To help farmers manage phosphorus, scientists need to come up with a test that can signal when phosphorus in soil is too high and will run off into streams. "We need to be able to tell the farmer how much soil phosphorus is too much," says Andrew Sharpley. "Nationally, farms of certain sizes must have nutrient management plans that include phosphorus in place by 2002, and Maryland's legislation says they need plans in less than a year. Getting these numbers now is critical."

The tubes sticking out of the ground are piezometers. USDA-ARS scientists use them to measure where and how much groundwater and chemicals flow through the agricultural landscape. Soil scientist Andrew Sharpley (left) talks with technician David Otto (plaid shirt), hydrologist Bill Gburek (white hat), and Earl Jacoby (right), superintendent of USDA-ARS's Klingerstown Research Station.

Sharpley's lab is leading a national effort to come up with threshold soil phosphorus levels for a wide range of soil types. "First, we need to devise a common way to do this research, so we can relate results from the Northeast, for instance, to results from Texas or the coastal plains. Once we gather data for a wide enough range of soils, we'll be able to extrapolate to other soils." Sharpley's lab is gathering data for sites all over the Northeast. Using a "rain machine," they soak soils that contain various concentrations of phosphorus, then measure how much phosphorus runs off. Similar research is taking place at 30 other labs across the country.

But the potential impact of runoff on water quality can't be based on soil phosphorus alone, Sharpley explains. "The phosphorus needs a way to get from the field to the stream. We need to take into account such factors as the slope of the land, distance from a stream, and whether the farmer plows the manure into the soil." To identify places where high soil phosphorus and high "transport potential" coincide, Sharpley and Doug Beegle are developing a simple tool called a phosphorus index. The index will help farmers identify areas on their land with the highest potential for phosphorus loss. By focusing on those critical areas, farmers can control phosphorus in a way that's practical and economically feasible. "Farmers will be able to draw lines on a map of their farm and know that within those lines, they really have to make sure they use all the conservation practices and don't apply excess fertilizer," Beegle says. "Outside of those areas, farmers will have more flexibility. They can base their plans on nitrogen, and the excess phosphorus probably won't cause any problems."

Often, a phosphorus hot spot is a fairly steep field near a channel that has some erosion, or a field near a stream where a farmer has applied manure year after year and soil phosphorus levels have built up. "We look for three things," says Sharpley. "First, has there been a history of manure or fertilizer application, so that the soil has built up a larger reserve of phosphorus than the crop needs? Second, what conservation practices, such as buffer strips and cover crops, is the farmer using? Third, does the farmer have the luxury of applying manure when he or she wants to? Many poultry farmers work under contract for other companies. The birds may only stay on the farm for six weeks, with a two-day gap between when they take out one flock and bring new chicks in. These farmers don't have the flexibility to wait until after it stops raining to spread manure."

During fall and winter 2000, Beegle and Sharpley plan to field-test a prototype of the index with farmers and nutrient management planners. "By 2001, we hope to have an index that farmers and planners can use," Beegle says.

But all of these practices, from adding phytase to feeds to managing manure, address the symptoms of the nutrient problem, not the causes. "Practices like buffer strips, riparian areas, and reduced tillage are just band-aids to keep phosphorus from running off into streams," says Sharpley. "We're putting additives in the manure to tie up the phosphorus, or planting crops with different genetics. These methods are all little pieces in the puzzle, but the only way to really get at the cause of the problem is to reduce the amount of phosphorus coming onto farms."