Jennifer Weld, a Penn State doctoral student and project associate, speaks about the phosphorus index.

GRANTVILLE, Pa. — Pennsylvania’s next version of a fertilizer planning calculation will give farmers better site-specific information for preventing phosphorus loss.

“We’re revising the tool, but we’re not revising what the basic concept is,” said Jennifer Weld, a Penn State doctoral student and project associate.

Weld spoke at the Keystone Crops and Soils Conference on Oct. 23 at the Holiday Inn Harrisburg.

The phosphorus index is a tool used in nutrient balance sheets and nutrient management plans to identify areas where phosphorus pollution is most likely.

It evaluates the availability of phosphorus sources such as soil nutrients, fertilizer, manure and biosolids, as well as ways phosphorus moves off the farm, namely erosion and runoff.

Likely trouble spots, called critical source areas, occur where there’s both a lot of phosphorus and a high likelihood of the nutrient getting transported away.

Generally, 90 percent of the phosphorus comes from about 10 percent of the land.

“It’s not going to be the entire farm, most likely,” Weld said.

The phosphorus index dictates how farmers can manage fertility.

On soils with run-of-the-mill phosphorus levels, farmers can apply fertilizer based on the nitrogen needs of the crop.

But on soils with a high phosphorus index — often occurring in critical source areas — nutrients can only be applied to meet the crop’s phosphorus removal.

When the index is very high, no phosphorus may be applied.

In addition to using the calculation for nutrient management plans, Pennsylvania farmers must run the index calculation for all fields that receive winter manure.

Things got more complicated when the Environmental Protection Agency and USDA Natural Resources Conservation Service did some soul-searching about the index and its effectiveness from 2010 to 2012.

In some cases, they acknow-ledged, the index allows phosphorus to be applied in areas where it isn’t needed for crop yields.

Deploying excess nutrients raises some environmental concerns, but the agencies mainly wondered if such a policy was wise given the world’s finite supply of phosphorus, Weld said.

Some analysts believe phosphorus supplies could get tight decades from now.

The agencies also found that the parameters for low, medium, high and very high phosphorus are not consistent across the country.

That’s mostly an issue in the West, where water quantity is a bigger issue than quality, Weld said.

Since the federal rethink, Pennsylvania has been working on its new version of the index.

One of the biggest changes will be considering two types of pollution separately.

Particulate phosphorus is moved by erosion and is generally attached to the soil, while dissolved phosphorus flows with runoff.

The two types of pollution make phosphorus environmentally available to different degrees and can be reduced with different conservation measures.

“We look at everything together,” Weld said. “We don’t separate it, which makes it difficult to recommend best management practices.”

In a 2015 survey, nutrient management planners gave a strong message that the phosphorus index needed to do more to encourage best management practices.

The new version of the index should do that, Weld said.

The revamp could also change the way soil loss is calculated.

The index currently averages soil loss across the years of a crop rotation. But crops like corn allow much more soil loss than, say, hay.

This difference is particularly obvious in rotations that have multiple years of hay or forage, Weld said.

In a study watershed in Northumberland County, modeled sediment phosphorus loss ranged from less than 1 to more than 15 kilograms per hectare.

Basing soil loss on the annual crop, not the average of the entire rotation, could lead to more realistic recommendations.

New York has already changed its phosphorus index to annual sediment phosphorus loss, but it’s still just a possibility at this point for Pennsylvania, Weld said.