Iowa State Researchers Work with Farmers to Cut Fertilizer Rates

March 26th, 2002

AMES, Iowa — Research during the past decade has shown that most corn producers could substantially reduce rates of commercial fertilizer if they delayed application until after plants emerge. Additional supporting evidence was collected in precision farming trials in 2001.

"Mounting evidence suggests it is easier to develop reliable recommendations for in-season applications of nitrogen than for preseason applications," says Alfred Blackmer, agronomy professor at Iowa State University. "The primary reason is that delaying applications minimizes nitrogen loss during spring rainfalls."

Blackmer worked with private farmers during the 2001 growing season to study this issue. Thirteen fields planted to corn following soybeans were used in the study. None had received recent applications of animal manure or more than 30 pounds of nitrogen per acre since harvest of the previous crop.

Fertilizer was applied at rates of 75 and 125 pounds of nitrogen per acre in alternating strips after corn plants were at least three inches tall. Each treatment was replicated five times in strips that were the length of the field and at least two combine-swaths in width. The strips were harvested by cooperating producers who had combines equipped with yield monitors and global positioning system receivers.

"These studies were conducted to evaluate the possibility that, for corn after soybean, profits can be maximized by fertilizing at a rate of 100 pounds of nitrogen per acre after the crop has emerged from the soil," Blackmer says. "Such a simple recommendation would not apply to all situations, but it makes sense to start learning where it would be reasonable."

Results showed the average difference in yield over all sites was 4.1 bushels per acre. The value of four bushels of corn is about the same as the cost of 50 pounds of nitrogen. So the Iowa State researchers concluded the two rates of nitrogen applied in the field trials provided equal profits.

Blackmer says that means applying nitrogen at a rate of 100 pounds per acre maximizes profits and leaves a small margin for error. "This rate is substantially less than would be called for by recommendations based on yield goals and credits for nitrogen supplied by a previous soybean crop," he says.

The field trials included analyses to determine where yield differences occurred within fields. Remote sensing, digitized soil survey maps, differential elevation maps and maps of electrical conductivity were used to relate soil characteristics to spatial patterns in yield differences.

This information is being used to develop recommendations for variable-rate fertilizer application.

"Delaying application of nitrogen until plants are a few inches in height deserves attention as a way for Iowa corn producers to address economic and environmental problems associated with nitrogen fertilizer," Blackmer says.

Blackmer will work this year with a number of producers on similar field trials. "These studies demonstrate how Iowa farmers are using powerful new technologies to find practical ways to minimize losses of nitrates to our water supplies," he says.

The precision farming trials are funded by the Iowa Corn Promotion Board and the Iowa Soybean Promotion Board.


Alfred Blackmer, Agronomy, (515) 294-7284

Susan Thompson, Agriculture Communications, (515) 294-0705