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.
A recently-completed summary of soil samples from last fall through this spring from the SDSU Soil Testing Laboratory illustrate this carryover effect. Average carryover nitrogen levels are 65, 87 and 119 pounds of nitrate in the top two feet for the east, central and west river areas of South Dakota, respectively. Last season, drought stress was more severe as one traveled west and this led to less crop growth and plant use of available soil nitrogen, Gelderman says.
Gelderman cautions producers not to use averages for their own fields or assume that they need no fertilizer for next year. For example, even though the average soil test from one farm may average 75 pounds of nitrate-nitrogen, individual fields from that same farm may range from 40 to 160 pounds nitrogen. This variance in carryover nitrogen depends on factors such as rainfall, crop planted, crop growth, tillage, 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 this spring for producers, Gelderman says, especially with the current high nitrogen prices.