After drought: How much N credit for next year?
The dry conditions and reductions in yield in many fields this season have resulted in lots of questions about plans for next year. Are nitrogen (N) rate adjustments needed for the 2013 corn crop?
First, the easy answers. In areas where corn yields were somewhat reduced but overall production practices have been normal, there should be no need to adjust the typical practices for next year's crop. For corn following corn, use the maximum return to nitrogen (MRTN) or profitable N rate range calculated with the corn N rate calculator. Similarly for soybean fields, even if they yielded nothing or very little, no special change should be made in nitrogen-management decisions. The corn N rate calculator has the option to select soybean as the previous crop when calculating MRTN for the corn crop in rotation. The reason you should not worry about adjusting the MRTN value according to the condition of the soybean crop is that soybeans do not leave excess N in the soil. Corn following soybean often needs less nitrogen because, among other factors, the quantity and quality of soybean residue compared to corn residue reduce the amount of immobilization and increase the amount of nitrogen mineralization from crop residue and the soil. Using nitrogen response data for corn following soybean, the calculator already adjusts for what we used to call an "N credit." Compared to corn following corn for Illinois, the calculator shows that the N rate for corn following soybean for the north, central, and south regions of the state can be reduced approximately 50, 30, and 20 lb N/acre, respectively.
Now let's talk about the difficult answers--those for fields where the corn crop was severely affected by drought and the amount of N taken up was reduced. In these situations it is likely that large amounts of unused nitrate-N would be left in the field. What is hard to know is how much of that nitrate to consider available for next year, and thus how much to adjust N rate applications. While there is no simple guideline, it is possible to approximate how much N would be available either indirectly, by determining how much was removed in grain, or directly, by measuring soil nitrate-N (NO3-N) levels.
The indirect measurement using grain yield is easier, but I believe it provides less certainty. On average, a bushel of corn removes 0.66 lb N. Multiplying this removal rate by the bushels produced per acre and subtracting it from the amount of N applied per acre would give an idea of how much N remains in the field. Of course, not all that N will be plant-available the next growing season. Under low N-loss potential (normal precipitation or drier conditions during the time between harvest and next spring), one should anticipate approximately 50% of the total remaining N to be available for the 2013 crop. For example, if a field received 180 lb N/acre and only yielded 70 bushels/acre, then the remaining N would be 180 - 46 (N in grain: 70 x 0.66) = 134 lb N/acre. Multiplying that difference by 0.5 (50% availability) equals 67 lb N/acre that could be subtracted from the nitrogen rate for next year.