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After drought: How much N credit for next year?

Fabián Fernández, University of Illinois | September 5, 2012

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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.

The direct measurement using soil nitrate-N levels is, in my opinion, more reliable than the indirect measurement, but there still is a lot of uncertainty in the estimation since N availability for the 2013 crop will largely be a function of how much N-loss potential exists between fall and spring. To obtain a reliable measurement of nitrate-N, a composite of at least 12 cores should be collected, taken to a depth of two to three feet from representative portions of the field and at different positions with respect to the crop-row. Each foot of sampling depth should be kept as a separate sample for analysis. Where little nitrate movement is expected, the 2-foot depth should be adequate, while the 3-foot depth would be more appropriate to quantify nitrate-N in fields where rain might have moved the nutrient deeper in the profile. In any given year, nitrate is present in the soil at the end of the growing season. Research has shown that for a 2-foot sample, only any remaining amount above 40 lb nitrate-N/acre should be subtracted from the application rate.

Because of the large uncertainty on how much carryover N will be present next spring for the 2013 crop, perhaps the best approach to adjusting N rates based on how much carryover N is present is to collect soil samples in the spring to adjust preplant or sidedress rates. This approach, of course, implies doing N applications in the spring. As with fall sampling, large variability can be present in the field, so a reliable measurement is possible only if the sampling strategy represents the field properly. In normal years a 1-foot sample is recommended for spring samples, but given present conditions I would recommend a 2-foot sample to quantify nitrate-N in the lower profile.

Another approach that can be used to reduce N rates when some carryover N is suspected but is difficult to estimate is to choose the lower end of the profitable N rate range in the corn N rate calculator.

Finally, I have received questions about using the end-of-season lower corn stalk nitrate test to determine adjustments on N rates for the next year. I do not recommend using this test because values will probably be artificially high because of the dry conditions that reduced plant growth and grain production.

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