Phosphorus concerns force conservation focus

It’s well known that the world is running out of phosphorus.

Geologists and scientists argue about when it will happen, possibly 100, 200 or 600 years from now, but most accept that the Earth has a limited supply of phosphate rock.

Considering how phosphorus is a critical nutrient for plant growth, it’s surprising how little attention is paid to phosphorus use efficiency, says Bill Plaxton, a Queen’s University research chair in plant biochemistry.

He said many plant experts and crop science companies ignore phosphorus because they’re obsessed with another nutrient.

“When I’ve talked to big biotech companies … for whatever reason, they’re not interested in improving phosphorus uptake by plants. They’re all concerned with nitrogen,” Plaxton said.

“(But) we’re never going to run out of nitrogen fertilizer as long as there are nitrogen fixing bacteria around…. Phosphorus is a whole (different) story. It’s a limited resource and it’s incredibly inefficient. I’ve read typically, at best, 20 percent of applied phosphate gets taken up by the crop. The rest either gets locked in … by being chelated to cations or runs off (the land) and pollutes rivers and lakes.”

Plaxton has been studying phosphorus use efficiency for 30 years. He focuses on how things work at the molecular and biochemical level to understand how plants acquire and use phosphorus.

“My philosophy is you really can’t fix a car engine until you know how it works.”

Basic research has taught plant scientists that certain plants and crops are particularly good at “scavenging” for phosphorus in the soil.

Research at the University of Manitoba found that soybeans don’t respond to added phosphorus fertilizer. Instead, they prefer to seek out phosphorus in the soil.

“In sites with soil tests as low as 3 parts per million Olson P, the control plot where we didn’t apply any phosphorus had a very good yield, as high as the other plots where we applied some fertilizer,” said Gustavo Bardella, a U of M grad student.

Most plants need help from bacteria to pull phosphorus out of the soil. Mycorrhizal fungi convert organic forms of phosphorus into inorganic forms that can be used by the plant.

“Those fungi for most plants and trees are really important for acquiring phosphate from the soil,” Plaxton said.

“They (fungi) are quite happy to (provide) P … in exchange for sucrose.”

Ninety percent of plants have a symbiotic relationship with mycorrhizae, but some do not.

Instead, many plants in the non-mycorrhizal category form root systems known as proteoid, or cluster roots, that resemble a bottle brush.

The plants can grow in soil with limited nutrients because the roots are able to free normally inaccessible phosphorus from the soil.

“Some of the plants that grow (well) in the most nutrient impoverished soils on the planet are non-mycorrhizzal,” Plaxton said.

Scientists at the University of Western Australia have studied two such species, Hakea and Banskia, to understand how plants pull off the trick.

It turns out the cluster roots a compounds called carboxylates, which are a salt or ester of an organic acid. They help the plant access the needed phosphorus.

“Carboxylates are negatively charged organic molecules that … (make) poorly available phosphorus available for uptake by plant roots,” said Hans Lambers of the University of Western Australia.

Buckwheat is one of the crops grown in Canada that is known for its ability to remove phosphorus from the soil. Research from Cornell University has found that buckwheat roots release mild acids that “activate” rock phosphate in the soil.

Plaxton said plant scientists should focus their attention on plants and crops that possess the unique ability to use phosphorus more efficiently so that those traits can be engineered into broad acre crops.

“I think nature has a lot to teach us,” he said.

“There’s got to be a lot more attention given to the extremophiles.”

How much phosphate rock remains on earth?

  • Geologists previously assumed that production of rock phosphate would hit a peak in 2035-40 and then slowly dwindle to nothing.
  • However, a 2010 report by the International Fertilizer Development Centre determined that there are more reserves and resources than was previous estimated.
  • The paper said there are sufficient phosphate rock reserves to produce fertilizer for 300 to 400 years. Most the world’s phosphate rock is in Morocco, which is sometimes called the Saudi Arabia of phosphorus.


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