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What You Must Know About Phosphorus

February 4, 2010
By: Sara Schafer, Farm Journal Media Business and Crops Editor

When you grow corn, the first nutrient you think about is nitrogen (N). But phosphorus (P) is equally impor-tant. The right timing and placement can boost yield—up to 30 bu. to 40 bu. per acre in years of late planting, such as 2008 and 2009.

In July, Farm Journal Field Agronomist Ken Ferrie explained P management to attendees at Farm Journal Corn College. Demonstration plots at the site near Bloomington, Ill., reinforced his points about timing and placement.

Why crops need phosphorus. To set the stage, Ferrie says, "phosphorus is required for the elongation and division of cells and for the transfer of starches and especially sugars. If a corn plant can't transfer sugars, which are produced in the upper part of the plant, to other areas, the plant stops growing and turns purple. Something similar happens to trees in the fall; the tree stops transferring sugar, and we see the leaves change color.

"By the time a corn plant reaches 25% of its dry-matter weight, it needs to have 75% of its phosphorus on board,” Ferrie continues. "If it doesn't have enough phosphorus, the plant will have problems. Nitrogen can be taken aboard throughout the growing season, but phosphorus must be early.”

Think of P as fuel for the growing process. Whether it is in plants, microbes or animals, the compounds inside cells, called ATP and ADP, provide energy to drive growth. Those compounds contain P.

There's more to it than that, of course. Ultimately, energy for plants comes from the sun, Ferrie explains. Plants use photosynthesis to turn sunlight into energy. But that process is inefficient without P.

P picked up by plants from the soil is eventually transferred to the grain during grain fill. Then it leaves the field at harvest and is consumed by animals or humans. The P in soil is essential to everything living on Earth.

How phosphorus reaches roots. Roots come in contact with P by interception, mass flow and diffusion, which occur simultaneously. In interception, as roots grow through the soil they come in contact with P.

Definitions You Need to Know
■Carbon penalty: Phosphorus fertilizer immobilized by soil microbes during decomposition early in the growing season.

■Immobilization: The process by which soil microbes render phos-phorus unavailable to plants by converting it to the organic form.

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FEATURED IN: Farm Journal - January 2010

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