Tapping Into Legacy P Can Reduce Fertilizer Costs And Overuse

Phosphorus (P) and potassium (K) management get easier when you understand what a soil test reading is, and what it isn’t, says Farm Journal Field Agronomist Ken Ferrie. ( Lindsey Benne )

Lake Erie is only a hop, skip and jump north of David Myerholtz’s Gibsonburg, Ohio, farm. The Great Lake often garners national attention for its algae blooms and was a driving force in Myerholtz’s decision nearly a decade ago to address the levels of accumulated phosphorus (P), commonly referred to as legacy P, on his corn-soybean operation.

Today, he says he is confident he has cut his overall P use at least 20% by adhering to the 4R principles— the right fertilizer source, at the right rate, the right time, and in the right place—and tapping into legacy P levels.

“This legacy P issue haunts us,” he says. “We’d like to use even more than what we do, but the soil doesn’t readily release it for crop uptake.”

Part of the legacy P problem exists because there are two types of phosphorus in soils, Bray P1 and Bray P2. The former is readily taken up by crops. In some scenarios, the right amount of P can boost corn yields by up to 40 bu. per acre.

On the other hand, Bray P2, is not readily available to crops as it tends to bind tightly with soil particles. However, just like Bray P1, P2 will show up in nutrient runoff, an issue that perplexes and often confuses farmers, according to Anne Cook and Adam Lovelace, both with The Andersons.

“Farmers and retailers know they’re using less P, so there’s sometimes a gap of understanding about why it’s still coming out of the drainage tiles at the same levels as before,” explains Cook, senior environmental, health and safety manager for The Andersons.

Unfortunately, there is no silver bullet to fix legacy P, says Lamonte Garber, Watershed Restoration Coordinator for the Stroud Water Research Center, Avondale, Pa. “A farmer’s ability to draw down the nutrient requires multiple strategies that work together,” he notes.

Some of those strategies include growing cover crops to extract nutrients and sequester organic material (thus improving the nutrient cycle) and using soil tests extensively to identify where accumulated P is located so farmers, like Myerholtz, can more accurately tap into it.

“You can’t manage it if you don’t know how much you have and where it is,” says Myerholtz, who soil tests every other year and also uses products that help “unbind” legacy P.

Lovelace cautions that the draw-down process takes time. “You should be able to detect and quantify the amount of phosphorus dropping in your soil in a three- to five-year period,” says Lovelace, director of farm centers for The Andersons.

Better phosphorus management and nutrient stewardship are good for the present state of production agriculture, and it can also help protect the long-term viability of farming operations for future generations, Garber notes.

“We don’t know where regulations will go in the future,” he says. “We also don’t want farmers backing themselves into a corner with high soil phosphorus levels that could become one of those things a future buyer might look at or could affect the farm real-estate value.”


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