Maryland’s Phosphorus Management Tool imposes the toughest restrictions on how farmers apply and handle manure on their fields of all the Chesapeake Bay watershed states. But maybe not for long, as the other states are looking at revising their phosphorus limits as a result of new research and new funding to look more closely at the issue.

The Chesapeake Bay Commission issued a report in January comparing Maryland’s new phosphorus management tool, or PMT, to the index approaches that Virginia and Pennsylvania have taken. Virginia has not updated its index since 2005; Pennsylvania’s has not been updated since 2007. In the intervening years, scientists have learned more about the movement of phosphorus, which was once thought to bind to the soil and could be controlled by preventing erosion. They now know that phosphorus moves with water through subsurface pathways and can be a particular problem in artificial drainage structures, like tile drains, or in sandy soils.

“The scientific understanding of how to best evaluate and manage phosphorus continues to evolve. Not surprisingly, there is strong interest in revising state-specific phosphorus indexes to reflect improved understanding of a complex issue,” the commission’s report states.

Though the commission’s chart compares only the three main Chesapeake Bay states, it notes that West Virginia, Delaware and New York are working with the U.S. Department of Agriculture on a grant to develop the “scientific underpinnings” of how phosphorus moves in those watersheds. That understanding may lead to stronger indexes there.

Here are some of the highlights of the comparisons:

  • Maryland: The Phosphorus Management Tool is triggered when a soil test shows phosphorus is above 150 parts per million. The phosphorus management tool looks at the amount of phosphorus in the soil, as opposed to location and topography. It does not allow for a nitrogen-based manure plan, in which a farmer applies manure based on nitrogen requirements for yield and could thus over-apply phosphorus.
  • Virginia: The phosphorus index is triggered when a soil test for phosphorus is above 127 ppm. It does not factor in soil saturation — the point at which dissolved phosphorus will move because the soil no longer binds with it. Instead, it focuses on steep slopes, sensitive waterways and best management practices to control phosphorus. It allows for a nitrogen-based manure plan.
  • Pennsylvania: The phosphorus index is triggered when a soil test shows phosphorus is about 200 ppm, or when the concentration of phosphorus is less than 150 feet from water. It does not factor in soil saturation; instead it considers the soil test, soil loss, distance to water and buffers. It allows for a nitrogen-based manure plan.

The commission was instrumental in the adoption of Maryland’s phosphorus management tool, and it will continue to explore advances in the other two states, said commission executive director Ann Swanson. She cautioned that states must consider four factors when updating their index: artificial drainage (tile drains and open ditches); manure spreading on snow-covered or frozen ground; erosion estimation over a single year or a crop rotation; and soil phosphorus measurement.

“In any re-examination of the phosphorus index,” she said, “the state should pay particular attention to those.”

The report is viewable at