Producers who apply manure to fields must decide if the corn they’ll plant also will 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 show that soil tests taken after corn plants emerge is an effective tool to help 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 of estimating nitrogen fertilizer needs after manure application. One method is to follow general guidelines based on the estimated difference between nitrogen amounts in the manure and the crop’s fertilizer needs at expected yield levels. The other method is to test the soil for nitrates 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.

“Producers who use the soil-testing approach have a way to determine these losses,” says Alfred Blackmer, Iowa State agronomy professor and study leader. “Soil testing helps identify management practices that are the most likely to increase a producers’ 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.

You can access the Agronomy Journal article on the Internet at: