Relating Soil Nitrogen to Corn Crop N Need.

Despite logistical challenges, using inorganic N levels from soil samples taken during vegetative crop development remains a practice and a goal of those developing and marketing the means to apply N to the crop at any stage of growth. There is a strong temporal component to this relationship: crop N uptake rates of 6 to 8 kg N/ha/day during rapid vegetative development mean a concomitant drop of up to 2 ppm/day of inorganic N in the upper 60 cm of soil, with this rate of decrease moderated by ongoing mineralization of soil organic N. In one series of experiments involving different forms and application times of fertilizer N, we found that soil N levels associated with full yield were only about 12 ppm in the top 60 cm in mid-June (stage V9-V10), and were less than 10 ppm by VT. In another study with N rates, the soil nitrate level associated with optimum yield dropped from about 40 ppm at stage V5 to 30 ppm at stage V9 to 10 ppm at stage VT. Such inconsistency, and the rapid change in critical soil N levels over time, point to the difficulties inherent in setting these levels as a way to guide application of more fertilizer during crop development.