Soil tests taken after corn emerges can effectively help growers who have already applied manure to determine if they’ll also need to add commercial nitrogen.

That’s according to on-farm research trials led by Iowa State University researchers. A study was published in the January-February 2004 issue of the Agronomy Journal.

It compared the performance of two methods for estimating nitrogen fertilizer needs after the application of manure. One method follows general guidelines based on the estimated difference between amounts of N 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 in. tall.

The methods were compared for their ability to predict corn yield responses to added fertilizer N 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 agronomist who led the study, says 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 says. “Soil testing helps them identify management practices that are the most likely to increase their profits while reducing nitrate pollution in surface waters.”