If the corn plant vegetation and/or grain yield was drastically affected by drought conditions, then N uptake would have been reduced and unused nitrate-N could be accounted for in determining the N fertilization rate for the 2013 corn crop. There are two methods to estimate carryover N. The direct method is to sample the soil profile (a minimum of 2 ft.) after harvest and measure the nitrate-N concentration. Sampling would be by 1-ft. increments. If dry conditions persist, most applied N should remain in the top 2 ft. Sampling to 3 ft. would be preferable, especially where rainfall was enough to move nitrate deeper in the profile. To add up nitrate-N in the sampled profile, multiply the concentration in each foot by four to get the nitrate-N amount per foot and then add the amounts together. One would not want to account for all of the nitrate-N as a subtraction from the next crop N recommendation as there is always some nitrate in the profile at the end of the season. A suggestion from research conducted in Wisconsin (which should be appropriate for Iowa) is to only account for nitrate-N greater than 40 lbs. nitrate-N (2-ft. depth) or 50 lbs. nitrate-N/acre (3-ft. depth), with the remaining amount then subtracted from the normal rate recommendation. A second method to estimate carryover nitrate-N is to use the 2012 corn grain yield. Take the total N applied for the 2012 corn crop and subtract the 2012 grain yield in bushels per acre. Then assume 50% of that amount will remain available to the 2013 crop if precipitation is normal or below normal for the fall/winter/early spring. The remaining nitrate-N amount will vary depending on the actual rainfall and potential losses from fall through spring. For example, if the total N application for the 2012 crop was 190 lbs. N/acre and the 2012 corn yield was 50 bu./acre, then the unused N would be 190 - 50 = 140 lbs. N/acre. The 140 lbs. N/acre times 50% leaves 70 lbs. N/acre to subtract from the 2013 rate recommendation.

As a conservative approach, and due to uncertainty with either estimation method, a minimum rate recommendation of 50 lbs. N/acre should be considered. If fall/spring precipitation is well above normal, then the carryover nitrate would not be likely, especially in soils with high leaching potential. Sandy soils are not likely to retain carryover nitrate.

Spring soil profile sampling for nitrate-N is an option, especially with concerns about residual nitrate remaining after the fall/winter. In addition, such sampling could allow for a spring preplant or sidedress N application based on spring profile nitrate-N results, and instead of a fall application. Use of the late spring soil nitrate test (LSNT) to determine carryover nitrate may miss considerable nitrate deeper than in just the top foot. Therefore, it would be better to sample the deeper profile before planting.

There could be considerable variation in nitrate levels across fields, due to yield level, banded N application, and soil/topography. Therefore, many cores (at least 12) should be collected per sample, and multiple samples per field from representative areas. Since the cores are by 1-ft. depths, mixing in the field will be needed to obtain a representative sample for each depth. Keep the soil from each depth as a separate sample to send to the lab.