News Releases

Farmers who apply manure to fields must decide if the corn they'll plant will also need an application of commercial nitrogen fertilizer in order to get the best yields. How do they decide?

On-farm research trials led by Iowa State University researchers have shown that soil tests taken after corn plants emerge is an effective tool to help them make that decision. The study was published in the January-February 2004 issue of the Agronomy Journal.

The study compared the performance of two methods for estimating nitrogen fertilizer needs after the application of manure. One method is following general guidelines based on the estimated difference between amounts of nitrogen in the manure and the fertilizer needs of the crop at expected yield levels. The other method is soil testing for nitrate when corn plants are about 6 inches tall. 

The methods were compared for their ability to predict corn yield responses to added fertilizer nitrogen across 205 on-farm trials in 28 Iowa counties. The soil-testing approach explained five times more of the variability in yield response than did the general-guideline approach.

Alfred Blackmer, the Iowa State agronomy professor who led the study, said the general-guideline approach cannot predict losses of manure-nitrogen from fields before corn crops start rapid growth in June.

"Farmers who use the soil-testing approach have a way to determine these losses," Blackmer said. "Soil testing helps them identify management practices that are the most likely to increase their profits while reducing nitrate pollution in surface waters."   

The work was supported by grants from the Leopold Center for Sustainable Agriculture and the Iowa Corn Promotion Board.

The Agronomy Journal is published by the American Society of Agronomy.