Think like a plant when planning phosphorus

Managing phosphorus is a careful balancing act between giving crops enough to thrive, but not so much that runoff pollutes waterways and harms the creatures living in them.

“Too little bioavailable phosphorus reduces crop yields but too much contributes to water quality loss,” said Barbara Cade-Menun. “But the important fact is while there might be a lot of phosphorus in the soil, not all phosphorus forms are bioavailable.”

Barbara Cade-Menun is a soil scientist at Agriculture Canada’s Swift Current Research and Development Centre where she specializes in phosphorus. She was speaking at the Prairie Certified Crop Advisor Board online agronomy research update in December.

All organisms need phosphorus. Its compounds show up in DNA and RNA, as well as adenosine triphosphate, the compound living cells use to transport energy. Cade-Menun cited data that show rice grown in phosphorus-poor soils is stunted.

Getting phosphorus management right is complicated and comes with high stakes both for producers’ bottom lines and the environment.

Most farmland in Western Canada drains into Lake Winnipeg, where runoff-driven blooms of blue-green algae have killed or driven off fish in whole sections of the giant lake and made the water unsafe for recreation.

Key for growing crops is bioavailabilty, Cade-Menun said, and not all phosphorus falls into this category. She gave her favourite gemstone, turquoise — a hydrated phosphate of copper and aluminum — as an example.

“It’s nice to be able to wear your favourite element,” she said.

Whether a soil is acid or alkaline (pH) plays an important role, since this affects what phosphorus compounds can form. Cade-Menun said the best range is a pH value of six or seven for maximum solubility.

The critical compound is phosphate, which is the only form plants can take up and use. Unfortunately, phosphorus can form many other compounds in the soil and the element can be absorbed and desorbed. Plants themselves turn phosphate into different forms depending on their current needs.

“This is what makes phosphorus cycling really, really complex,” she said. “All of these different processes will go on simultaneously.”

Confounding matters even more is weather — temperature and moisture. Phosphorus moves more quickly in wet soils than dry, and more slowly in the cold, which is why signs of deficiency in crops show up more in the spring.

It all makes for a tough soil testing challenge. Cade-Menun said soil test phosphorus, which is the value most familiar to producers and agrologists, is designed to predict crop response. It is limited, though, in that it doesn’t measure how much plant-available phosphorus is in the soil.

“There’s a lot of phosphorus that is not measured by soil test phosphorus that may be potentially available to crops,” she said.

Phosphorus can also vary widely depending on soil depth, so it’s important to sample from the root zone of the crop that will be going into a particular field. Cade-Menun said it’s common to get a soil test back calling for more phosphorus, but if it’s applied above where the plant can get to it, it will simply end up running off into waterways.

This approach allows producers to potentially reduce fertilizer inputs and draw down “legacy phosphorous” that has accumulated in the soil.

This can be a significant resource. When researchers stopped applying phosphorus to some of Ag Canada’s Swift Current research plots in 1995, it set up a natural experiment for Cade-Menun. When they looked at the data, they discovered no loss of yield in 15 years with no supplemental phosphorus.

Further research showed a cause: ammonium fertilizer had changed the pH of the soil, making it slightly more acidic and allowing phosphorus that was bound up in the soil to go into solution where the crop roots could get at it. The soil microbe profile also changed.

Legacy phosphorus should also be considered when judging the effectiveness of alternative approaches such as regenerative agriculture.

“I think that is what’s happening. They’re getting this legacy phosphorus,” Cade Menun said. “We’re just not completely sure what the mechanisms are.”

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