Look Before You Cut
Global English [ Change Region ]

Look Before You Cut

April 14, 2009   |   ShareThis

By Gil Gullickson, Crops Technology Editor, Successful Farming

High fertilizer prices amid plunging commodity prices are prompting some of you to consider cutting back or maintaining fertilizer rates for 2009. This may work in a number of cases. Still, slicing fertilizer rates too much can also slice yields. Here's a roundup of what to do for nitrogen (N), phosphorus (P), and potassium (K) needs in 2009.

Nitrogen Needs

In many areas, pegging the optimum N rate for corn mimics trying to snag a slippery salmon swimming in a swift stream.

"It's difficult to do, especially in states like Illinois, Iowa and Indiana, because there is no soil test that really works," says Bob Hoeft, University of Illinois (U of I) Extension soils specialist.

Western Corn Belt areas have successfully used the nitrate-N soil test to peg N needs. This soil test isn't as accurate in the central and eastern Corn Belt, though. That's because more rainfall makes it difficult to pinpoint crop needs for this mobile nutrient that can rapidly leach through soils.

For example, the optimum N rate at a U of I research site at Monmouth in west-central Illinois has ranged in recent years between 70 and 240 pounds per acre, Hoeft says.

Your best bet in these areas is an N-rate calculator developed by soil scientists in Iowa, Illinois, Indiana, Minnesota and Wisconsin, Hoeft says. This calculator takes into account expected corn and N prices as it calculates the N rate most likely to provide your best investment return.

You can access the website at http://extension.agron.iastate.edu/soilfertility/nrate.aspx.

Phosphorus and Potassium Needs

"The tendency for a lot of farmers facing the high input costs of phosphorus and potassium is to cut back on those and keep their nitrogen levels up," says Dan Froehlich, director of agronomy for The Mosaic Company. "That may be OK as long as there are adequate levels of soil phosphorus and potassium."

To find out for sure, soil test. The recommended soil P level is 20 parts per million (ppm) for 180-bushel-per-acre corn. Below this level, yields can decline.

"If you have just 10 ppm in your soils and can't meet the critical 20 ppm level, you may have 20% less corn yields," says Froehlich.

It's a similar scenario for K at the 180-bushel-per-acre level.

"If you are below the critical level of 165 ppm for potassium, you definitely want to add to that," says Froehlich.

"With 180-bushel corn, soil P and K levels drop an annual 4 ppm and 6 ppm, respectively, with no P and K applications," he adds. "Several years of not adding P and K could take farmers below these levels."

Learn more at www.agriculture.com