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Preplant soil nitrate test saves money, protects groundwater (Research Brief #2)

Posted July 1992

The preplant soil nitrate test can save farmers money while reducing the risk of groundwater contamination

A two-year, on-farm research project found that the preplant test, conducted on more than 100 Wisconsin corn fields, showed an average potential cost savings to farmers of $11.90 per acre in 1990 and $5.90 per acre in 1991.

“The goal of the test is to help farmers increase their profits while decreasing the risk of nitrate loss to groundwater,” says lead researcher Larry Bundy, a University of Wisconsin-Madison professor of soil science.

Wisconsin farmers spend more than $50 million each year for nitrogen fertilizer. Thus, even a small reduction in nitrogen use would save them a lot of money. In addition, there is evidence that nitrogen from fertilizer and non-fertilizer sources not used by crops seeps through the soil and pollutes the groundwater.

The preplant test measures the amount of residual or carryover nitrate in the root zone before planting. Samples for the preplant nitrate test are taken in early spring to a depth of two feet.

The preplant test allows farmers to adjust nitrogen applications to meet the needs of each specific field. Traditionally, farmers apply a standard amount of nitrogen to fields with similar soils and management histories.

“The test confirmed what I thought might be the case; there was plenty of nitrogen in the ground,” said Ken Congdon, a Trempealeau County farmer. “I cut my nitrogen inputs by 20 percent and I plan to cut back even more this year.”

Farmers conducted the preplant nitrate test on 134 fields in 15 counties in 1990 and 73 fields in 14 counties in 1991. Of the sites tested, 20 percent in 1990 and 4 percent in 1991 needed no additional nitrogen fertilizer to achieve optimum yields. Meanwhile, 90 percent of the sites in 1990 and 65 percent in 1991 could have achieved optimum yields with reduced nitrogen rates. Preplant soil nitrate-nitrogen content on the farms averaged 153 pounds per acre in 1990 and 80 pounds per acre in 1991.

UW-Madison nitrogen recommendations for corn vary from 80 to 200 pounds of nitrogen per acre, depending upon soil type, number of growing degree days, percent of soil organic matter and irrigation. For optimum yields, corn grown on a typical silt loam soil in southern Wisconsin with 2 to 5 percent organic matter needs 160 pounds of nitrogen per acre.

The study included four crop rotations: first-year corn following alfalfa, second-year corn following alfalfa, corn following soybeans, and continuous corn.

Cropping sequence significantly affects the amount of nitrogen in the soil available to corn. The preplant test is most useful in continuous corn, second-year fields, and fields with a history of manure applications. Researchers found the highest nitrogen carryover in second-year corn following alfalfa. Most plots in the study received manure before the alfalfa was plowed under.

The fact that there is excess nitrate-nitrogen in second-year corn shows that the legume and manure nitrogen credits often exceed the nitrogen needs of first-year corn, Bundy says. The preplant test provides a good method of adjusting nitrogen rates for second-year corn, he adds.

The study also showed fairly high residual nitrate levels in continuous corn. Sites where manure was applied to continuous corn had nearly twice as much nitrate-nitrogen available at planting time as sites where no manure was applied.

The preplant nitrate test doesn’t measure nitrogen from manure applied or alfalfa plowed down the current spring. Therefore, the test may not reflect all the nitrogen available to first-year corn grown after alfalfa. In many cases, Bundy notes, first-year corn following alfalfa needs little or no additional nitrogen.

The preplant test is most useful on medium- or heavy-textured soils and during years when precipitation is normal or below normal. Below normal precipitation in autumn and winter leads to higher spring preplant soil nitrate levels. But the test can provide farmers with substantial savings even in years with above normal precipitation, says Bundy.

Soil labs charge $10 to analyze a sample from a 20-acre field for preplant soil nitrate levels. Crop consultants typically charge an additional $1 per acre to collect soil for the preplant test. A 20-acre field takes about an hour to sample when 10 soil samples are taken.

In the future, Bundy hopes to improve the preplant test and make it more convenient for Wisconsin farmers to use. Further research is also needed to determine if current alfalfa and manure nitrogen credit recommendations are adequate for second-year corn.

For more information about this research, contact:

Larry Bundy
Department of Soil Science
University of Wisconsin-Madison
Madison, WI 53706
(608) 263-2889

Published as Research Brief #2
July, 1992