Corn is now in the seed filling growth stages (R2-R5) when nutrient demands are put on plant leaves as the seed kernels develop. At R2 blister stage, approximately 75% of the corn plant’s required nitrogen (N) is in the plant. The remaining 25% still needs to be taken up by the roots. When the plant is water stressed, N uptake is limited, resulting in more N being transported from the leaves to the seed kernels. Leaves below the ear are the main sources for N in the seed when the corn plant is stressed for water and N or in the case when the applied N rate is too low.
Although plant tissue testing is probably not the best tool for evaluating N status of the plant during these late grain filling growth stages, visual observations can give good indications of plant N status. N deficiency appears first on the lower leaves of the plant as leaf yellowing and tissue die-off which starts at the leaf tip and forms a V-shape pointing down the mid-rib of the leaf (see nitrogen leaf symptom at: http://www.sdstate.edu/ps/extension/soil-fert/corn-deficiency-photos.cfm).
A long-term study at the SE Research Farm near Beresford evaluated nitrogen rates for corn. In this study, % leaf greenness was visually estimated for each leaf below the ear from 10 plants in each N rate plot. Percent leaf greenness was then compared (regressed) with grain yield and it was determined that if the third and fourth leaf below the primary ear leaf were green (without visual N deficiency) for corn following corn and soybean respectively, yield should not have been limited due to lack of N (for more research information see the Soil/Water Research: 2009 Progress Report.
The late-season stalk nitrate-N test is another way to evaluate corn N status. For more information see the full Iowa State University report. With this test, stalk samples are taken when corn plants reach maturity and submitted to a laboratory for nitrate-N analysis. The interpretation of these results does not result in an exact determination of the correct N rate, but can be helpful in determining if the corn plant N status was low, marginal, optimal or excessive. Post season soil samples obtained from the two foot depth, can also be a way to determine how much nitrate-N is remaining in the soil and potentially not utilized by the corn plant. Using the check book method of accounting for nitrogen in the soil and taken up by corn plants is not a good approach.
N mineralization from the soils organic matter plays a large role in contributing to the variability of the amount of nitrate-N that can exist in the soil profile. It is hard to predict how much N will be mineralized because it is controlled mainly by previous plant residue and manure additions and changes in soil temperature and moisture. Across the last 10 years in South Dakota, the average nitrate-N remaining in the soil after corn grain harvest has been 86 lbs N/a (0-2 ft depth). Hopefully, these suggestions for evaluating late season nitrogen deficiency will help to determine more effective use of nitrogen for corn production.