gypsum boosts soil structure. As infiltration improves, phosphorus moves downward into soil instead of a lateral slide into runoff. In tandem, microorganisms such as E. coli are held in soil and have less of a chance to escape.
“Most people don’t realize plants needs calcium and sulfur, sometimes in greater masses than nitrogen, phosphorus and potassium,” says Darrell Norton, a retired soil scientist with USDA–Agricultural Research Service National Soil Erosion Research Laboratory at Purdue University. “Calcium builds strong cell walls in plants. Sulphur is needed for photosynthesis and provides necessary lipid synthesis.”
Gypsum can help manage water, Norton adds. “In flooded soils, gypsum can keep plants alive longer and keep roots from dying,” he says. “Just soluble calcium alone has doubled the biomass of plant roots and increased surface area by a factor of 10 in some soils. That allows a plant to take up more water from the same volume of soil during dry periods.”
Some producers mix gypsum into liquid manure and immediately apply it across fields. Norton warns against putting gypsum on ground where livestock are set to forage. High sulfate content can cause rumen issues in cattle and, in some cases, death. “Don’t turn cattle loose after application until a few inches of rain have fallen,” he says.
“As the natural gradual-release amendment, gypsum only dissolves when it rains, and it helps soil in so many ways,” Norton says. “It’s a Swiss army knife for farmers.”