The state-federal Bay Program will soon give states working to meet their water improvement plan goals greater nutrient reduction credits for farms that implement nutrient management plans. But scientists and environmentalists worry that the credits overstate the amount of phosphorus reductions the plans will achieve.

Nutrient management plans may slow the rate at which phosphorus builds up in the soils, but they won’t stop it, critics say, and on many farms the phosphorus increases will ultimately result in more of the nutrient entering rivers and the Chesapeake.

Several independent experts I spoke to agree with that assessment, though they said it is a difficult issue to unravel.

The issue came to a head at the end of September when the Bay Program approved new recommendations from an Expert Panel it had appointed in 2011 to determine the amount of nitrogen and phosphorus reductions that should be credited to nutrient management plans. Those plans seek to optimize the amount of nutrients placed on a field to grow a particular crop.

For phosphorus, the panel’s new recommendations would give a 6 percent reduction credit to certain farms that implement  advanced phosphorus reduction actions based on soil tests and site conditions.

Environmental groups said there was too little science available to justify those credits, which would be used in Bay Program computer models to estimate the amount of nutrient reduction progress made by states in the watershed.

But a major part of their concern is that the recommendation adds to an error they contend the Expert Panel made two years ago. In late 2013, the panel approved an even larger 10 percent phosphorus reduction credit for more basic nutrient management plans for most fields where manure was applied. For some fields, the new 6 percent reduction will now be added onto the 10 percent reduction.

But critics say the earlier 10 percent phosphorus reduction was flawed. It stemmed from changes in the advice that state land grant universities had given to farmers over the years about the amount of nitrogen needed to grow a crop.

Based on computer model estimates, the Expert Panel calculated that if farmers were following those nitrogen recommendations, they would have reduced, on average, the amount of nitrogen runoff by 9.5 percent (from typical application rates on farms without nutrient management plans).

Although the land grant university recommendations applied to nitrogen, the panel estimated that for fields where manure was applied, the reduced manure applications would also result in a 10 percent reduction in phosphorus runoff.

But with phosphorus, critics say, reduced manure applications do not necessarily mean reduced runoff. Almost any type of animal manure has a higher ratio of phosphorus to nitrogen than is needed by a crop. That means, when enough manure is applied to meet the nitrogen needs of a crop, more phosphorus is going on the ground than that crop will use.

Unlike nitrogen, which tends to leave the soil relatively quickly — either through uptake by crops, surface runoff or by sinking into the groundwater — phosphorus can continue to build up in the soil. Until, that is, the soil becomes so saturated that it can no longer hold the phosphorus. At that point, large amounts can begin leaking into waterways, as is the case on parts of the lower Eastern Shore and other portions of the watershed with large amounts of animal agriculture.

In other words, even if less phosphorus is applied, it can contribute to a long-term buildup in the soil — and contribute to water quality problems —as long as applications exceed what the crop uses.

“It is directionally incorrect,” Tom Simpson, a retired soil scientist with the University of Maryland, said of the phosphorus recommendations. “It is giving a 10 percent reduction for something that if done, would result in a continued increase in phosphorus loss. The rate of increase may slow down.”

Simpson is a hired consultant by environmental groups in this case.

So, I called three other phosphorus experts, both in and out of the watershed, and they agreed, with some caveats.

For instance, when manure is freshly applied to a field, some portion of the phosphorus can run off before it mixes into the soil. Applying less manure, and therefore less phosphorus, can reduce that short-term runoff. Also, following a nutrient management plan may eliminate manure applications altogether for some crops, such as soybeans. That would also reduce phosphorus runoff.

But on most croplands, the bulk of the phosphorus remains on the ground where it builds up in soils. That has been a problem not just around the Bay, but also for other waterbodies with intense animal operations nearby, including Lake Erie and rivers in Arkansas where long-term phosphorus buildups are contaminating waterways.

“Basically, the reason we’ve got into a problem with phosphorus runoff, especially in the Bay in most areas, was because we used nitrogen-based applications, so phosphorus built up until it started to get to levels where it would enrich runoff. That is what got us here,” said Andrew Sharpley, professor of soils and water quality at the University of Arkansas, and a nationally recognized phosphorus expert.

“It would just aggravate the problem.”

Two other experts I spoke to concurred, though they did not want to be drawn into a public debate over the controversy.

Because the original Expert Panel nutrient management recommendations had already been approved when those concerns were raised, many Bay Program participants have been reluctant to set a precedent by revisiting them — especially as they are going to be replaced by new recommendations, by a new Expert Panel, in 2018.

But critics say over-crediting phosphorus exacerbates what almost everyone recognizes is a problem in how the Bay Program tracks phosphorus.

Right now, the Bay Program’s computer modeling shows that phosphorus levels are declining, and states are likely to meet their phosphorus reduction goals for the end of 2017. That is when states are supposed to have taken actions that would achieve 60 percent of their ultimate 2025 phosphorus reduction goals.

But water quality monitoring tells a different story, with phosphorus levels holding steady or increasing for many Bay tributaries.

That’s because the Bay Program’s computer models don’t handle some phosphorus issues, such as phosphorus buildup in soils, very well. Those issues are being addressed in a new version of the model that goes into use in 2018.

Other issues that are adding to the Bay’s phosphorus loads, such as the impact of the sediment buildup behind Conowingo Dam, will also be addressed at that time.

Critics say overcounting nutrient management credits for phosphorus only worsens the problem. Others contend that not giving farmers credit for things like using a phosphorus site index sends a message that such tools are useless.

One thing is pretty clear.

When 2017 comes to an end, states might have met their interim 60 percent phosphorus implementation goal on paper. But the story will likely look a lot different when the calendar turns to January 2018, and states may face a huge phosphorus gap — and have only 8 years to address it.

The nutrient management issue is one — of many — issues that will have contributed to that problem.

To read about research on the impact of phosphorus buildup in soils in Maryland’s Green Run watershed, click here.