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Corn+Soybean Digest

Soil Test To Save Nitrogen Costs

South Dakota farmers can save money this spring by soil testing to learn whether they can cut back on nitrogen, a South Dakota State University specialist said.

SDSU Extension Soils Specialist Jim Gerwing said several years of limited rainfall have minimized losses of nitrogen from the leaching and denitrification (the conversion of nitrate to a gas by soil microbes) that take place during wet years. Reduced yields in some areas also removed less nitrogen from soil. As a result, nitrate carryover from fertilizer is high in some fields.

"With the nitrogen fertilizer costs we're seeing, it would be an easy profit to do some soil testing and perhaps cut back the nitrogen rate a bit where carryover levels are high," Gerwing said.

Gerwing said not only are central South Dakota producers seeing high nitrogen carryovers, so are some farmers in the northeast, east and southeast – even though those parts of the state typically had more rain.

"Apparently efforts to achieve high or 'maximum' yields have resulted in higher nitrogen fertilizer applications than needed the last few years," Gerwing added.

Gerwing stressed that some farmers still will have to fertilize as usual. About 46% of soil samples coming in to SDSU's Soil Lab show less than 60 lbs./acre of nitrogen carryover in the top 2 ft. of soil, which is about normal.

But 29% of soil samples tested show nitrogen carryover of greater than 100 lbs./acre in the top 2 ft., and 6% of samples show above 200 lbs.

Gerwing added that soil testing is also a good indication of whether crops had enough nitrogen last year. If a field has at least 50 lbs. of nitrogen carryover in the top 2 ft., there was enough.

Gerwing said that farmers who worry about sacrificing yield by cutting back on nitrogen should be aware that most of the plant response comes from the first increments. In other words, a farmer who applies 120 lbs. of nitrogen to the acre in one shot can expect to get the biggest crop response from the first 40 lbs., less of a response from the second 40 lbs., and still less response from the final 40 lbs.

Every situation is different, Gerwing said, but farmers may come out ahead financially by sacrificing a few bushels of yield in order to trim fertilizer costs.

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