Hold your P! Measuring phosphorus loss from cover crops

Person handling dead leaves in a fall cover crop

James Cober, Merrin Macrae, and Laura Van Eerd. 2018. Nutrient release from living and terminated cover crops under variable freeze-thaw cycles. Climatology and Water Management. doi: 10.2134/agronj2017.08.0449
James Cober, Merrin Macrae, and Laura Van Eerd. 2019. Winter phosphorus release from cover crops and linkages with runoff chemistry. Journal of Environmental Quality. doi: 10.2134/jeq2018.08.0307
Study Summary by Jordan Grigg and Cameron Ogilvie

key messages

  • Cover crops that are killed by southern Ontario winters are more likely to leak phosphorus than cover crops that survive the winter
  • The amount of phosphorus in soil has a greater influence on phosphorus loss than cover crops
  • Cover crops remain an important practice to reduce soil and phosphorus loss

Phosphorus is an essential nutrient for plant growth – including the algae that grow in rivers and lakes. When phosphorus is lost from agricultural soils into waterways, it can allow algae to grow rapidly, which causes problems for fish and humans. Cover crops are often recommended as a way of preventing the loss of nutrients from soil over the non-growing season. But the freezing and thawing that happens in our region may cause some of the phosphorus that cover crops have taken up to be released. In both a lab and field study, a group of researchers investigated how cover crop species and the intensity of freeze-thaw cycles influenced the amount of phosphorus that was released over the non-growing season.


The lab-based portion of this experiment exposed 5 different cover crop species (cereal rye, oilseed radish, red clover, hairy vetch, and oat) to several freeze-thaw cycles with different intensities. As the temperature difference between the coldest and warmest part of the cycle increased (to a max of -18 to 10°C), more phosphorus was lost. In smaller temperature differences (-4 to 4°C) this was not seen. The amount of leaked phosphorus also depended on the cover crop species: frost-tolerant crops (ex. hairy vetch, red clover, cereal rye) did not lose as much phosphorus as did the frost-intolerant crops (ex. oat and oilseed radish).

These same results were seen in the field, with frost-tolerant species releasing less phosphorus than frost-intolerant species. However, the phosphorus that leaked from the cover crops was not detected in the water samples that were taken within 24 hours after a rainstorm. Instead, the water samples were more related to the amount of phosphorus that was in the soil rather than the cover crops. Where cover crops had between 0.08 to 1.98 kg ha-1 of phosphorus that could be lost, the soil had between 1.10 to 3.46 kg ha-1 of phosphorus that could be lost.


Cover crops that are killed by our winters in southern Ontario are more likely to leak phosphorus than those that aren’t. Less phosphorus loss can be expected from cover crops such as cereal rye, hairy vetch, or red clover. However, soil erosion is more of a concern than cover crops as a cause of phosphorus losses in southern Ontario. For this reason, cover crops remain an important management practice to prevent soil and nutrient losses over the non-growing season.


Living plant samples from five growing cover crops (cereal rye, oilseed radish, red clover, oat, and hairy vetch) were taken to the lab for experimentation. In the lab, plant samples were exposed to a range of freeze-thaw cycles: no freezing (held at 4°C), -4 to 4°C, -18 to 4°C, and -18 to 10°C. Water-extractable phosphorus (ie. the phosphorus that could be removed by a simple water bath) was then collected and measured.

For the field experiment, the same cover crops were planted in plots after wheat harvest at a field near Bloomingdale, Ontario. Plant and soil samples were collected when cover crops were at their largest, and water-extractable phosphorus was measured from each. Water sample collection units were also installed to measure phosphorus concentrations in surface runoff and shallow groundwater after rainfall events.

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