Apr. 8, 1998 WEST LAFAYETTE, Ind. -- Purdue University research is helping farmers choreograph a balancing act with phosphorus, giving plants and livestock just enough of the nutrient so they grow properly, but no more.
That's important because excess phosphorus can find its way into surface water, where it causes algal blooms and fish kills and hastens the aging process in lakes.
Part of Purdue's role has been to identify the problem. The next step is to help farmers use phosphorus efficiently.
U.S. researchers knew years ago that excess phosphorus in rivers could cause trouble, but they thought most of it came from urban and industrial waste, not farm fields.
"Even 10 years ago the prevailing thought was that phosphorus in fields didn't move, unless it eroded away with soil," says Purdue agronomist Brad Joern.
Phosphorus already has been banned from detergents and laundry soaps. But enough phosphorus was still making its way into streams last fall to warrant federal attention. An Oct. 18, 1997, message from Vice President Al Gore directed the Environmental Protection Agency to "identify the major sources of nitrogen and phosphorus in our waters, and identify actions to address these sources."
Even before federal eyes turned back to phosphorus, Purdue researchers suspected that old theories about the nutrient's movement weren't accurate. In 1992, Joern started measuring phosphorus levels in fields and found that phosphorus can move down through soils.
"Fields with a long history of phosphorus loading from manure or other fertilizer sources can lose phosphorus in surface water runoff and potentially even through tile drains three feet down," Joern says. From there it can be a quick trip to a neighboring river or lake.
Joern and colleagues from Michigan and Ohio are developing a set of new environmental guidelines that should keep phosphorus out of surface water while still giving good crop yields. The guidelines will tell producers how much manure they can spread on land without over-applying phosphorus.
The guidelines will help, Joern says, but as they start to measure phosphorus in manure, some livestock operations will find that they produce more manure than they can spread on the land they own.
"When manure is applied to meet the nitrogen needs of the crop, it exceeds the phosphorus requirements for the plants," says Purdue animal scientist Alan Sutton. "Repeated applications of manure on the same fields can result in a significant buildup of soil phosphorus, which might increase the potential for surface water contamination."
In part because of phosphorus, the practice of spreading manure on farm land is coming under increasing scrutiny.
At the same time that Gore sent out his October message, the Environmental Protection Agency presented a Draft Proposed Strategy for Strengthening Nonpoint Source Management. In the proposal, the agency said it "will expand the scope of regulatory requirements to address application of animal waste."
Farmers historically measured the nitrogen content of manure, then based field applications on the nitrogen need of the crop. In the future, it's likely farmers also will have to measure the phosphorus in manure before they spread it. Then they'll apply the manure at levels that avoid over-application of either nutrient, and phosphorus may become the limiting factor.
When farmers base manure applications on phosphorus content, they'll only put on one-quarter to one-half as much manure, or spread manure on fields only once every three or four years instead of every year, Sutton says. They also will have to find more land to handle the excess manure, and they'll apply commercial nitrogen to meet the crop's nitrogen needs. That will take more time and money, and the additional tractor trips across fields to spread the nitrogen will increase the chances of compacting soil.
Purdue researchers are looking for alternatives. For example, they tried to reduce the phosphorus content of manure by changing animals' diets. Because Indiana producers are high on raising hogs, that's where they've turned their attention.
Sixty percent to 80 percent of the phosphorus in most grain fed to hogs is in a form that passes right through animals, never getting picked up and used by their bodies.
Joern and animal scientist Layi Adeola are testing a new type of corn that is naturally higher in easy-to-absorb forms of phosphorus and low in hard-to-absorb phosphorus. Such corn could reduce phosphorus levels in manure and decrease the need for supplemental phosphorus in animal diets. Joern says the preliminary results look promising.
In separate but related research, Sutton and Adeola fed pigs a diet containing the phytase enzyme, which makes hard-to-absorb phosphorus in grain easier for pigs to absorb. In their tests, pigs fed the phytase enzyme and a low-phosphorus diet gained weight and grew just as well as pigs on a normal-phosphorus diet. Pigs could better use the phosphorus in the grain, so less passed into the manure. Manure from the pigs that ate the phytase enzyme held a balance of nutrients that more nearly matched crop needs, a balance less likely to leach phosphorus into water.
Overall, Joern says, the phytase enzyme and the new, high-available phosphorus corn look like good bets for farmers who have lots of manure to spread and little land to put it on.
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