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Primer Nitrogen: Rethink Fertilizer Rates, Tech

05/09/2014 @ 1:15pm

Wetter springs, more technology, cheaper natural gas, and more environmental pressure. All are things that could change the way you fertilize corn and other crops. Peter Scharf, University of Missouri crop nutrient professor, says the time is right for a refresher course on fertilizer – particularly nitrogen (N) fertilizer – and how to get the most from it.

N still greatly improves productivity. You know that from watching what it does in your cornfields, but Scharf has a much starker example. “North Korea uses about one third as much N fertilizer as it did in 1989,” he says. “That was when the Soviet Union collapsed and stopped supplying them with crop fertilizer. Their grain production dropped in half. The result has been real, literal starvation. N is crucial to putting food on the world’s table.”

Gorilla vs. beetle

If you take a typical Midwest soil and measure all of the N in 1 acre to a depth of 3 feet, it will be about 11,000 pounds on average. “That’s the gorilla,” says Scharf. “Comparatively, the 150 pounds we might add to an acre in a year it grows corn is the beetle. It’s tiny compared to what is already there.” 

Of course, the hard part is getting the soil to give up its N gorilla in any given year. How much it gives is largely determined by weather, so you apply the N beetle to make up for what the soil holds onto. 

“We estimate that of the N we apply to a field, about 40% of it gets into the crop, 35% replaces what is used from soil organic matter, and 25% is lost,” says Scharf. “We’re getting better on that last part, but it’ll never be zero.”

Variability is huge

On a Missouri field that Scharf studied, yield with zero N fertilizer varied across the field from 13 bushels an acre to 145 bushels. What makes the difference?

“It’s the amount of N that the soil is kicking in or not kicking in,” says Scharf. “When we fertilize that field, the yield goes way up and the yield variability goes way down. Honestly, we don’t know why the soil kicks in more N in some places than in others. Soil type, weather, and management history are probably all factors.”

There’s another way to look at this variability, Scharf says. “We may have a field in which the best rate at the whole-field level is 140 pounds of applied N per acre. But some areas of that field need 60 pounds to maximize yield and some need 220 pounds. That degree of variability within fields is very common.”

Variable-rate technology (VRT) continues to grow in favor with fertilizer users. It manages soil N variability by using such things as crop color sensors to determine if more fertilizer is needed. 

In corn experiments at 55 locations around Missouri, crop color sensors and VRT saved 14 pounds of N per acre on average and yielded 2 bushels per acre better. It may not sound like much, but it was worth about $18 an acre. 

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