The standard nitrogen (N) rule of thumb, which for corn following soybeans was 1 lb. N/bu. of corn, is no longer the standard, according to Purdue University experts.
“The yield response to nitrogen is not really a straight line like the rule would imply,” says Bob Nielsen, Purdue Extension agronomist.
Purdue research has found the N rate needed for maximum yield (or agronomic optimum) for corn following soybean rotation is about 173 lbs. N/acre. But if N costs 60¢/lb. and the grain price for corn is $4, then the economic optimum rate drops to only 147 lbs. N/acre, Nielsen says. For corn following corn, agronomic and economic optimum N rates are about 30 lbs. more.
However, Nielsen points out that it's important to recognize these N rates are a midpoint and that in any given year, the actual optimum rate may vary plus or minus 30 lbs., depending on rainfall and soil types.
“If growers think they've had a lot of nitrogen loss prior to the time of sidedressing, they may want to bump the nitrogen rates up 20-30 lbs. to account for the nitrogen they've lost,” Nielsen explains. “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 20-30 lbs.”
Using the midpoint numbers, the new N rate recommendations would increase the dollar return from N fertilizer by about $5/acre for corn following soybeans or corn following corn with N fertilizer priced at 60¢/lb. For a 1,000 acre farm, that's a savings of at least $5,000.