Phosphorus — which long played second-fiddle to nitrogen in the Bay cleanup — is likely to continue moving closer to center stage in the future, at least for agriculture.

For years, conventional wisdom was that any excess phosphorus applied to the fields would bind to the soils and stay put. As a result, more attention was paid to controlling water-soluble nitrogen, which can be readily washed into streams, sink into groundwater or even volatilize into the air only to land somewhere else.

But the view that phosphorus stays put while nitrogen runs off is changing as a result of what a new Chesapeake Bay Commission white paper calls the “new science” — recent findings that phosphorus, especially when it builds up in the soil, is more likely to run off the fields and pollute surface waters than previously thought.

The issue was the focus of discussion at the commission’s November meeting. The commission is an advisory body made up of general assembly members from the Bay states.

Maryland is already addressing the issue and Pennsylvania and Virginia will have to follow, the white paper suggests.

Paying more attention to phosphorus will create a host of problems — Maryland is struggling to find ways to deal with its excess — but the situation may be eased if efforts are aimed primarily at the relatively small areas that account for most of the phosphorus runoff.

“It seems that where you have agricultural land, that a very large proportion of the phosphorus is coming off a relatively small area,” said Douglas Beegle, a Penn State University professor of agronomy, who made a presentation at a recent commission meeting.

Beegle, who with colleagues has been studying phosphorus runoff from land for years, said if farmers can identify areas most likely to lose the nutrient, they could restrict the use of phosphorus-based nutrient management plans to those sites.

Plants need both nitrogen and phosphorus to grow. But they often require a higher ratio of nitrogen to phosphorus than is found in some types of animal manure, such as that from chickens and hogs.

As a result, farmers who apply enough manure to supply the nitrogen needs of their crops may be placing four times as much phosphorus on the ground as is needed.

Historically, that wasn’t thought to be much of a problem because phosphorus tends to bind to the soil. Therefore, if farmers used practices to curb runoff, it was thought any excess phosphorus would stay immobilized on the ground.

But the new science has shown that isn’t always the case, and areas with huge buildups in the soils are leaking phosphorus.

That led the Maryland General Assembly last year to pass legislation requiring farmers to develop nutrient management plans based on phosphorus, rather than nitrogen. Nutrient management plans are farm-specific guides to how much fertilizer or manure is needed, based on soil types, crops and other variables.

Because some animal manure has so much phosphorus relative to nitrogen, phosphorus-based plans can be more difficult to implement. When only enough manure is spread to meet the phosphorus needs of the plants, the available manure must be spread over more ground — maybe four times as many acres.

In regions with large animal populations, such as Maryland’s Eastern Shore, there may not be enough land to apply all the manure. Maryland officials are exploring burning, transporting manure elsewhere and other options to deal with the surplus.

Further, when phosphorus-based plans are used, the manure alone doesn’t provide enough nitrogen for the crops. That means the farmers would have to buy chemical nitrogen fertilizer to apply to the land, in addition to the manure.

The result: Phosphorus-based plans, at least when some types of animal manure are used, create disposal problems and add to the farmers’ costs because they have to buy additional fertilizer.

Beegle and others are working to develop a “phosphorus site index” to rank specific fields within a farm to determine the likelihood of phosphorus runoff. The index accounts for such factors as the amount of erosion, runoff, soil tests, whether fertilizer is added, and how it is added, and whether manure is added.

Fields with the greatest potential to lose phosphorus would then be targeted for phosphorus-based plans. Beegle said identifying the worst problem areas can help minimize the problems for farmers.

“What I’m afraid of is if we put a strict limit across-the-board, [on phosphorus] we’re going to wipe out a lot of the ag industry,” Beegle said. “That will solve the problem, but I’m not sure that is the way we want to solve it. This gives us an alternative. We can get a lot of the benefits with less pain.”

But the phosphorus site index, Beegle cautioned, is not a final solution. Ultimately, far more phosphorus is coming into agricultural watersheds than is leaving. New approaches need to be found to restore phosphorus balances in the watershed.

New technologies, such as changing animal feeds to reduce phosphorus in manure can help, but won’t solve the problem, Beegle said. Large-scale solutions may be required, such as redistributing where animal populations are located.

“They’re buying us some time, and that’s all,” he said. “If we don’t work on the bigger problem, someday the bill is going to come due.”

The commission’s white paper says both Pennsylvania and Virginia are moving to give phosphorus more importance.

Pennsylvania, for example, is expected to soon have a task force exploring the problem, which could then recommend changes in the state’s nutrient management program. In Virginia, legislation under review would make it easier to incorporate phosphorus considerations into nutrient management plans. Meanwhile, the Virginia Polytechnic Institute has received a grant to develop a Phosphorus Site Index for the state.

The white paper said that resistance to phosphorus-based planning “appears to be less a repudiation of the science and more out of genuine concern for the economic and management implications of too rapid a transition.”

As a result, the paper suggests that a “limited approach,” which focuses first on the most problematic sites, would be a way to minimize agricultural disruption while reducing water pollution.

Copies of the white paper, “Phosphorus: New Challenges for the Next Century,” are available from the commission by calling 410-263-3420.