Fall-applied Nitrogen May Not Help Corn

February 10th, 2002

AMES, Iowa — There is evidence that nitrogen fertilizer applied last fall may not be much help to corn plants this summer, according to Iowa State University researchers.

Soil samples were collected in mid-December from three fields that received anhydrous ammonia in mid-November. The testing was done by Alfred Blackmer, agronomy professor, and Peter Kyveryga, a soil fertility graduate student.

The fields were in Greene County. Soil samples were collected from 25 areas to a depth of 18 inches in and between the bands of soil where fertilizer was injected. The test areas were selected to represent the range in soil characteristics within the fields.

The soils were tested because the weather was unusually warm. Warm weather enables bacteria in the soil to convert the fertilizer to nitrate by a process called nitrification.

Concentrations of ammonium and nitrate were measured in each sample. The researchers found that nitrification ranged from 21 to 50 percent, depending on the soil pH level. Percentage nitrification tended to increase with soil pH. Blackmer says the key finding in this study is that nitrification is occurring much more rapidly than generally believed, especially in soils with pH values greater than 6.5.

Blackmer says rapid conversion of fall-applied nitrogen from ammonium to nitrate is undesirable. "Nitrate is a form used by plants, but it is likely to be lost from fields by leaching when soils have excess water, which often occurs in March, April and May," he says. "Because plants don't really need the fertilizer until June, the problem can be minimized by delaying fertilization."

Blackmer says it's impossible to accurately predict how much nitrogen will be lost from Iowa cornfields before crops are planted this spring. "But evidence collected in this project, and during the past few years, suggests that more than half will be lost from many fields if we have above-average amounts of rainfall," he says.

Farmers often choose to apply fertilizer in the fall to make efficient use of their equipment and time. But Blackmer says efficient application to soils shouldn't be confused with efficient use by crops. "The costs of nitrogen losses before crops grow often are much greater than the dollars paid for the fertilizer," he says.

Blackmer says producers who apply fertilizer in the fall should check soil nitrate levels when plants are 6 to 12 inches tall if above-average spring rainfall occurs. Guidelines for testing are given in ISU Extension pamphlet 1714. The results should be interpreted by using nitrogen recommendations for manured soils.

This research is being funded by the Iowa Corn Promotion Board. The project also involves a search for alternatives to fall application of nitrogen.


Alfred Blackmer, Agronomy, (515) 294-7284

Ed Adcock, Communications Service, (515) 294-2314