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What's a pound of nitrogen worth to you?

Agriculture.com Staff 01/22/2008 @ 1:54pm

The standard nitrogen rule of thumb, which for corn following soybeans was one pound of nitrogen per bushel of corn, is no longer the standard, according to Purdue University specialists.

"The cost of nitrogen ranges from 43 cents to 60 cents per pound, but the bottom line is that nitrogen is expensive and its price continues to increase," says Purdue Extension agronomist and corn management specialist Bob Nielsen in a university report. "In the last year or so, nitrogen fertilizer has become one of the most costly variable input costs for corn production, which is much of the reason behind the nitrogen work we've been doing for the last couple of years."

As nitrogen fertilizer became more expensive, economics needed to be factored into the development of nitrogen recommendations, Nielsen says.

"The yield response to nitrogen is not really a straight line like the rule would imply," he says. "If you take the rule of thumb to its limit, you could apply 400 pounds of nitrogen and get 400 bushels an acre, and we all know that's not true.

"The response curve is generally more of a curved line, meaning you get about a pound per bushel on the first 50 or 100 pounds of nitrogen that you put on. Then that curve starts to flatten out and as you get to higher and higher yields; it actually requires more and more pounds of nitrogen to get those last few bushels. At some point near the plateau will be the economic rate beyond which you just can't afford to apply more nitrogen."

Nitrogen rate trials were conducted by Nielsen and Jim Camberato, Purdue Extension soil fertility specialist, for the past two years on many of the Purdue research farms and more than 30 sites around the state.

"We've found the nitrogen rate needed for maximum yield (or agronomic optimum) for corn following soybean rotation is about 173 pounds of nitrogen per acre," Nielsen says. "But if nitrogen costs 60 cents per pound and the grain price for corn is $4, then the economic optimum rate drops to only 147 pounds of nitrogen per acre. For corn following corn, agronomic and economic optimum nitrogen rates are about 30 pounds more."

However, Nielsen points out that it is important to recognize these nitrogen rates are a midpoint and that in any given year the actual optimum rate may vary plus or minus 30 pounds, depending on rainfall and soil types.

"If growers think they've had a lot of nitrogen loss prior to the time of side-dressing, they may want to bump the nitrogen rates up 20 to 30 pounds to account for the nitrogen they've lost," Nielsen says. "Conversely, in a drier year where we expect much less nitrogen loss from the soil, growers may actually be able to back off those midpoint recommendations by 20 to 30 pounds."

Using the midpoint numbers, the new nitrogen rate recommendations would increase the dollar return from nitrogen fertilizer by about $5 per acre for corn following soybeans or corn following corn with nitrogen fertilizer priced at 60 cents per pound. For a 1,000-acre farm, that's a savings of at least $5,000.

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