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Field Notes: How nitrogen fertilizer killed crop rotation

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John Pesek spent 42 years with the agronomy department at Iowa State University, retiring in 1992. He said no amount of nitrogen fertilizer allowed for continuous corn production. (Courtesy John Pesek)
John Pesek spent 42 years with the agronomy department at Iowa State University, retiring in 1992. He said no amount of nitrogen fertilizer allowed for continuous corn production. (Courtesy John Pesek)

This is the latest installment of Harvest Public Media’s Field Notes, in which reporters talk to newsmakers and experts about important issues related to food production.

For this episode of Field Notes, we hear from an agronomist with a unique perspective on Midwest agriculture. John Pesek began his career at Iowa State University in Ames in 1950. He retired in 1992 after serving as head of the agronomy department for 26 of those years. He recently spoke with Harvest Public Media’s Amy Mayer about how the introduction of nitrogen fertilizer some 60 years ago fundamentally changed farming.

Today, Midwest farms often rotate between just corn and soybeans. But Pesek said that decades ago, oats, clover and alfalfa were a big part of the mix—and more farms also had livestock. The combination allowed for replenishment of nitrogen in the soil, both through the spreading of manure and by varying what types of crops went into the land. 

But after World War II, nitrogen fertilizer became widely available,  and that made rotations and livestock less necessary. 

"So farmers all of a sudden could convert some of their land to more valuable corn than oats, which was the most common recent crop that was in rotation," Pesek said.

The big question arose: Could that corn now be planted year after year? Pesek said university researchers jumped on that question. 

"And we have a set of plots on the old agronomy farm south of town that has been producing corn after corn since 1916 and after going down, down, down on un-amended soil plots, adding nitrogen made a tremendous increase in yield," he said.

But ultimately  the research showed that no amount of fertilizer could make continuous corn production practical. Pesek said even when all the nitrogen the plants needed was added,the yields dropped 10 percent with continuous corn plantings. And that may be why so many farmers today rotate corn with soybeans. 

"Even growing soybeans increases the yield of the following corn crop about 10-15 percent more than is produced by corn after corn in the same field instead of corn after soybeans," Pesek said.