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How the legacy of phosphorus can assist farmers to reduce fertilizer applications

June 16, 2023  By Agriculture and Agri-Food Canada

Alley of mature corn plants interseeded with Italian ryegrass to protect the soil and help tap into legacy phosphorus at Agassiz Research and Development Centre. Courtesy of Agriculture and Agri-Food Canada

Did you know phosphorus fertilizer has a legacy? It’s not a great one, but scientists are working to turn that around. Phosphorus has been a popular additive for decades, supplying the soil with the primary nutrients that a crop requires. But when it’s overused, especially in excess to what the plant needs, it can build up in the soil – a phenomenon known as legacy phosphorus or legacy P. This can have a range of negative effects on the environment, notably with chemical run off into nearby water streams.

At Agriculture and Agri-Food Canada (AAFC), research scientist Dr. Aimé J. Messiga has come up with solutions that can help farmers mitigate this issue by tapping into the phosphorus that’s already in the soil. In doing so, farmers will not only reduce the environmental risk, but they’ll also save on costs.

“Legacy P can be a valuable source for plant growth and can offer a substitution for fertilizers,” says Messiga. “Since there is enough legacy phosphorus built up in the soil, it’s possible for farmers to take advantage of it and decrease the amount of starter fertilizer they add.”


Tapping into legacy phosphorus

Considering the surplus of phosphorus that is already in the soil, limiting the amount that is added on top is one of the most effective methods to addressing legacy P. One way farmers can do this is by reducing the amount of starter phosphorus fertilizer that they use. Starter phosphorus fertilizer refers to a small amount of phosphorus which is placed close to the seed at time of planting. Messiga and his team have developed a recommended guide to help farmers determine how much starter phosphorus fertilizer is actually needed. Fortunately, the research shows that reductions can be made without impacting crop growth. In a study conducted with corn crops in fields with legacy P, Messiga found that phosphorus applications could be cut by 75 per cent without affecting yield.

Messiga is also working on developing best management practices to drawdown phosphorus levels in soils high in legacy P, such as interseeding cover crops in corn fields. Interseeding refers to the practice of planting cover crops in between rows of the cash crop. Cover crops can be helpful since they have a better tendency to take up legacy P and cycle it back to the cash crop.

To put this practice to the test, Messiga established long-term experiments in 2018 in contrasting soils with high legacy P levels to compare two different cropping systems: (1) Italian Ryegrass interseeded between young silage corn plants; and (2) silage corn followed by Italian ryegrass planted during the fall season following corn harvest. The first cropping system was found to be the most successful, enhancing cover crop establishment without competing with the corn. This system can also help farmers avoid having to rush to the field to seed in the short window between the fall harvest and when the first rains fall. For dairy farmers, the cover crops can even offer extra feed for dairy cows in early spring. This method can even scavenge residual soil nutrients, including phosphorus, which helps to drawdown soil phosphorus.

Messiga’s research will also assist farmers in mitigating phosphorus loss to the environment by identifying fields at risk, implementing a nutrient management plan, as well as monitoring how this risk changes with time. A new Code of Practice for Agricultural Environmental Management was published in early 2019 to guide farmers and professionals in British Columbia (B.C.) with recommendations on methods to be included when designing a nutrient management plan. Additionally, Messiga and his team are adapting a phosphorus saturation index to assess the risk for phosphorus loss from agricultural soils in B.C.

In the long run, Messiga hopes to reduce starter phosphorus fertilizer applications in B.C. by 75 per cent relative to current recommendations, a goal which will help farmers save on fertilizer costs and minimize environmental risks. Reduced applications of starter phosphorus fertilizer can also help lower the demand for mining phosphorus reserves, providing further environmental benefits.

Key discoveries/benefits:

  • Legacy phosphorus is a term used to describe the accumulation of excess phosphorus in the soil. This happens when phosphorus fertilizer is overused, especially in abundance to what the plant actually needs.
  • Messiga and his team are looking into ways to take advantage of legacy phosphorus and limit its risk to the wider environment.
  • Messiga’s research will help provide farmers with a number of methods to mitigate the issue, including a recommended guide on how much starter phosphorus fertilizer they should apply; best management practices to drawdown phosphorus levels, such as interseeding cover crops; and implementing a nutrient management plan to address the risk of phosphorus loss to the wider environment.


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