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Phosphate products vary, but fate the same

Over time, they all want to become plant-unavailable phosphate rock

All phosphate fertilizers might not be created equal — but in the end they all wind up that way.

That was the message Don Flaten, a University of Manitoba soil science professor, shared with farmers last month during a session at Ag Days.

That’s because it’s a highly reactive compound and over time, the very different sources of phosphate applied to farm fields interacts with soil and organic material to eventually revert to phosphate rock. In some cases, that means several transformations for the products, Flaten said.

“It’s a very reactive compound,” Flaten said. “Over time, a lot of different sources of P become the same old stuff.”

That can mean there’s ample phosphate present in the soils, but little available for plant use, he said.

“There’s lots of P in our soils, as much as 1,000 to 2,000 pounds, but most of it is tied up,” Flaten said. “P gets stuck onto all sorts of different stuff all the time.”

That means that in the year of application, phosphate fertilizer efficacy is somewhere in the neighbourhood of 10 to 20 per cent.

“A lot of it winds up down here attached to soil particles or incorporated into soil solids,” he said.

That’s because there’s only one type of phosphorus that’s usable by the plant — orthophosphorus.

“It’s not in chains, there’s a single P atom per molecule,” Flaten said.

It’s difficult to predict how quickly it will be released in other forms bound to the soil.

Farmers are advised to continue to fertilize since common phosphorus fertilizers, such as 11-52-0, follow predictable pathways. It first reacts very quickly with the soil calcium and magnesium, which is typically in ample supply in Manitoba, to form dical (dicalcium phosphate dihydrate) which is similar to mineral supplements fed to livestock. It later forms octacalcium, which we all carry around in our teeth and bones. Neither are readily plant available, but are still better than the next, and final, step in the transformation, when it becomes phosphate rock, Flaten said.

“This is the form phosphate wants to be,” Flaten said. “It’s about as useful as gravel as a fertilizer.”

As needed

This inevitable transition means some of the best strategies for producers equal giving the plant exactly what it needs when it needs it. For example starter phosphorus on wheat crops, Flaten said, which seems to work well in research trials.

“It made a big difference in the early-season vigour of that wheat crop,” Flaten said, pointing to trial results. “Even a small amount of starter P can make a big difference in cold soils in the spring, even just 10 pounds.”

Flaten also said farmers in Western Canada seem to have a good handle on what works in their conditions. Monoammonium phosphate (MAP), typically 11-52-0, currently holds about 90 per cent of the market in the region.

“The last 75 years have set the stage for this,” Flaten said. “Farmers didn’t just pick this out of thin air.”

On the U.S. side of the line, the preferred phosphate source is triple super, or calcium dihydrogen phosphate as it’s also known. Little is sold on this side of the line, and Flaten said that’s a good thing. Soil calcium in this region is already doing a too-effective job of tying up soil phosphate, he said.

“Calcium goes after phosphate ions like a fox to a chicken,” he said. “Triple super has calcium in it, so you’re buying foxes with your chickens.

Flaten also noted that the most cost-effective phosphorus source was hog manure, and he was encouraged that many farmers seem to be adopting it. He also noted that the City of Winnipeg has signalled it intends to begin recycling nutrients from waste water.

“This is a very good news story,” he said. “We don’t really have a cycle going on here, and most of our phosphate comes from rock deposits in western Africa. Over time, it’s being mined out.”

About the author


Gord Gilmour

Gord Gilmour is Editor of the Manitoba Co-operator.

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