10/19/00

Contacts:
Gerald Miller, College of Agriculture, (515) 294-4333
Antonio Mallarino, Agronomy, (515) 294-6200
John Sawyer, Agronomy, (515) 294-1923
Barbara Stewart, NRCS, (515) 323-2260
Susan Thompson, Communications Service, (515) 294-0705

SCIENTIFIC INDEX DEVELOPED TO ASSIST IN PHOSPHORUS MANAGEMENT

AMES, Iowa -- A team of Iowa State University scientists and Natural Resources Conservation Service (NRCS) staff have developed a new tool to help producers manage phosphorus (P). The Iowa Phosphorus Index uses a number of factors to calculate the potential for phosphorus loss from a field.

Research has shown phosphorus that moves from farm fields and into water bodies can cause algae blooms and other water quality impairments.

A process that began more than a year ago culminated October 19 when the P-Index was presented at a meeting of the USDA state technical committee. That committee had asked its nutrient management subcommittee to develop the index, which will be used by the state NRCS staff to provide assistance on federal nutrient management guidelines.

Public seminars were held last winter on the ISU campus where the scientists and others discussed issues that impact phosphorus levels in Iowa's soil and water.

Gerald Miller, associate dean of the ISU College of Agriculture, coordinated the effort. He says the P-Index should be seen as a risk assessment tool. "It provides a loss rating, and can be useful in understanding the process of phosphorus delivery to surface water," he says. "It helps identify management practices that can reduce the movement of phosphorus to surface water."

The index is designed to be used on a field basis or on Conservation Management Units (CMU). A CMU is a portion of a field, a field or group of fields that have the same land use, similar treatment needs and planned management.

Three components are included in the P-Index equation. The erosion component is made up of five factors that help determine the potential amount of phosphorus delivered to surface water with sediment. Those factors include such things as conservation and management practices.

The second component of the P-Index deals with the potential phosphorus delivered to surface water in runoff. It takes into account precipitation, soil tests, recent phosphorus applications and the method and timing of those applications.

The third component considers the potential for phosphorus to move through subsurface drainage. Precipitation, the existence of underground tiles, soil type and soil test phosphorus levels impact this third component.

The P-Index is designed for use by NRCS staff and others on laptop computers in the field. By plugging each of the factors and components into a spreadsheet, a final number is reached that translates into the potential risk of phosphorus movement.

Five risk assessment classes have been established. If the P-Index places a field or CMU in the very low or low class, impacts on surface water resources from P losses will be small.

In the medium class, the P delivery potential may produce some water quality impairment. Once a field or CMU falls in this class, changes may be necessary in soil and water conservation practices.

An index number in the high class means impacts on surface water resources will be large and remedial action should be taken. New soil and water conservation and/or P management practices would be necessary to reduce potential offsite movement of P, thus lowering the P-Index value.

A P-Index number in the very high class means impacts on surface water resources would be extreme and remedial action would be needed. All necessary soil and water conservation practices, plus a phosphorus management plan that might require discontinuing P applications, should be put in place to reduce the potential of water quality impairments.

Miller cautions the index has its limits. "The P-Index is a tool to be used by planners and land users to assess the risk that exists for phosphorus moving off agricultural fields toward a surface water body. It's intended to be part of the NRCS nutrient management planning process that takes place between the land user and resource planner," he says.

Miller says the P-Index isn't an evaluation scale for determining whether land users are within water quality or nutrient management standards established by government agencies. "Any attempt to use this index as a regulatory tool would be beyond its intent," Miller says. "This index needs further testing and will need to be modified periodically as new research data becomes available."

The USDA state technical committees must recommend final rules on phosphorus to the state NRCS conservationist by April 1, 2001. It is expected that some states will establish a flat rate for soil-test P in a field, and place limits on P application rates based on soil tests. Miller says Iowa chose to develop a P-Index instead, because it allows many factors to be considered.

The USDA state technical committee will make recommendations on any modifications to the P-Index before it is put into use in Iowa.

Once recommended by the USDA state technical committee and adopted by NRCS, an electronic version of the Iowa Phosphorus Index and user's guide will be available on the Web at www.ia.nrcs.usda.gov

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