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.