Deep nitrate-nitrogen soil testing can provide a producer with the level of carryover nitrogen after a failed crop year, a South Dakota State University (SDSU) specialist says.
Ron Gelderman, the soil testing lab manager and plant science professor, says soil testing, a proven crop production practice, 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 utilized. This carryover nitrogen is available for next year's crop.
Recent samples at the SDSU Soil Testing Laboratory illustrate this carryover effect. Those samples that were 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, crop use was a factor in how much carryover occurred, Gelderman says.
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 nitrogen. Fields, even on the same farm, will 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.
Because soils are still mineralizing nitrogen, producers should wait until after Sept. 15 to take the deep nitrate-nitrogen soil test. However, for fall-seeded crops, samples can be taken earlier and an adjustment will be applied for early sampling.