Nitrate losses through leaching and denitrification, and the soil’s ability to supply nitrogen through mineralization of organic matter, are probably the two largest variables in selecting the best nitrogen application rates for corn production. Temperature and moisture are the controlling factors that drive these processes and, while the weather is beyond our control, nitrate and ammonium soil testing are valuable tools that can guide us to better management decisions. When compared to traditional soil sampling for nutrient content please consider some critical process changes that need to be made when analyzing for nitrate and ammonium. Soil cores must be taken at a depth of 0”-12” and it is best to collect 15-20 cores per sample because of the higher variability of nitrogen content across the landscape. The samples must be shipped to the lab as quickly as possible (1-2 days) for best results.
Results are normally reported as NO3-N (nitrate-nitrogen) and NH4-N (ammonium-nitrogen) measured as a concentration in parts per million. While these samples have been analyzed for the nitrate and ammonium compounds, the results have been converted by calculation to reflect the actual nitrogen content of each nitrogen form, and they can be added together to estimate the combined nitrogen. Please remember that the results are based on a 12” sample and must be multiplied by 4 to estimate nitrogen in pounds per acre. Additional samples can be collected from deeper in the soil profile (12”-24” and 24”-36”) to track mobile nitrate which can leach downward with soil moisture.
Jim Camberato and Robert Neilsen with Purdue Extension have calculated the theoretical nitrogen levels that can be expected based on fertilizer nitrogen applied. These values are a good starting point when estimating losses that may have occurred prior to the nitrate and ammonium soil sampling date and can be used to fine-tune side-dress target rates.
Monday morning quarterbacks can usually do an admirable job determining the ideal nitrogen rate for last year’s corn crop but estimating nitrogen losses, soil nitrogen mineralization, and dialing in the perfect nitrogen rates for the current crop has many challenges. Soil nitrate and ammonium testing can be a useful tool to aid in these decisions.