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Phosphorus: on the land, not in the water

High levels of phosphorus in ponds and lakes can lead to algae blooms.     Photo by Keith Reid, AAFC.

On a farm field, phosphorus is an essential crop nutrient. But in water bodies, too much phosphorus can cause serious problems, including increased algae growth.

Algae blooms can result in oxygen depletion, leading to fish kills, and blue-green algae can release toxins into the water. So a project is underway to develop a field-scale tool to help farmers keep phosphorus on the land.

“There has been a lot of talk in the news recently about more algae blooms in Lake Erie. After it had been cleaned up in the 1990s, it’s getting worse again. Lake Winnipeg is having a lot of algae bloom problems. Lake Champlain’s Missisquoi Bay has blue-green algae problems. The culprit is excess phosphorus. And agriculture is in the crosshairs to some extent as being blamed for part of that excess phosphorus,” says Keith Reid, a soil scientist with Agriculture and Agri-Food Canada (AAFC) who is leading the project.

He adds, “Excess phosphorus in the water doesn’t translate into a terribly large loss from land, but we want to try to reduce that loss to do agriculture’s part in keeping our lakes swimmable and fishable.”
Called Project P, this research is funded by AAFC. The tool is being designed for conditions in eastern Canada, and Reid is working with several other AAFC research scientists in eastern Canada, as well as provincial government and university researchers in Ontario and Quebec.

The tool aims to predict fields, or parts of fields, at high risk of phosphorus (P) loss. Reid explains, “If you’re going to ask a farmer to do something, let’s do it where it’s going to be effective, where it’s actually going to make a difference. This tool should identify where the high-risk areas are so farmers can focus their attention on those areas.”

A next-generation phosphorus index
“Essentially, we’re looking to improve the phosphorus index,” notes Reid. “The P index was developed as a tool to try to identify hot spots – areas with the greatest risk of phosphorus losses from agricultural land. An example would be a livestock operation that has a small land base with fields near a stream, and they’ve been putting their manure on those fields continually and building up the soil test phosphorus. So those fields have a lot of phosphorus that’s available for loss and easy transport into the water.”

The idea of a phosphorus index was first proposed about two decades ago. Since then, various American states and several provinces, including Ontario, have adopted a P index and adapted it to their own conditions and needs.

“[Ontario’s] current P index, on a large scale, accounts for most of the risk factors. But no one has taken the time to determine if the index’s predictions compare well with what is actually showing up in the water. The current index also misses some of the factors that are a big part of agriculture in Ontario, including tile drains,” says Reid.

He stresses that Project P is not about creating a new index to replace P indexes that Ontario, Quebec or the Atlantic provinces already have. “Our aim is to provide the scientific background that the provinces can use when they adapt their P indexes.”

Reid and his research team will be filling some of the information gaps in the current index to develop an improved model. Then they’ll test the prototype model in the field to make sure it works as expected.

Developing and testing the prototype
The researchers are already working on the prototype. “We have been gathering all the information we can out of the scientific literature, from studies in Ontario and in regions with similar climates, soils and cropping systems. We’re examining what those studies are finding, and we’re putting that together in a model,” Reid says.

Predicting phosphorus loss from agricultural land is challenging because it is influenced by many factors, such as nutrient application rate, placement and timing, soil test P, erodibility of the soil surface, rainfall and snowmelt amounts and surface runoff and subsurface flow patterns, including connectivity with nearby water bodies.

Some practices to reduce the risk of phosphorus loss are already clear. Reid notes, “For instance, we know we can reduce the risk by just getting a phosphorus application worked into the soil or subsurface placed, rather than leaving it on the surface. And we know we can reduce the risk by waiting until the ground is dried up in the spring before applying phosphorus.”

But the effect of tile drainage on phosphorus loss is not so clear. “I’ve done comparisons of phosphorus indexes across U.S. states in particular, which have most of the information [on tile drainage considerations], and they treat tile drains as either all good or all bad,” says Reid.  

“The one mindset is: tile drains divert what would be surface runoff into the tiles so you reduce soil erosion. Therefore, tile drains reduce the risk of phosphorus loss. The other mindset is: tile drains have more connectivity between the field and the water, so it is easier for phosphorus applied in the field to move off the field and into the water. Therefore, tile drainage increases the risk of phosphorus loss. But, in reality, both of those are happening simultaneously.”

As well, he says phosphorus loss in tile drains varies with soil type and with the size of the area drained by the tiles.

The researchers hope to have their prototype ready for field testing by the spring of 2015 and to finalize the tool in 2016.

“[In our tool,] we’re trying to simplify many factors so the tool can be used on the farm. We don’t want a tool that requires a whole lot of very complex measurements and inputs before you can run it,” explains Reid.

But those simplifications mean the model is being designed to predict general trends, not how many grams of phosphorus will be lost from a field.

“So, in our field testing, if the model identifies an area as a high risk for phosphorus loss, then we would expect to see higher levels of phosphorus in the downstream water. And if it identifies a low-risk area, we would expect to see lower concentrations of phosphorus.”

The researchers are in the process of looking for field sites where they can compare the model’s predictions with what is actually happening. The sites have to meet several criteria.

“We need sites with water quality measurements at a fine-enough time-scale that they give a good picture of what is showing up in the water. And the measurements have to be for a small-enough area that we can make some conclusions about what the water quality data mean relative to the land area draining into that stream. For instance, if the water quality is being measured at the mouth of the large river like the Grand River, it won’t be very useful for our purposes; there’s just way too much going on in that watershed. So the field sites need to be up in smaller sub-watersheds,” explains Reid.

“And then we have to combine [those criteria] with being able to get the information about the agricultural practices on the land. That will enable us to run the model and compare the results with the water quality data.”

Turning the tool’s results into practical action
According to Reid, the main users of the tool will likely be nutrient management planners. “We would expect to see the tool incorporated into, for example, [Ontario’s] NMAN software, once we’re happy with the way the model is working.”

He adds, “I would also hope the model is transparent enough that a farmer could use it, recognizing that some farmers will have an interest and some won’t because they’ve got a lot of other things on their minds.”

Reid expects the results generated by the tool will be provided to users in a way that indicates why a certain area is identified as having a high risk of phosphorus loss; for example, whether the risk is mainly due to soil erosion or to phosphorus application factors. That type of information would point the user towards which types of best-management practices would be most effective in reducing phosphorus loss.

He notes, “This project is working in the context of: how can we farm a little bit better to both keep the phosphorus on the land and keep the water clean, and also have a profitable cropping system?”

November 19, 2014  By Carolyn King