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Phosphorus Index score will tell MD farmers where, how to apply fertilizer

Topography, water table, soil type and soil test will be used to determine whether field is over or under threshold.

  • By Rona Kobell on October 01, 2012

University of Maryland scientists are revising the state's Phosphorus Index, a tool that takes into account a variety of factors to help a farmer determine where, and how, to apply phosphorus. The tool, known as the P Index, has been around since 2000. But new research on how phosphorus moves in surface water, groundwater and the air persuaded scientists to revise it and make it more accurate. But the phosphorus to nitrogen ratio in most manure is higher than is needed by crops. As a result, when farmers apply enough manure to satisfy the nitrogen needs of a crop, they typically overapply phosphorus.

The current index combines a variety of factors — including topography, the water table, the type of soil and a soil test — and gives a field a score. If fields score higher than a 150, farmers are not allowed to apply any phosphorus on their fields.

The new index will also look at how the phosphorus travels — both where it goes and how fast it gets there — in assessing the score. And, it will add a threshold number that a farmer cannot exceed. The threshold number will become part of a regulation. Farmers who have levels of phosphorus in their fields that are above the threshold will have to use the index to manage their phosphorus and base their nutrient management plan on phosphorus. Everyone else can have a nitrogen-based nutrient management plan.

Maryland Department of Agriculture Assistant Secretary Royden N. Powell III said he doesn't know what the threshold number will be yet because he hasn't seen the final index. Whatever the final number is, Powell is confident the new index will help to reduce the amount of phosphorus reaching waterways.

"It will assess risk pathways that were not captured under the existing index," Powell said. "We're going to have a tool that is more accurate across Maryland."

Phosphorus research has come a long way in the last several decades, and the new P Index reflects recent major breakthroughs. Since the 1950s, agronomists and scientists told farmers not to worry about phosphorus, because it bound to the soil and wouldn't run off. But in the last two decades that was shown not to be the case. At that point, many of the fields on the Eastern Shore were so loaded in phosphorus that they shouldn't have received any more. Still, many farmers continued to apply it.

Scientists know now that phosphorus leaves the soil in three ways: It can run off the top layer of soil in surface water; it can travel in shallow subsurface pathways and reach ditches; and it can dissolve in the groundwater.

The subsurface pathway finding was "really new science," said Josh McGrath, the University of Maryland soil fertility and nutrient management professor who is focusing on retooling the index. "We've been trying to understand it. We don't have an accurate scientific model to predict this kind of transport."

McGrath said scientists have also determined that the chance of phosphorus running off a field depends on how much iron and aluminum are in the soil. Those two elements help bind phosphorus to the soil. Figuring out the ratios will be important to farmers.

To complete the index, McGrath and his team sampled 420 fields. They looked at the transport mechanisms of the phosphorus — and the source. They also examined topographical differences, realizing that something that worked for the steep slopes of the Piedmont area would not necessarily work in the highly erodible soils of Southern Maryland or the low-lying Eastern Shore.

They also considered new technology that can prevent runoff. For example, scientists have developed a poultry manure injector, a technology previously used for dairy and hog manure. Farmers that have a surface runoff issue can consider injecting manure to keep their phosphorus on the fields.

Environmentalists and some regulators have long questioned the validity of the P Index, arguing that it allowed farmers to apply phosphorus to fields already saturated because it took into account so many variables. A soil test, they said, would be a better barometer. Over a certain number, no more phosphorus could be applied; under it, and the farmer could manage for nitrogen only and not worry about phosphorus.

The U.S. Department of Agriculture seemed to be listening. Two years ago, it floated the idea of a nationwide phosphorus standard. Soils that tested higher than 200 parts per million for phosphorus couldn't receive any more, according to their nutrient management guidance.

Some soil scientists and the poultry and beef industry railed against this change, and the government relented. It then asked the soil scientists, part of a group called SERA-17, to help them revise indices in their own states. Most states use the P Index approach to manage phosphorus.

In Maryland, the new P Index is a part of the watershed implementation plan that the state has submitted under the Chesapeake Bay's pollution diet. Powell said that he hopes to deliver it this fall. After he receives the index, he will incorporate it into proposed regulations, and the public will have a chance to comment.

Once the tool is finalized, the department will show its nutrient management experts how to use it to help farmers develop their management plans.

Many environmentalists still believe in the simple, soil-test method.

"The thing that makes the most sense to us is to have a threshold above which you don't go," said Tommy Landers, executive director of Environment Maryland.

McGrath agreed that the criticism of the index in the past was legitimate, and allowing farmers to apply more phosphorus on already saturated fields "didn't pass the smell test." But the index was not developed as a tool to reduce phosphorus runoff per se; it was developed as a tool to help farmers obtain better yields and lose less nutrients.

The new information about pathways, McGrath said, should be hugely helpful to farmers by protecting both waterways and their own investments, so they are not wasting fertilizer.

Because the index is so involved, McGrath is careful to say he doesn't know if it will reduce the amount of phosphorus going on fields. But, he said, it should reduce the amount of phosphorus reaching waterways.

"How much am I allowed to apply is the wrong question," he said. "The question should be, "How should I apply it?"

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About Rona Kobell

Rona Kobell is a former writer for the Baltimore Sun. .(JavaScript must be enabled to view this email address).

Read more articles by Rona Kobell

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