Farmers in parts of Missouri that were parched last summer had lower corn yields than normal, and a University of Missouri agronomist said one result has been nitrogen-fertilizer carryover in many of the hardest-hit fields.
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“A wide swath of Missouri, from the northwest corner down to central Missouri, received very little rain during the peak water uptake time for corn,” said Peter Scharf, MU associate professor of agronomy. “Corn yields were low for most fields in this area, meaning that nitrogen uptake also was low. The crop was not able to effectively use the nitrogen fertilizer that had been applied.”
Rainfall after harvest and throughout the winter has been “a little above average” in that area, he said. “There has been enough rain in most fields that water has moved down through the soil profile, possibly carrying unused nitrogen with it.”
In Atchison, Linn and northern Ray counties last month, Scharf undertook to find out how much of the unused nitrogen remained in the soil. He and his research team sampled soil to a depth of 3 feet in five fields where corn was grown last year. They found the average soil nitrogen content to be 105 lb/acre.
“Typically, we expect to find about 50 pounds of nitrogen per acre in a field that has not been fertilized, so our soil test results were about 55 pounds per acre higher than normal,” he said. “This suggests that if these fields are returned to corn, nitrogen rates could be cut back without hurting yield.”
Scharf cautioned that nitrogen now in the soil “could be lost before the corn has a change to get it, if we have a wet spring.”
Also, nitrogen isn’t the only thing that can carry over in a cornfield from year to year, he said. “In many years, there will be a rotation effect that leads to lower yields for corn following corn. This may be due to carryover of disease organisms or insect pests.”
Excess nitrogen aside, Scharf observed, “for many soils and fields in Missouri, soybeans on average are more profitable than corn, and in these fields it makes sense to plant soybeans even though the soil contains residual nitrogen that won’t benefit the beans.”
In March, Scharf also sampled three fields in which beans were grown last year, finding the average soil nitrogen content to be 52 pounds per acre, “which is just what we’d expect in a normal year.” Farmers planting corn following 2003 soybeans should use normal nitrogen rates, he said.