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Corn+Soybean Digest

Test For Nitrogen Carryover

You may have more nitrogen carryover on drought-stricken fields than you think.

Ron Gelderman, soil testing lab manager and plant science professor at South Dakota State University (SDSU), says soil testing is especially important after a drought year. Crop growth can be so limited during a drought that applied nitrogen and mineralized soil nitrogen is not fully used. This carryover nitrogen is available for next year's crop.

SDSU tests show that samples taken from failed crops had an average of 173 lbs/acre of carryover nitrogen in the top 2' of soil. The average from fields that were harvested was 73 lbs/acre of nitrate-nitrogen. Clearly, Gelderman says, crop use was a factor in how much carryover occurred.

Gelderman cautions producers not to use averages for their own fields or assume that they don't need fertilizer for next year. For example, even though the average soil test from harvested field samples was 73 lbs nitrate-nitrogen, they ranged from 29-163 lbs of nitrogen. Fields, even on the same farm, vary considerably in carryover nitrogen depending on factors such as rainfall, crop growth, nitrogen fertilizer applied and many other variables.

The only proven method to determine carryover nitrogen is with a soil test. These tests can save significant input dollars next spring for producers, Gelderman says.

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