A new study estimates that current manure application guidelines in approved nutrient management plans provided to farmers result in tens of millions of pounds of excess phosphorus being applied to croplands in the watershed, greatly increasing the risk of pollution to local waterways and the Bay.
The study estimated that manure containing about 37 million pounds of phosphorus beyond what's needed to grow crops was applied to fields in the 11 counties with the most intense animal agriculture within the Bay watershed annually.
Part of the problem, according to the Annapolis-based nonprofit organization Water Stewardship, which completed the study, is the widely used phosphorus site index, the guide that farmers use to determine how much manure to spread on their field. According to the study, the index allows the continued application of manure on lands already above a critical phosphorus saturation level.
The scientists who did the research called for switching to a strategy that would bar further manure applications on fields that exceed a certain phosphorus saturation threshold based on soil test results. They acknowledged that alternative uses would have to be developed for huge amounts of manure that could no longer be applied to fields.
"Our biggest recommendation is that you need alternative uses" for manure, said Tom Simpson, the executive director of the nonprofit, a retired soil scientist from the University of Maryland and former chair of the Bay Program's Nutrient Subcommittee.
Concern about phosphorus buildup in soils has increased for areas with large animal populations. As animal numbers have increased, manure is often applied to land at disposal rates in nutrient management plans, leading to a buildup of phosphorus in soils.
Even when used as fertilizer, the phosphorus in manure is problematic. Typically, manure has a greater phosphorus-to-nitrogen ratio than a crop needs, so when it is applied to meet the nitrogen needs of a crop, phosphorus is usually overapplied and tends to build up in the soil.
To manage phosphorus, farm agencies have promoted the use of the phosphorus site index in the last two decades to identify fields with the greatest risk for phosphorus runoff based on a variety of factors.
Nonetheless, the new report found that huge amounts of phosphorus were still being applied to fields, even where the phosphorus site index was used. In some cases, Simpson said, "you are 20 to 30 times above crop needs for phosphorus."
The U.S. Department of Agriculture is reviewing whether the phosphorus site index should be the primary tool for advising farmers, or whether a stricter standard should be applied. (See "Proposed national standard for phosphorus derailed by critics," February 2010).
It was once thought that excess phosphorus was mostly bound to the soil. As a result, it was not considered a significant water quality threat if erosion was controlled. But the amount of phosphorus that soils in the Bay region can hold is limited by the iron and aluminum compounds in the soil.
Research in recent decades has shown that as soil becomes saturated, a greater portion of the excess will be washed off when it rains. The amount of phosphorus in runoff begins to accelerate when soil is 20-30 percent phosphorus-saturated.
The scientists recommended that soils that reach the 30 percent phosphorus saturation threshold not receive any additional manure. That's still more than crops need, Simpson said, but "in our major production areas, most lands would not receive any phosphorus-containing material" if they were only meeting crop needs.
The scientists see that as a compromise between relying only on the phosphorus index, which too often allows continued phosphorus buildup, and strict adherence to only applying phosphorus to meeting crop needs, as that would leave farmers with mountains of unusable manure.
For soils that are 20-30 percent phosphorus-saturated, they recommend manure be applied at crop removal rates only two out of every four years to draw down phosphorus levels in the soil over time.
That would still leave farmers with a significant amount of excess manure in livestock-growing regions. The scientists recommended that any transition to the 30 percent threshold take place over a period of years, and be coupled with financial assistance to farmers-who will need to purchase nitrogen and potash if they cannot apply manure-along with stepped-up efforts to find alternative uses for manure.
"There has to be enough economics behind this so that the farmer gets something from this conversion, at least enough to cover nitrogen purchases," said Ron Korcak, program coordinator with Water Stewardship and the former associate director of the USDA's Beltsville Agricultural Research Center.
Finding ways to salvage and reuse phosphorus may become increasingly important over time, they noted, as some research suggests a global shortage of phosphorus could be looming-phosphorus is a mineral which must be mined, and known reserves are limited.
Korcak and Simpson said allowing farmers to continue to apply excess fertilizer to fields is harmful to farmers in the long run. As soils become more saturated, farmers may have fewer options if regulations are imposed in the future.
"You're not really being fair to the farmers," Korcak said. "You are digging them a bigger hole. We want to position them to be ready for more strict phosphorus regulations. And allowing them to keep on applying more and more phosphorus is not, in the long run, going to be fair to the farmer."
Indeed, a hint of what the future could hold in the Bay and other coastal watersheds came in March, when the EPA actually recommended an even stricter threshold-20 percent phosphorus saturation-in guidance for farms on federally owned land in the Bay watershed.
"This is our best recommendation based on what we know now," said J. Charles "Chuck" Fox, senior adviser to the EPA administrator for the Chesapeake Bay, as the guidance was released for public comment.
The full report can be found on Water Stewardship's website: http://waterstewardshipinc.org/.